mediawiki.js: Fix eslint valid-jsdoc

[lhc/web/wiklou.git] / resources / src / mediawiki / mediawiki.js

/**
 * Base library for MediaWiki.
 *
 * Exposed globally as `mediaWiki` with `mw` as shortcut.
 *
 * @class mw
 * @alternateClassName mediaWiki
 * @singleton
 */

( function ( $ ) {
        'use strict';

        var mw, StringSet, log,
                hasOwn = Object.prototype.hasOwnProperty,
                slice = Array.prototype.slice,
                trackCallbacks = $.Callbacks( 'memory' ),
                trackHandlers = [],
                trackQueue = [];

        /**
         * FNV132 hash function
         *
         * This function implements the 32-bit version of FNV-1.
         * It is equivalent to hash( 'fnv132', ... ) in PHP, except
         * its output is base 36 rather than hex.
         * See <https://en.wikipedia.org/wiki/FNV_hash_function>
         *
         * @private
         * @param {string} str String to hash
         * @return {string} hash as an seven-character base 36 string
         */
        function fnv132( str ) {
                /* eslint-disable no-bitwise */
                var hash = 0x811C9DC5,
                        i;

                for ( i = 0; i < str.length; i++ ) {
                        hash += ( hash << 1 ) + ( hash << 4 ) + ( hash << 7 ) + ( hash << 8 ) + ( hash << 24 );
                        hash ^= str.charCodeAt( i );
                }

                hash = ( hash >>> 0 ).toString( 36 );
                while ( hash.length < 7 ) {
                        hash = '0' + hash;
                }

                return hash;
                /* eslint-enable no-bitwise */
        }

        function defineFallbacks() {
                // <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set>
                StringSet = window.Set || ( function () {
                        /**
                         * @private
                         * @class
                         */
                        function StringSet() {
                                this.set = {};
                        }
                        StringSet.prototype.add = function ( value ) {
                                this.set[ value ] = true;
                        };
                        StringSet.prototype.has = function ( value ) {
                                return hasOwn.call( this.set, value );
                        };
                        return StringSet;
                }() );
        }

        /**
         * Alias property to the global object.
         *
         * @private
         * @static
         * @member mw.Map
         * @param {mw.Map} map
         * @param {string} key
         * @param {Mixed} value
         */
        function setGlobalMapValue( map, key, value ) {
                map.values[ key ] = value;
                log.deprecate(
                        window,
                        key,
                        value,
                        // Deprecation notice for mw.config globals (T58550, T72470)
                        map === mw.config && 'Use mw.config instead.'
                );
        }

        /**
         * Create an object that can be read from or written to via methods that allow
         * interaction both with single and multiple properties at once.
         *
         * @private
         * @class mw.Map
         *
         * @constructor
         * @param {boolean} [global=false] Whether to synchronise =values to the global
         *  window object (for backwards-compatibility with mw.config; T72470). Values are
         *  copied in one direction only. Changes to globals do not reflect in the map.
         */
        function Map( global ) {
                this.values = {};
                if ( global === true ) {
                        // Override #set to also set the global variable
                        this.set = function ( selection, value ) {
                                var s;
                                if ( arguments.length > 1 ) {
                                        if ( typeof selection !== 'string' ) {
                                                return false;
                                        }
                                        setGlobalMapValue( this, selection, value );
                                        return true;
                                }
                                if ( typeof selection === 'object' ) {
                                        for ( s in selection ) {
                                                setGlobalMapValue( this, s, selection[ s ] );
                                        }
                                        return true;
                                }
                                return false;
                        };
                }
        }

        Map.prototype = {
                constructor: Map,

                /**
                 * Get the value of one or more keys.
                 *
                 * If called with no arguments, all values are returned.
                 *
                 * @param {string|Array} [selection] Key or array of keys to retrieve values for.
                 * @param {Mixed} [fallback=null] Value for keys that don't exist.
                 * @return {Mixed|Object|null} If selection was a string, returns the value,
                 *  If selection was an array, returns an object of key/values.
                 *  If no selection is passed, a new object with all key/values is returned.
                 */
                get: function ( selection, fallback ) {
                        var results, i;
                        fallback = arguments.length > 1 ? fallback : null;

                        if ( Array.isArray( selection ) ) {
                                results = {};
                                for ( i = 0; i < selection.length; i++ ) {
                                        if ( typeof selection[ i ] === 'string' ) {
                                                results[ selection[ i ] ] = hasOwn.call( this.values, selection[ i ] ) ?
                                                        this.values[ selection[ i ] ] :
                                                        fallback;
                                        }
                                }
                                return results;
                        }

                        if ( typeof selection === 'string' ) {
                                return hasOwn.call( this.values, selection ) ?
                                        this.values[ selection ] :
                                        fallback;
                        }

                        if ( selection === undefined ) {
                                results = {};
                                for ( i in this.values ) {
                                        results[ i ] = this.values[ i ];
                                }
                                return results;
                        }

                        // Invalid selection key
                        return fallback;
                },

                /**
                 * Set one or more key/value pairs.
                 *
                 * @param {string|Object} selection Key to set value for, or object mapping keys to values
                 * @param {Mixed} [value] Value to set (optional, only in use when key is a string)
                 * @return {boolean} True on success, false on failure
                 */
                set: function ( selection, value ) {
                        var s;
                        // Use `arguments.length` because `undefined` is also a valid value.
                        if ( arguments.length > 1 ) {
                                if ( typeof selection !== 'string' ) {
                                        return false;
                                }
                                this.values[ selection ] = value;
                                return true;
                        }
                        if ( typeof selection === 'object' ) {
                                for ( s in selection ) {
                                        this.values[ s ] = selection[ s ];
                                }
                                return true;
                        }
                        return false;
                },

                /**
                 * Check if one or more keys exist.
                 *
                 * @param {Mixed} selection Key or array of keys to check
                 * @return {boolean} True if the key(s) exist
                 */
                exists: function ( selection ) {
                        var i;
                        if ( Array.isArray( selection ) ) {
                                for ( i = 0; i < selection.length; i++ ) {
                                        if ( typeof selection[ i ] !== 'string' || !hasOwn.call( this.values, selection[ i ] ) ) {
                                                return false;
                                        }
                                }
                                return true;
                        }
                        return typeof selection === 'string' && hasOwn.call( this.values, selection );
                }
        };

        /**
         * Object constructor for messages.
         *
         * Similar to the Message class in MediaWiki PHP.
         *
         * Format defaults to 'text'.
         *
         *     @example
         *
         *     var obj, str;
         *     mw.messages.set( {
         *         'hello': 'Hello world',
         *         'hello-user': 'Hello, $1!',
         *         'welcome-user': 'Welcome back to $2, $1! Last visit by $1: $3'
         *     } );
         *
         *     obj = new mw.Message( mw.messages, 'hello' );
         *     mw.log( obj.text() );
         *     // Hello world
         *
         *     obj = new mw.Message( mw.messages, 'hello-user', [ 'John Doe' ] );
         *     mw.log( obj.text() );
         *     // Hello, John Doe!
         *
         *     obj = new mw.Message( mw.messages, 'welcome-user', [ 'John Doe', 'Wikipedia', '2 hours ago' ] );
         *     mw.log( obj.text() );
         *     // Welcome back to Wikipedia, John Doe! Last visit by John Doe: 2 hours ago
         *
         *     // Using mw.message shortcut
         *     obj = mw.message( 'hello-user', 'John Doe' );
         *     mw.log( obj.text() );
         *     // Hello, John Doe!
         *
         *     // Using mw.msg shortcut
         *     str = mw.msg( 'hello-user', 'John Doe' );
         *     mw.log( str );
         *     // Hello, John Doe!
         *
         *     // Different formats
         *     obj = new mw.Message( mw.messages, 'hello-user', [ 'John "Wiki" <3 Doe' ] );
         *
         *     obj.format = 'text';
         *     str = obj.toString();
         *     // Same as:
         *     str = obj.text();
         *
         *     mw.log( str );
         *     // Hello, John "Wiki" <3 Doe!
         *
         *     mw.log( obj.escaped() );
         *     // Hello, John &quot;Wiki&quot; &lt;3 Doe!
         *
         * @class mw.Message
         *
         * @constructor
         * @param {mw.Map} map Message store
         * @param {string} key
         * @param {Array} [parameters]
         */
        function Message( map, key, parameters ) {
                this.format = 'text';
                this.map = map;
                this.key = key;
                this.parameters = parameters === undefined ? [] : slice.call( parameters );
                return this;
        }

        Message.prototype = {
                /**
                 * Get parsed contents of the message.
                 *
                 * The default parser does simple $N replacements and nothing else.
                 * This may be overridden to provide a more complex message parser.
                 * The primary override is in the mediawiki.jqueryMsg module.
                 *
                 * This function will not be called for nonexistent messages.
                 *
                 * @return {string} Parsed message
                 */
                parser: function () {
                        return mw.format.apply( null, [ this.map.get( this.key ) ].concat( this.parameters ) );
                },

                /**
                 * Add (does not replace) parameters for `$N` placeholder values.
                 *
                 * @param {Array} parameters
                 * @return {mw.Message}
                 * @chainable
                 */
                params: function ( parameters ) {
                        var i;
                        for ( i = 0; i < parameters.length; i++ ) {
                                this.parameters.push( parameters[ i ] );
                        }
                        return this;
                },

                /**
                 * Convert message object to its string form based on current format.
                 *
                 * @return {string} Message as a string in the current form, or `<key>` if key
                 *  does not exist.
                 */
                toString: function () {
                        var text;

                        if ( !this.exists() ) {
                                // Use ⧼key⧽ as text if key does not exist
                                // Err on the side of safety, ensure that the output
                                // is always html safe in the event the message key is
                                // missing, since in that case its highly likely the
                                // message key is user-controlled.
                                // '⧼' is used instead of '<' to side-step any
                                // double-escaping issues.
                                // (Keep synchronised with Message::toString() in PHP.)
                                return '⧼' + mw.html.escape( this.key ) + '⧽';
                        }

                        if ( this.format === 'plain' || this.format === 'text' || this.format === 'parse' ) {
                                text = this.parser();
                        }

                        if ( this.format === 'escaped' ) {
                                text = this.parser();
                                text = mw.html.escape( text );
                        }

                        return text;
                },

                /**
                 * Change format to 'parse' and convert message to string
                 *
                 * If jqueryMsg is loaded, this parses the message text from wikitext
                 * (where supported) to HTML
                 *
                 * Otherwise, it is equivalent to plain.
                 *
                 * @return {string} String form of parsed message
                 */
                parse: function () {
                        this.format = 'parse';
                        return this.toString();
                },

                /**
                 * Change format to 'plain' and convert message to string
                 *
                 * This substitutes parameters, but otherwise does not change the
                 * message text.
                 *
                 * @return {string} String form of plain message
                 */
                plain: function () {
                        this.format = 'plain';
                        return this.toString();
                },

                /**
                 * Change format to 'text' and convert message to string
                 *
                 * If jqueryMsg is loaded, {{-transformation is done where supported
                 * (such as {{plural:}}, {{gender:}}, {{int:}}).
                 *
                 * Otherwise, it is equivalent to plain
                 *
                 * @return {string} String form of text message
                 */
                text: function () {
                        this.format = 'text';
                        return this.toString();
                },

                /**
                 * Change the format to 'escaped' and convert message to string
                 *
                 * This is equivalent to using the 'text' format (see #text), then
                 * HTML-escaping the output.
                 *
                 * @return {string} String form of html escaped message
                 */
                escaped: function () {
                        this.format = 'escaped';
                        return this.toString();
                },

                /**
                 * Check if a message exists
                 *
                 * @see mw.Map#exists
                 * @return {boolean}
                 */
                exists: function () {
                        return this.map.exists( this.key );
                }
        };

        defineFallbacks();

        /* eslint-disable no-console */
        log = ( function () {
                /**
                 * Write a verbose message to the browser's console in debug mode.
                 *
                 * This method is mainly intended for verbose logging. It is a no-op in production mode.
                 * In ResourceLoader debug mode, it will use the browser's console if available, with
                 * fallback to creating a console interface in the DOM and logging messages there.
                 *
                 * See {@link mw.log} for other logging methods.
                 *
                 * @member mw
                 * @param {...string} msg Messages to output to console.
                 */
                var log = function () {},
                        console = window.console;

                // Note: Keep list of methods in sync with restoration in mediawiki.log.js
                // when adding or removing mw.log methods below!

                /**
                 * Collection of methods to help log messages to the console.
                 *
                 * @class mw.log
                 * @singleton
                 */

                /**
                 * Write a message to the browser console's warning channel.
                 *
                 * This method is a no-op in browsers that don't implement the Console API.
                 *
                 * @param {...string} msg Messages to output to console
                 */
                log.warn = console && console.warn && Function.prototype.bind ?
                        Function.prototype.bind.call( console.warn, console ) :
                        $.noop;

                /**
                 * Write a message to the browser console's error channel.
                 *
                 * Most browsers also print a stacktrace when calling this method if the
                 * argument is an Error object.
                 *
                 * This method is a no-op in browsers that don't implement the Console API.
                 *
                 * @since 1.26
                 * @param {Error|...string} msg Messages to output to console
                 */
                log.error = console && console.error && Function.prototype.bind ?
                        Function.prototype.bind.call( console.error, console ) :
                        $.noop;

                /**
                 * Create a property on a host object that, when accessed, will produce
                 * a deprecation warning in the console.
                 *
                 * @param {Object} obj Host object of deprecated property
                 * @param {string} key Name of property to create in `obj`
                 * @param {Mixed} val The value this property should return when accessed
                 * @param {string} [msg] Optional text to include in the deprecation message
                 * @param {string} [logName=key] Optional custom name for the feature.
                 *  This is used instead of `key` in the message and `mw.deprecate` tracking.
                 */
                log.deprecate = !Object.defineProperty ? function ( obj, key, val ) {
                        obj[ key ] = val;
                } : function ( obj, key, val, msg, logName ) {
                        var logged = new StringSet();
                        logName = logName || key;
                        msg = 'Use of "' + logName + '" is deprecated.' + ( msg ? ( ' ' + msg ) : '' );
                        function uniqueTrace() {
                                var trace = new Error().stack;
                                if ( logged.has( trace ) ) {
                                        return false;
                                }
                                logged.add( trace );
                                return true;
                        }
                        // Support: Safari 5.0
                        // Throws "not supported on DOM Objects" for Node or Element objects (incl. document)
                        // Safari 4.0 doesn't have this method, and it was fixed in Safari 5.1.
                        try {
                                Object.defineProperty( obj, key, {
                                        configurable: true,
                                        enumerable: true,
                                        get: function () {
                                                if ( uniqueTrace() ) {
                                                        mw.track( 'mw.deprecate', logName );
                                                        mw.log.warn( msg );
                                                }
                                                return val;
                                        },
                                        set: function ( newVal ) {
                                                if ( uniqueTrace() ) {
                                                        mw.track( 'mw.deprecate', logName );
                                                        mw.log.warn( msg );
                                                }
                                                val = newVal;
                                        }
                                } );
                        } catch ( err ) {
                                obj[ key ] = val;
                        }
                };

                return log;
        }() );
        /* eslint-enable no-console */

        /**
         * @class mw
         */
        mw = {
                redefineFallbacksForTest: function () {
                        if ( !window.QUnit ) {
                                throw new Error( 'Reset not allowed outside unit tests' );
                        }
                        defineFallbacks();
                },

                /**
                 * Get the current time, measured in milliseconds since January 1, 1970 (UTC).
                 *
                 * On browsers that implement the Navigation Timing API, this function will produce floating-point
                 * values with microsecond precision that are guaranteed to be monotonic. On all other browsers,
                 * it will fall back to using `Date`.
                 *
                 * @return {number} Current time
                 */
                now: ( function () {
                        var perf = window.performance,
                                navStart = perf && perf.timing && perf.timing.navigationStart;
                        return navStart && typeof perf.now === 'function' ?
                                function () { return navStart + perf.now(); } :
                                function () { return Date.now(); };
                }() ),

                /**
                 * Format a string. Replace $1, $2 ... $N with positional arguments.
                 *
                 * Used by Message#parser().
                 *
                 * @since 1.25
                 * @param {string} formatString Format string
                 * @param {...Mixed} parameters Values for $N replacements
                 * @return {string} Formatted string
                 */
                format: function ( formatString ) {
                        var parameters = slice.call( arguments, 1 );
                        return formatString.replace( /\$(\d+)/g, function ( str, match ) {
                                var index = parseInt( match, 10 ) - 1;
                                return parameters[ index ] !== undefined ? parameters[ index ] : '$' + match;
                        } );
                },

                /**
                 * Track an analytic event.
                 *
                 * This method provides a generic means for MediaWiki JavaScript code to capture state
                 * information for analysis. Each logged event specifies a string topic name that describes
                 * the kind of event that it is. Topic names consist of dot-separated path components,
                 * arranged from most general to most specific. Each path component should have a clear and
                 * well-defined purpose.
                 *
                 * Data handlers are registered via `mw.trackSubscribe`, and receive the full set of
                 * events that match their subcription, including those that fired before the handler was
                 * bound.
                 *
                 * @param {string} topic Topic name
                 * @param {Object} [data] Data describing the event, encoded as an object
                 */
                track: function ( topic, data ) {
                        trackQueue.push( { topic: topic, timeStamp: mw.now(), data: data } );
                        trackCallbacks.fire( trackQueue );
                },

                /**
                 * Register a handler for subset of analytic events, specified by topic.
                 *
                 * Handlers will be called once for each tracked event, including any events that fired before the
                 * handler was registered; 'this' is set to a plain object with a 'timeStamp' property indicating
                 * the exact time at which the event fired, a string 'topic' property naming the event, and a
                 * 'data' property which is an object of event-specific data. The event topic and event data are
                 * also passed to the callback as the first and second arguments, respectively.
                 *
                 * @param {string} topic Handle events whose name starts with this string prefix
                 * @param {Function} callback Handler to call for each matching tracked event
                 * @param {string} callback.topic
                 * @param {Object} [callback.data]
                 */
                trackSubscribe: function ( topic, callback ) {
                        var seen = 0;
                        function handler( trackQueue ) {
                                var event;
                                for ( ; seen < trackQueue.length; seen++ ) {
                                        event = trackQueue[ seen ];
                                        if ( event.topic.indexOf( topic ) === 0 ) {
                                                callback.call( event, event.topic, event.data );
                                        }
                                }
                        }

                        trackHandlers.push( [ handler, callback ] );

                        trackCallbacks.add( handler );
                },

                /**
                 * Stop handling events for a particular handler
                 *
                 * @param {Function} callback
                 */
                trackUnsubscribe: function ( callback ) {
                        trackHandlers = trackHandlers.filter( function ( fns ) {
                                if ( fns[ 1 ] === callback ) {
                                        trackCallbacks.remove( fns[ 0 ] );
                                        // Ensure the tuple is removed to avoid holding on to closures
                                        return false;
                                }
                                return true;
                        } );
                },

                // Expose Map constructor
                Map: Map,

                // Expose Message constructor
                Message: Message,

                /**
                 * Map of configuration values.
                 *
                 * Check out [the complete list of configuration values](https://www.mediawiki.org/wiki/Manual:Interface/JavaScript#mw.config)
                 * on mediawiki.org.
                 *
                 * If `$wgLegacyJavaScriptGlobals` is true, this Map will add its values to the
                 * global `window` object.
                 *
                 * @property {mw.Map} config
                 */
                // Dummy placeholder later assigned in ResourceLoaderStartUpModule
                config: null,

                /**
                 * Empty object for third-party libraries, for cases where you don't
                 * want to add a new global, or the global is bad and needs containment
                 * or wrapping.
                 *
                 * @property
                 */
                libs: {},

                /**
                 * Access container for deprecated functionality that can be moved from
                 * from their legacy location and attached to this object (e.g. a global
                 * function that is deprecated and as stop-gap can be exposed through here).
                 *
                 * This was reserved for future use but never ended up being used.
                 *
                 * @deprecated since 1.22 Let deprecated identifiers keep their original name
                 *  and use mw.log#deprecate to create an access container for tracking.
                 * @property
                 */
                legacy: {},

                /**
                 * Store for messages.
                 *
                 * @property {mw.Map}
                 */
                messages: new Map(),

                /**
                 * Store for templates associated with a module.
                 *
                 * @property {mw.Map}
                 */
                templates: new Map(),

                /**
                 * Get a message object.
                 *
                 * Shortcut for `new mw.Message( mw.messages, key, parameters )`.
                 *
                 * @see mw.Message
                 * @param {string} key Key of message to get
                 * @param {...Mixed} parameters Values for $N replacements
                 * @return {mw.Message}
                 */
                message: function ( key ) {
                        var parameters = slice.call( arguments, 1 );
                        return new Message( mw.messages, key, parameters );
                },

                /**
                 * Get a message string using the (default) 'text' format.
                 *
                 * Shortcut for `mw.message( key, parameters... ).text()`.
                 *
                 * @see mw.Message
                 * @param {string} key Key of message to get
                 * @param {...Mixed} parameters Values for $N replacements
                 * @return {string}
                 */
                msg: function () {
                        return mw.message.apply( mw.message, arguments ).toString();
                },

                // Expose mw.log
                log: log,

                /**
                 * Client for ResourceLoader server end point.
                 *
                 * This client is in charge of maintaining the module registry and state
                 * machine, initiating network (batch) requests for loading modules, as
                 * well as dependency resolution and execution of source code.
                 *
                 * For more information, refer to
                 * <https://www.mediawiki.org/wiki/ResourceLoader/Features>
                 *
                 * @class mw.loader
                 * @singleton
                 */
                loader: ( function () {

                        /**
                         * Fired via mw.track on various resource loading errors.
                         *
                         * @event resourceloader_exception
                         * @param {Error|Mixed} e The error that was thrown. Almost always an Error
                         *   object, but in theory module code could manually throw something else, and that
                         *   might also end up here.
                         * @param {string} [module] Name of the module which caused the error. Omitted if the
                         *   error is not module-related or the module cannot be easily identified due to
                         *   batched handling.
                         * @param {string} source Source of the error. Possible values:
                         *
                         *   - style: stylesheet error (only affects old IE where a special style loading method
                         *     is used)
                         *   - load-callback: exception thrown by user callback
                         *   - module-execute: exception thrown by module code
                         *   - resolve: failed to sort dependencies for a module in mw.loader.load
                         *   - store-eval: could not evaluate module code cached in localStorage
                         *   - store-localstorage-init: localStorage or JSON parse error in mw.loader.store.init
                         *   - store-localstorage-json: JSON conversion error in mw.loader.store.set
                         *   - store-localstorage-update: localStorage or JSON conversion error in mw.loader.store.update
                         */

                        /**
                         * Fired via mw.track on resource loading error conditions.
                         *
                         * @event resourceloader_assert
                         * @param {string} source Source of the error. Possible values:
                         *
                         *   - bug-T59567: failed to cache script due to an Opera function -> string conversion
                         *     bug; see <https://phabricator.wikimedia.org/T59567> for details
                         */

                        /**
                         * Mapping of registered modules.
                         *
                         * See #implement and #execute for exact details on support for script, style and messages.
                         *
                         * Format:
                         *
                         *     {
                         *         'moduleName': {
                         *             // From mw.loader.register()
                         *             'version': '########' (hash)
                         *             'dependencies': ['required.foo', 'bar.also', ...]
                         *             'group': 'somegroup', (or) null
                         *             'source': 'local', (or) 'anotherwiki'
                         *             'skip': 'return !!window.Example', (or) null
                         *             'module': export Object
                         *
                         *             // Set from execute() or mw.loader.state()
                         *             'state': 'registered', 'loaded', 'loading', 'ready', 'error', or 'missing'
                         *
                         *             // Optionally added at run-time by mw.loader.implement()
                         *             'skipped': true
                         *             'script': closure, array of urls, or string
                         *             'style': { ... } (see #execute)
                         *             'messages': { 'key': 'value', ... }
                         *         }
                         *     }
                         *
                         * State machine:
                         *
                         * - `registered`:
                         *    The module is known to the system but not yet required.
                         *    Meta data is registered via mw.loader#register. Calls to that method are
                         *    generated server-side by the startup module.
                         * - `loading`:
                         *    The module was required through mw.loader (either directly or as dependency of
                         *    another module). The client will fetch module contents from the server.
                         *    The contents are then stashed in the registry via mw.loader#implement.
                         * - `loaded`:
                         *    The module has been loaded from the server and stashed via mw.loader#implement.
                         *    If the module has no more dependencies in-flight, the module will be executed
                         *    immediately. Otherwise execution is deferred, controlled via #handlePending.
                         * - `executing`:
                         *    The module is being executed.
                         * - `ready`:
                         *    The module has been successfully executed.
                         * - `error`:
                         *    The module (or one of its dependencies) produced an error during execution.
                         * - `missing`:
                         *    The module was registered client-side and requested, but the server denied knowledge
                         *    of the module's existence.
                         *
                         * @property
                         * @private
                         */
                        var registry = {},
                                // Mapping of sources, keyed by source-id, values are strings.
                                //
                                // Format:
                                //
                                //     {
                                //         'sourceId': 'http://example.org/w/load.php'
                                //     }
                                //
                                sources = {},

                                // For queueModuleScript()
                                handlingPendingRequests = false,
                                pendingRequests = [],

                                // List of modules to be loaded
                                queue = [],

                                /**
                                 * List of callback jobs waiting for modules to be ready.
                                 *
                                 * Jobs are created by #enqueue() and run by #handlePending().
                                 *
                                 * Typically when a job is created for a module, the job's dependencies contain
                                 * both the required module and all its recursive dependencies.
                                 *
                                 * Format:
                                 *
                                 *     {
                                 *         'dependencies': [ module names ],
                                 *         'ready': Function callback
                                 *         'error': Function callback
                                 *     }
                                 *
                                 * @property {Object[]} jobs
                                 * @private
                                 */
                                jobs = [],

                                /**
                                 * For #addEmbeddedCSS() and #addLink()
                                 *
                                 * @private
                                 * @property {HTMLElement|null} marker
                                 */
                                marker = document.querySelector( 'meta[name="ResourceLoaderDynamicStyles"]' ),

                                // For addEmbeddedCSS()
                                cssBuffer = '',
                                cssBufferTimer = null,
                                cssCallbacks = [],
                                rAF = window.requestAnimationFrame || setTimeout;

                        /**
                         * Create a new style element and add it to the DOM.
                         *
                         * @private
                         * @param {string} text CSS text
                         * @param {Node|null} [nextNode] The element where the style tag
                         *  should be inserted before
                         * @return {HTMLElement} Reference to the created style element
                         */
                        function newStyleTag( text, nextNode ) {
                                var el = document.createElement( 'style' );
                                el.appendChild( document.createTextNode( text ) );
                                if ( nextNode && nextNode.parentNode ) {
                                        nextNode.parentNode.insertBefore( el, nextNode );
                                } else {
                                        document.head.appendChild( el );
                                }
                                return el;
                        }

                        /**
                         * Add a bit of CSS text to the current browser page.
                         *
                         * The CSS will be appended to an existing ResourceLoader-created `<style>` tag
                         * or create a new one based on whether the given `cssText` is safe for extension.
                         *
                         * @private
                         * @param {string} [cssText=cssBuffer] If called without cssText,
                         *  the internal buffer will be inserted instead.
                         * @param {Function} [callback]
                         */
                        function addEmbeddedCSS( cssText, callback ) {
                                function fireCallbacks() {
                                        var i,
                                                oldCallbacks = cssCallbacks;
                                        // Reset cssCallbacks variable so it's not polluted by any calls to
                                        // addEmbeddedCSS() from one of the callbacks (T105973)
                                        cssCallbacks = [];
                                        for ( i = 0; i < oldCallbacks.length; i++ ) {
                                                oldCallbacks[ i ]();
                                        }
                                }

                                if ( callback ) {
                                        cssCallbacks.push( callback );
                                }

                                // Yield once before creating the <style> tag. This lets multiple stylesheets
                                // accumulate into one buffer, allowing us to reduce how often new stylesheets
                                // are inserted in the browser. Appending a stylesheet and waiting for the
                                // browser to repaint is fairly expensive. (T47810)
                                if ( cssText ) {
                                        // Don't extend the buffer if the item needs its own stylesheet.
                                        // Keywords like `@import` are only valid at the start of a stylesheet (T37562).
                                        if ( !cssBuffer || cssText.slice( 0, '@import'.length ) !== '@import' ) {
                                                // Linebreak for somewhat distinguishable sections
                                                cssBuffer += '\n' + cssText;
                                                if ( !cssBufferTimer ) {
                                                        cssBufferTimer = rAF( function () {
                                                                // Wrap in anonymous function that takes no arguments
                                                                // Support: Firefox < 13
                                                                // Firefox 12 has non-standard behaviour of passing a number
                                                                // as first argument to a setTimeout callback.
                                                                // http://benalman.com/news/2009/07/the-mysterious-firefox-settime/
                                                                addEmbeddedCSS();
                                                        } );
                                                }
                                                return;
                                        }

                                // This is a scheduled flush for the buffer
                                } else {
                                        cssBufferTimer = null;
                                        cssText = cssBuffer;
                                        cssBuffer = '';
                                }

                                newStyleTag( cssText, marker );

                                fireCallbacks();
                        }

                        /**
                         * @private
                         * @param {Array} modules List of module names
                         * @return {string} Hash of concatenated version hashes.
                         */
                        function getCombinedVersion( modules ) {
                                var hashes = modules.map( function ( module ) {
                                        return registry[ module ].version;
                                } );
                                return fnv132( hashes.join( '' ) );
                        }

                        /**
                         * Determine whether all dependencies are in state 'ready', which means we may
                         * execute the module or job now.
                         *
                         * @private
                         * @param {Array} modules Names of modules to be checked
                         * @return {boolean} True if all modules are in state 'ready', false otherwise
                         */
                        function allReady( modules ) {
                                var i;
                                for ( i = 0; i < modules.length; i++ ) {
                                        if ( mw.loader.getState( modules[ i ] ) !== 'ready' ) {
                                                return false;
                                        }
                                }
                                return true;
                        }

                        /**
                         * Determine whether all dependencies are in state 'ready', which means we may
                         * execute the module or job now.
                         *
                         * @private
                         * @param {Array} modules Names of modules to be checked
                         * @return {boolean} True if no modules are in state 'error' or 'missing', false otherwise
                         */
                        function anyFailed( modules ) {
                                var i, state;
                                for ( i = 0; i < modules.length; i++ ) {
                                        state = mw.loader.getState( modules[ i ] );
                                        if ( state === 'error' || state === 'missing' ) {
                                                return true;
                                        }
                                }
                                return false;
                        }

                        /**
                         * A module has entered state 'ready', 'error', or 'missing'. Automatically update
                         * pending jobs and modules that depend upon this module. If the given module failed,
                         * propagate the 'error' state up the dependency tree. Otherwise, go ahead and execute
                         * all jobs/modules now having their dependencies satisfied.
                         *
                         * Jobs that depend on a failed module, will have their error callback ran (if any).
                         *
                         * @private
                         * @param {string} module Name of module that entered one of the states 'ready', 'error', or 'missing'.
                         */
                        function handlePending( module ) {
                                var j, job, hasErrors, m, stateChange;

                                if ( registry[ module ].state === 'error' || registry[ module ].state === 'missing' ) {
                                        // If the current module failed, mark all dependent modules also as failed.
                                        // Iterate until steady-state to propagate the error state upwards in the
                                        // dependency tree.
                                        do {
                                                stateChange = false;
                                                for ( m in registry ) {
                                                        if ( registry[ m ].state !== 'error' && registry[ m ].state !== 'missing' ) {
                                                                if ( anyFailed( registry[ m ].dependencies ) ) {
                                                                        registry[ m ].state = 'error';
                                                                        stateChange = true;
                                                                }
                                                        }
                                                }
                                        } while ( stateChange );
                                }

                                // Execute all jobs whose dependencies are either all satisfied or contain at least one failed module.
                                for ( j = 0; j < jobs.length; j++ ) {
                                        hasErrors = anyFailed( jobs[ j ].dependencies );
                                        if ( hasErrors || allReady( jobs[ j ].dependencies ) ) {
                                                // All dependencies satisfied, or some have errors
                                                job = jobs[ j ];
                                                jobs.splice( j, 1 );
                                                j -= 1;
                                                try {
                                                        if ( hasErrors ) {
                                                                if ( typeof job.error === 'function' ) {
                                                                        job.error( new Error( 'Module ' + module + ' has failed dependencies' ), [ module ] );
                                                                }
                                                        } else {
                                                                if ( typeof job.ready === 'function' ) {
                                                                        job.ready();
                                                                }
                                                        }
                                                } catch ( e ) {
                                                        // A user-defined callback raised an exception.
                                                        // Swallow it to protect our state machine!
                                                        mw.track( 'resourceloader.exception', { exception: e, module: module, source: 'load-callback' } );
                                                }
                                        }
                                }

                                if ( registry[ module ].state === 'ready' ) {
                                        // The current module became 'ready'. Set it in the module store, and recursively execute all
                                        // dependent modules that are loaded and now have all dependencies satisfied.
                                        mw.loader.store.set( module, registry[ module ] );
                                        for ( m in registry ) {
                                                if ( registry[ m ].state === 'loaded' && allReady( registry[ m ].dependencies ) ) {
                                                        // eslint-disable-next-line no-use-before-define
                                                        execute( m );
                                                }
                                        }
                                }
                        }

                        /**
                         * Resolve dependencies and detect circular references.
                         *
                         * @private
                         * @param {string} module Name of the top-level module whose dependencies shall be
                         *  resolved and sorted.
                         * @param {Array} resolved Returns a topological sort of the given module and its
                         *  dependencies, such that later modules depend on earlier modules. The array
                         *  contains the module names. If the array contains already some module names,
                         *  this function appends its result to the pre-existing array.
                         * @param {StringSet} [unresolved] Used to track the current dependency
                         *  chain, and to report loops in the dependency graph.
                         * @throws {Error} If any unregistered module or a dependency loop is encountered
                         */
                        function sortDependencies( module, resolved, unresolved ) {
                                var i, deps, skip;

                                if ( !hasOwn.call( registry, module ) ) {
                                        throw new Error( 'Unknown dependency: ' + module );
                                }

                                if ( registry[ module ].skip !== null ) {
                                        // eslint-disable-next-line no-new-func
                                        skip = new Function( registry[ module ].skip );
                                        registry[ module ].skip = null;
                                        if ( skip() ) {
                                                registry[ module ].skipped = true;
                                                registry[ module ].dependencies = [];
                                                registry[ module ].state = 'ready';
                                                handlePending( module );
                                                return;
                                        }
                                }

                                if ( resolved.indexOf( module ) !== -1 ) {
                                        // Module already resolved; nothing to do
                                        return;
                                }
                                // Create unresolved if not passed in
                                if ( !unresolved ) {
                                        unresolved = new StringSet();
                                }
                                // Tracks down dependencies
                                deps = registry[ module ].dependencies;
                                for ( i = 0; i < deps.length; i++ ) {
                                        if ( resolved.indexOf( deps[ i ] ) === -1 ) {
                                                if ( unresolved.has( deps[ i ] ) ) {
                                                        throw new Error( mw.format(
                                                                'Circular reference detected: $1 -> $2',
                                                                module,
                                                                deps[ i ]
                                                        ) );
                                                }

                                                unresolved.add( module );
                                                sortDependencies( deps[ i ], resolved, unresolved );
                                        }
                                }
                                resolved.push( module );
                        }

                        /**
                         * Get names of module that a module depends on, in their proper dependency order.
                         *
                         * @private
                         * @param {string[]} modules Array of string module names
                         * @return {Array} List of dependencies, including 'module'.
                         * @throws {Error} If an unregistered module or a dependency loop is encountered
                         */
                        function resolve( modules ) {
                                var i, resolved = [];
                                for ( i = 0; i < modules.length; i++ ) {
                                        sortDependencies( modules[ i ], resolved );
                                }
                                return resolved;
                        }

                        /**
                         * Like #resolve(), except it will silently ignore modules that
                         * are missing or have missing dependencies.
                         *
                         * @private
                         * @param {string[]} modules Array of string module names
                         * @return {Array} List of dependencies.
                         */
                        function resolveStubbornly( modules ) {
                                var i, saved, resolved = [];
                                for ( i = 0; i < modules.length; i++ ) {
                                        saved = resolved.slice();
                                        try {
                                                sortDependencies( modules[ i ], resolved );
                                        } catch ( err ) {
                                                // This module is unknown or has unknown dependencies.
                                                // Undo any incomplete resolutions made and keep going.
                                                resolved = saved;
                                                mw.track( 'resourceloader.exception', {
                                                        exception: err,
                                                        source: 'resolve'
                                                } );
                                        }
                                }
                                return resolved;
                        }

                        /**
                         * Load and execute a script.
                         *
                         * @private
                         * @param {string} src URL to script, will be used as the src attribute in the script tag
                         * @param {Function} [callback] Callback to run after request resolution
                         */
                        function addScript( src, callback ) {
                                var script = document.createElement( 'script' );
                                script.src = src;
                                script.onload = script.onerror = function () {
                                        if ( script.parentNode ) {
                                                script.parentNode.removeChild( script );
                                        }
                                        script = null;
                                        if ( callback ) {
                                                callback();
                                                callback = null;
                                        }
                                };
                                document.head.appendChild( script );
                        }

                        /**
                         * Queue the loading and execution of a script for a particular module.
                         *
                         * @private
                         * @param {string} src URL of the script
                         * @param {string} moduleName Name of currently executing module
                         * @param {Function} callback Callback to run after addScript() resolution
                         */
                        function queueModuleScript( src, moduleName, callback ) {
                                pendingRequests.push( function () {
                                        if ( hasOwn.call( registry, moduleName ) ) {
                                                // Emulate runScript() part of execute()
                                                window.require = mw.loader.require;
                                                window.module = registry[ moduleName ].module;
                                        }
                                        addScript( src, function () {
                                                // 'module.exports' should not persist after the file is executed to
                                                // avoid leakage to unrelated code. 'require' should be kept, however,
                                                // as asynchronous access to 'require' is allowed and expected. (T144879)
                                                delete window.module;
                                                callback();
                                                // Start the next one (if any)
                                                if ( pendingRequests[ 0 ] ) {
                                                        pendingRequests.shift()();
                                                } else {
                                                        handlingPendingRequests = false;
                                                }
                                        } );
                                } );
                                if ( !handlingPendingRequests && pendingRequests[ 0 ] ) {
                                        handlingPendingRequests = true;
                                        pendingRequests.shift()();
                                }
                        }

                        /**
                         * Utility function for execute()
                         *
                         * @ignore
                         * @param {string} [media] Media attribute
                         * @param {string} url URL
                         */
                        function addLink( media, url ) {
                                var el = document.createElement( 'link' );

                                el.rel = 'stylesheet';
                                if ( media && media !== 'all' ) {
                                        el.media = media;
                                }
                                // If you end up here from an IE exception "SCRIPT: Invalid property value.",
                                // see #addEmbeddedCSS, T33676, T43331, and T49277 for details.
                                el.href = url;

                                if ( marker && marker.parentNode ) {
                                        marker.parentNode.insertBefore( el, marker );
                                } else {
                                        document.head.appendChild( el );
                                }
                        }

                        /**
                         * @private
                         * @param {string} code JavaScript code
                         */
                        function domEval( code ) {
                                var script = document.createElement( 'script' );
                                script.text = code;
                                document.head.appendChild( script );
                                script.parentNode.removeChild( script );
                        }

                        /**
                         * Executes a loaded module, making it ready to use
                         *
                         * @private
                         * @param {string} module Module name to execute
                         */
                        function execute( module ) {
                                var key, value, media, i, urls, cssHandle, checkCssHandles, runScript,
                                        cssHandlesRegistered = false;

                                if ( !hasOwn.call( registry, module ) ) {
                                        throw new Error( 'Module has not been registered yet: ' + module );
                                }
                                if ( registry[ module ].state !== 'loaded' ) {
                                        throw new Error( 'Module in state "' + registry[ module ].state + '" may not be executed: ' + module );
                                }

                                registry[ module ].state = 'executing';

                                runScript = function () {
                                        var script, markModuleReady, nestedAddScript;

                                        script = registry[ module ].script;
                                        markModuleReady = function () {
                                                registry[ module ].state = 'ready';
                                                handlePending( module );
                                        };
                                        nestedAddScript = function ( arr, callback, i ) {
                                                // Recursively call queueModuleScript() in its own callback
                                                // for each element of arr.
                                                if ( i >= arr.length ) {
                                                        // We're at the end of the array
                                                        callback();
                                                        return;
                                                }

                                                queueModuleScript( arr[ i ], module, function () {
                                                        nestedAddScript( arr, callback, i + 1 );
                                                } );
                                        };

                                        try {
                                                if ( Array.isArray( script ) ) {
                                                        nestedAddScript( script, markModuleReady, 0 );
                                                } else if ( typeof script === 'function' ) {
                                                        // Pass jQuery twice so that the signature of the closure which wraps
                                                        // the script can bind both '$' and 'jQuery'.
                                                        script( $, $, mw.loader.require, registry[ module ].module );
                                                        markModuleReady();

                                                } else if ( typeof script === 'string' ) {
                                                        // Site and user modules are legacy scripts that run in the global scope.
                                                        // This is transported as a string instead of a function to avoid needing
                                                        // to use string manipulation to undo the function wrapper.
                                                        domEval( script );
                                                        markModuleReady();

                                                } else {
                                                        // Module without script
                                                        markModuleReady();
                                                }
                                        } catch ( e ) {
                                                // Use mw.track instead of mw.log because these errors are common in production mode
                                                // (e.g. undefined variable), and mw.log is only enabled in debug mode.
                                                registry[ module ].state = 'error';
                                                mw.track( 'resourceloader.exception', { exception: e, module: module, source: 'module-execute' } );
                                                handlePending( module );
                                        }
                                };

                                // Add localizations to message system
                                if ( registry[ module ].messages ) {
                                        mw.messages.set( registry[ module ].messages );
                                }

                                // Initialise templates
                                if ( registry[ module ].templates ) {
                                        mw.templates.set( module, registry[ module ].templates );
                                }

                                // Make sure we don't run the scripts until all stylesheet insertions have completed.
                                ( function () {
                                        var pending = 0;
                                        checkCssHandles = function () {
                                                // cssHandlesRegistered ensures we don't take off too soon, e.g. when
                                                // one of the cssHandles is fired while we're still creating more handles.
                                                if ( cssHandlesRegistered && pending === 0 && runScript ) {
                                                        if ( module === 'user' ) {
                                                                // Implicit dependency on the site module. Not real dependency because
                                                                // it should run after 'site' regardless of whether it succeeds or fails.
                                                                mw.loader.using( [ 'site' ] ).always( runScript );
                                                        } else {
                                                                runScript();
                                                        }
                                                        runScript = undefined; // Revoke
                                                }
                                        };
                                        cssHandle = function () {
                                                var check = checkCssHandles;
                                                pending++;
                                                return function () {
                                                        if ( check ) {
                                                                pending--;
                                                                check();
                                                                check = undefined; // Revoke
                                                        }
                                                };
                                        };
                                }() );

                                // Process styles (see also mw.loader.implement)
                                // * back-compat: { <media>: css }
                                // * back-compat: { <media>: [url, ..] }
                                // * { "css": [css, ..] }
                                // * { "url": { <media>: [url, ..] } }
                                if ( registry[ module ].style ) {
                                        for ( key in registry[ module ].style ) {
                                                value = registry[ module ].style[ key ];
                                                media = undefined;

                                                if ( key !== 'url' && key !== 'css' ) {
                                                        // Backwards compatibility, key is a media-type
                                                        if ( typeof value === 'string' ) {
                                                                // back-compat: { <media>: css }
                                                                // Ignore 'media' because it isn't supported (nor was it used).
                                                                // Strings are pre-wrapped in "@media". The media-type was just ""
                                                                // (because it had to be set to something).
                                                                // This is one of the reasons why this format is no longer used.
                                                                addEmbeddedCSS( value, cssHandle() );
                                                        } else {
                                                                // back-compat: { <media>: [url, ..] }
                                                                media = key;
                                                                key = 'bc-url';
                                                        }
                                                }

                                                // Array of css strings in key 'css',
                                                // or back-compat array of urls from media-type
                                                if ( Array.isArray( value ) ) {
                                                        for ( i = 0; i < value.length; i++ ) {
                                                                if ( key === 'bc-url' ) {
                                                                        // back-compat: { <media>: [url, ..] }
                                                                        addLink( media, value[ i ] );
                                                                } else if ( key === 'css' ) {
                                                                        // { "css": [css, ..] }
                                                                        addEmbeddedCSS( value[ i ], cssHandle() );
                                                                }
                                                        }
                                                // Not an array, but a regular object
                                                // Array of urls inside media-type key
                                                } else if ( typeof value === 'object' ) {
                                                        // { "url": { <media>: [url, ..] } }
                                                        for ( media in value ) {
                                                                urls = value[ media ];
                                                                for ( i = 0; i < urls.length; i++ ) {
                                                                        addLink( media, urls[ i ] );
                                                                }
                                                        }
                                                }
                                        }
                                }

                                // Kick off.
                                cssHandlesRegistered = true;
                                checkCssHandles();
                        }

                        /**
                         * Add one or more modules to the module load queue.
                         *
                         * See also #work().
                         *
                         * @private
                         * @param {string|string[]} dependencies Module name or array of string module names
                         * @param {Function} [ready] Callback to execute when all dependencies are ready
                         * @param {Function} [error] Callback to execute when any dependency fails
                         */
                        function enqueue( dependencies, ready, error ) {
                                // Allow calling by single module name
                                if ( typeof dependencies === 'string' ) {
                                        dependencies = [ dependencies ];
                                }

                                // Add ready and error callbacks if they were given
                                if ( ready !== undefined || error !== undefined ) {
                                        jobs.push( {
                                                // Narrow down the list to modules that are worth waiting for
                                                dependencies: dependencies.filter( function ( module ) {
                                                        var state = mw.loader.getState( module );
                                                        return state === 'registered' || state === 'loaded' || state === 'loading' || state === 'executing';
                                                } ),
                                                ready: ready,
                                                error: error
                                        } );
                                }

                                dependencies.forEach( function ( module ) {
                                        var state = mw.loader.getState( module );
                                        // Only queue modules that are still in the initial 'registered' state
                                        // (not ones already loading, ready or error).
                                        if ( state === 'registered' && queue.indexOf( module ) === -1 ) {
                                                // Private modules must be embedded in the page. Don't bother queuing
                                                // these as the server will deny them anyway (T101806).
                                                if ( registry[ module ].group === 'private' ) {
                                                        registry[ module ].state = 'error';
                                                        handlePending( module );
                                                        return;
                                                }
                                                queue.push( module );
                                        }
                                } );

                                mw.loader.work();
                        }

                        function sortQuery( o ) {
                                var key,
                                        sorted = {},
                                        a = [];

                                for ( key in o ) {
                                        a.push( key );
                                }
                                a.sort();
                                for ( key = 0; key < a.length; key++ ) {
                                        sorted[ a[ key ] ] = o[ a[ key ] ];
                                }
                                return sorted;
                        }

                        /**
                         * Converts a module map of the form `{ foo: [ 'bar', 'baz' ], bar: [ 'baz, 'quux' ] }`
                         * to a query string of the form `foo.bar,baz|bar.baz,quux`.
                         *
                         * See `ResourceLoader::makePackedModulesString()` in PHP, of which this is a port.
                         * On the server, unpacking is done by `ResourceLoaderContext::expandModuleNames()`.
                         *
                         * Note: This is only half of the logic, the other half has to be in #batchRequest(),
                         * because its implementation needs to keep track of potential string size in order
                         * to decide when to split the requests due to url size.
                         *
                         * @private
                         * @param {Object} moduleMap Module map
                         * @return {Object}
                         * @return {string} return.str Module query string
                         * @return {Array} return.list List of module names in matching order
                         */
                        function buildModulesString( moduleMap ) {
                                var p, prefix,
                                        str = [],
                                        list = [];

                                function restore( suffix ) {
                                        return p + suffix;
                                }

                                for ( prefix in moduleMap ) {
                                        p = prefix === '' ? '' : prefix + '.';
                                        str.push( p + moduleMap[ prefix ].join( ',' ) );
                                        list.push.apply( list, moduleMap[ prefix ].map( restore ) );
                                }
                                return {
                                        str: str.join( '|' ),
                                        list: list
                                };
                        }

                        /**
                         * Resolve indexed dependencies.
                         *
                         * ResourceLoader uses an optimization to save space which replaces module names in
                         * dependency lists with the index of that module within the array of module
                         * registration data if it exists. The benefit is a significant reduction in the data
                         * size of the startup module. This function changes those dependency lists back to
                         * arrays of strings.
                         *
                         * @private
                         * @param {Array} modules Modules array
                         */
                        function resolveIndexedDependencies( modules ) {
                                var i, j, deps;
                                function resolveIndex( dep ) {
                                        return typeof dep === 'number' ? modules[ dep ][ 0 ] : dep;
                                }
                                for ( i = 0; i < modules.length; i++ ) {
                                        deps = modules[ i ][ 2 ];
                                        if ( deps ) {
                                                for ( j = 0; j < deps.length; j++ ) {
                                                        deps[ j ] = resolveIndex( deps[ j ] );
                                                }
                                        }
                                }
                        }

                        /**
                         * @private
                         * @param {Object} params Map of parameter names to values
                         * @return {string}
                         */
                        function makeQueryString( params ) {
                                return Object.keys( params ).map( function ( key ) {
                                        return encodeURIComponent( key ) + '=' + encodeURIComponent( params[ key ] );
                                } ).join( '&' );
                        }

                        /**
                         * Create network requests for a batch of modules.
                         *
                         * This is an internal method for #work(). This must not be called directly
                         * unless the modules are already registered, and no request is in progress,
                         * and the module state has already been set to `loading`.
                         *
                         * @private
                         * @param {string[]} batch
                         */
                        function batchRequest( batch ) {
                                var reqBase, splits, maxQueryLength, b, bSource, bGroup, bSourceGroup,
                                        source, group, i, modules, sourceLoadScript,
                                        currReqBase, currReqBaseLength, moduleMap, currReqModules, l,
                                        lastDotIndex, prefix, suffix, bytesAdded;

                                /**
                                 * Start the currently drafted request to the server.
                                 *
                                 * @ignore
                                 */
                                function doRequest() {
                                        // Optimisation: Inherit (Object.create), not copy ($.extend)
                                        var query = Object.create( currReqBase ),
                                                packed = buildModulesString( moduleMap );
                                        query.modules = packed.str;
                                        // The packing logic can change the effective order, even if the input was
                                        // sorted. As such, the call to getCombinedVersion() must use this
                                        // effective order, instead of currReqModules, as otherwise the combined
                                        // version will not match the hash expected by the server based on
                                        // combining versions from the module query string in-order. (T188076)
                                        query.version = getCombinedVersion( packed.list );
                                        query = sortQuery( query );
                                        addScript( sourceLoadScript + '?' + makeQueryString( query ) );
                                }

                                if ( !batch.length ) {
                                        return;
                                }

                                // Always order modules alphabetically to help reduce cache
                                // misses for otherwise identical content.
                                batch.sort();

                                // Query parameters common to all requests
                                reqBase = {
                                        skin: mw.config.get( 'skin' ),
                                        lang: mw.config.get( 'wgUserLanguage' ),
                                        debug: mw.config.get( 'debug' )
                                };
                                maxQueryLength = mw.config.get( 'wgResourceLoaderMaxQueryLength', 2000 );

                                // Split module list by source and by group.
                                splits = {};
                                for ( b = 0; b < batch.length; b++ ) {
                                        bSource = registry[ batch[ b ] ].source;
                                        bGroup = registry[ batch[ b ] ].group;
                                        if ( !hasOwn.call( splits, bSource ) ) {
                                                splits[ bSource ] = {};
                                        }
                                        if ( !hasOwn.call( splits[ bSource ], bGroup ) ) {
                                                splits[ bSource ][ bGroup ] = [];
                                        }
                                        bSourceGroup = splits[ bSource ][ bGroup ];
                                        bSourceGroup.push( batch[ b ] );
                                }

                                for ( source in splits ) {

                                        sourceLoadScript = sources[ source ];

                                        for ( group in splits[ source ] ) {

                                                // Cache access to currently selected list of
                                                // modules for this group from this source.
                                                modules = splits[ source ][ group ];

                                                // Query parameters common to requests for this module group
                                                // Optimisation: Inherit (Object.create), not copy ($.extend)
                                                currReqBase = Object.create( reqBase );
                                                // User modules require a user name in the query string.
                                                if ( group === 'user' && mw.config.get( 'wgUserName' ) !== null ) {
                                                        currReqBase.user = mw.config.get( 'wgUserName' );
                                                }

                                                // In addition to currReqBase, doRequest() will also add 'modules' and 'version'.
                                                // > '&modules='.length === 9
                                                // > '&version=1234567'.length === 16
                                                // > 9 + 16 = 25
                                                currReqBaseLength = makeQueryString( currReqBase ).length + 25;

                                                // We may need to split up the request to honor the query string length limit,
                                                // so build it piece by piece.
                                                l = currReqBaseLength;
                                                moduleMap = {}; // { prefix: [ suffixes ] }
                                                currReqModules = [];

                                                for ( i = 0; i < modules.length; i++ ) {
                                                        // Determine how many bytes this module would add to the query string
                                                        lastDotIndex = modules[ i ].lastIndexOf( '.' );
                                                        // If lastDotIndex is -1, substr() returns an empty string
                                                        prefix = modules[ i ].substr( 0, lastDotIndex );
                                                        suffix = modules[ i ].slice( lastDotIndex + 1 );
                                                        bytesAdded = hasOwn.call( moduleMap, prefix ) ?
                                                                suffix.length + 3 : // '%2C'.length == 3
                                                                modules[ i ].length + 3; // '%7C'.length == 3

                                                        // If the url would become too long, create a new one, but don't create empty requests
                                                        if ( maxQueryLength > 0 && currReqModules.length && l + bytesAdded > maxQueryLength ) {
                                                                // Dispatch what we've got...
                                                                doRequest();
                                                                // .. and start again.
                                                                l = currReqBaseLength;
                                                                moduleMap = {};
                                                                currReqModules = [];

                                                                mw.track( 'resourceloader.splitRequest', { maxQueryLength: maxQueryLength } );
                                                        }
                                                        if ( !hasOwn.call( moduleMap, prefix ) ) {
                                                                moduleMap[ prefix ] = [];
                                                        }
                                                        l += bytesAdded;
                                                        moduleMap[ prefix ].push( suffix );
                                                        currReqModules.push( modules[ i ] );
                                                }
                                                // If there's anything left in moduleMap, request that too
                                                if ( currReqModules.length ) {
                                                        doRequest();
                                                }
                                        }
                                }
                        }

                        /**
                         * @private
                         * @param {string[]} implementations Array containing pieces of JavaScript code in the
                         *  form of calls to mw.loader#implement().
                         * @param {Function} cb Callback in case of failure
                         * @param {Error} cb.err
                         */
                        function asyncEval( implementations, cb ) {
                                if ( !implementations.length ) {
                                        return;
                                }
                                mw.requestIdleCallback( function () {
                                        try {
                                                domEval( implementations.join( ';' ) );
                                        } catch ( err ) {
                                                cb( err );
                                        }
                                } );
                        }

                        /**
                         * Make a versioned key for a specific module.
                         *
                         * @private
                         * @param {string} module Module name
                         * @return {string|null} Module key in format '`[name]@[version]`',
                         *  or null if the module does not exist
                         */
                        function getModuleKey( module ) {
                                return hasOwn.call( registry, module ) ?
                                        ( module + '@' + registry[ module ].version ) : null;
                        }

                        /**
                         * @private
                         * @param {string} key Module name or '`[name]@[version]`'
                         * @return {Object}
                         */
                        function splitModuleKey( key ) {
                                var index = key.indexOf( '@' );
                                if ( index === -1 ) {
                                        return {
                                                name: key,
                                                version: ''
                                        };
                                }
                                return {
                                        name: key.slice( 0, index ),
                                        version: key.slice( index + 1 )
                                };
                        }

                        /* Public Members */
                        return {
                                /**
                                 * The module registry is exposed as an aid for debugging and inspecting page
                                 * state; it is not a public interface for modifying the registry.
                                 *
                                 * @see #registry
                                 * @property
                                 * @private
                                 */
                                moduleRegistry: registry,

                                /**
                                 * @inheritdoc #newStyleTag
                                 * @method
                                 */
                                addStyleTag: newStyleTag,

                                /**
                                 * Start loading of all queued module dependencies.
                                 *
                                 * @protected
                                 */
                                work: function () {
                                        var q, batch, implementations, sourceModules;

                                        batch = [];

                                        // Appends a list of modules from the queue to the batch
                                        for ( q = 0; q < queue.length; q++ ) {
                                                // Only load modules which are registered
                                                if ( hasOwn.call( registry, queue[ q ] ) && registry[ queue[ q ] ].state === 'registered' ) {
                                                        // Prevent duplicate entries
                                                        if ( batch.indexOf( queue[ q ] ) === -1 ) {
                                                                batch.push( queue[ q ] );
                                                                // Mark registered modules as loading
                                                                registry[ queue[ q ] ].state = 'loading';
                                                        }
                                                }
                                        }

                                        // Now that the queue has been processed into a batch, clear the queue.
                                        // This MUST happen before we initiate any eval or network request. Otherwise,
                                        // it is possible for a cached script to instantly trigger the same work queue
                                        // again; all before we've cleared it causing each request to include modules
                                        // which are already loaded.
                                        queue = [];

                                        if ( !batch.length ) {
                                                return;
                                        }

                                        mw.loader.store.init();
                                        if ( mw.loader.store.enabled ) {
                                                implementations = [];
                                                sourceModules = [];
                                                batch = batch.filter( function ( module ) {
                                                        var implementation = mw.loader.store.get( module );
                                                        if ( implementation ) {
                                                                implementations.push( implementation );
                                                                sourceModules.push( module );
                                                                return false;
                                                        }
                                                        return true;
                                                } );
                                                asyncEval( implementations, function ( err ) {
                                                        var failed;
                                                        // Not good, the cached mw.loader.implement calls failed! This should
                                                        // never happen, barring ResourceLoader bugs, browser bugs and PEBKACs.
                                                        // Depending on how corrupt the string is, it is likely that some
                                                        // modules' implement() succeeded while the ones after the error will
                                                        // never run and leave their modules in the 'loading' state forever.
                                                        mw.loader.store.stats.failed++;

                                                        // Since this is an error not caused by an individual module but by
                                                        // something that infected the implement call itself, don't take any
                                                        // risks and clear everything in this cache.
                                                        mw.loader.store.clear();

                                                        mw.track( 'resourceloader.exception', { exception: err, source: 'store-eval' } );
                                                        // Re-add the failed ones that are still pending back to the batch
                                                        failed = sourceModules.filter( function ( module ) {
                                                                return registry[ module ].state === 'loading';
                                                        } );
                                                        batchRequest( failed );
                                                } );
                                        }

                                        batchRequest( batch );
                                },

                                /**
                                 * Register a source.
                                 *
                                 * The #work() method will use this information to split up requests by source.
                                 *
                                 *     mw.loader.addSource( 'mediawikiwiki', '//www.mediawiki.org/w/load.php' );
                                 *
                                 * @param {string|Object} id Source ID, or object mapping ids to load urls
                                 * @param {string} loadUrl Url to a load.php end point
                                 * @throws {Error} If source id is already registered
                                 */
                                addSource: function ( id, loadUrl ) {
                                        var source;
                                        // Allow multiple additions
                                        if ( typeof id === 'object' ) {
                                                for ( source in id ) {
                                                        mw.loader.addSource( source, id[ source ] );
                                                }
                                                return;
                                        }

                                        if ( hasOwn.call( sources, id ) ) {
                                                throw new Error( 'source already registered: ' + id );
                                        }

                                        sources[ id ] = loadUrl;
                                },

                                /**
                                 * Register a module, letting the system know about it and its properties.
                                 *
                                 * The startup modules contain calls to this method.
                                 *
                                 * When using multiple module registration by passing an array, dependencies that
                                 * are specified as references to modules within the array will be resolved before
                                 * the modules are registered.
                                 *
                                 * @param {string|Array} module Module name or array of arrays, each containing
                                 *  a list of arguments compatible with this method
                                 * @param {string|number} version Module version hash (falls backs to empty string)
                                 *  Can also be a number (timestamp) for compatibility with MediaWiki 1.25 and earlier.
                                 * @param {string|Array} dependencies One string or array of strings of module
                                 *  names on which this module depends.
                                 * @param {string} [group=null] Group which the module is in
                                 * @param {string} [source='local'] Name of the source
                                 * @param {string} [skip=null] Script body of the skip function
                                 */
                                register: function ( module, version, dependencies, group, source, skip ) {
                                        var i, deps;
                                        // Allow multiple registration
                                        if ( typeof module === 'object' ) {
                                                resolveIndexedDependencies( module );
                                                for ( i = 0; i < module.length; i++ ) {
                                                        // module is an array of module names
                                                        if ( typeof module[ i ] === 'string' ) {
                                                                mw.loader.register( module[ i ] );
                                                        // module is an array of arrays
                                                        } else if ( typeof module[ i ] === 'object' ) {
                                                                mw.loader.register.apply( mw.loader, module[ i ] );
                                                        }
                                                }
                                                return;
                                        }
                                        if ( hasOwn.call( registry, module ) ) {
                                                throw new Error( 'module already registered: ' + module );
                                        }
                                        if ( typeof dependencies === 'string' ) {
                                                // A single module name
                                                deps = [ dependencies ];
                                        } else if ( typeof dependencies === 'object' ) {
                                                // Array of module names
                                                deps = dependencies;
                                        }
                                        // List the module as registered
                                        registry[ module ] = {
                                                // Exposed to execute() for mw.loader.implement() closures.
                                                // Import happens via require().
                                                module: {
                                                        exports: {}
                                                },
                                                version: version !== undefined ? String( version ) : '',
                                                dependencies: deps || [],
                                                group: typeof group === 'string' ? group : null,
                                                source: typeof source === 'string' ? source : 'local',
                                                state: 'registered',
                                                skip: typeof skip === 'string' ? skip : null
                                        };
                                },

                                /**
                                 * Implement a module given the components that make up the module.
                                 *
                                 * When #load() or #using() requests one or more modules, the server
                                 * response contain calls to this function.
                                 *
                                 * @param {string} module Name of module and current module version. Formatted
                                 *  as '`[name]@[version]`". This version should match the requested version
                                 *  (from #batchRequest and #registry). This avoids race conditions (T117587).
                                 *  For back-compat with MediaWiki 1.27 and earlier, the version may be omitted.
                                 * @param {Function|Array|string} [script] Function with module code, list of URLs
                                 *  to load via `<script src>`, or string of module code for `$.globalEval()`.
                                 * @param {Object} [style] Should follow one of the following patterns:
                                 *
                                 *     { "css": [css, ..] }
                                 *     { "url": { <media>: [url, ..] } }
                                 *
                                 * And for backwards compatibility (needs to be supported forever due to caching):
                                 *
                                 *     { <media>: css }
                                 *     { <media>: [url, ..] }
                                 *
                                 * The reason css strings are not concatenated anymore is T33676. We now check
                                 * whether it's safe to extend the stylesheet.
                                 *
                                 * @protected
                                 * @param {Object} [messages] List of key/value pairs to be added to mw#messages.
                                 * @param {Object} [templates] List of key/value pairs to be added to mw#templates.
                                 */
                                implement: function ( module, script, style, messages, templates ) {
                                        var split = splitModuleKey( module ),
                                                name = split.name,
                                                version = split.version;
                                        // Automatically register module
                                        if ( !hasOwn.call( registry, name ) ) {
                                                mw.loader.register( name );
                                        }
                                        // Check for duplicate implementation
                                        if ( hasOwn.call( registry, name ) && registry[ name ].script !== undefined ) {
                                                throw new Error( 'module already implemented: ' + name );
                                        }
                                        if ( version ) {
                                                // Without this reset, if there is a version mismatch between the
                                                // requested and received module version, then mw.loader.store would
                                                // cache the response under the requested key. Thus poisoning the cache
                                                // indefinitely with a stale value. (T117587)
                                                registry[ name ].version = version;
                                        }
                                        // Attach components
                                        registry[ name ].script = script || null;
                                        registry[ name ].style = style || null;
                                        registry[ name ].messages = messages || null;
                                        registry[ name ].templates = templates || null;
                                        // The module may already have been marked as erroneous
                                        if ( registry[ name ].state !== 'error' && registry[ name ].state !== 'missing' ) {
                                                registry[ name ].state = 'loaded';
                                                if ( allReady( registry[ name ].dependencies ) ) {
                                                        execute( name );
                                                }
                                        }
                                },

                                /**
                                 * Execute a function as soon as one or more required modules are ready.
                                 *
                                 * Example of inline dependency on OOjs:
                                 *
                                 *     mw.loader.using( 'oojs', function () {
                                 *         OO.compare( [ 1 ], [ 1 ] );
                                 *     } );
                                 *
                                 * Example of inline dependency obtained via `require()`:
                                 *
                                 *     mw.loader.using( [ 'mediawiki.util' ], function ( require ) {
                                 *         var util = require( 'mediawiki.util' );
                                 *     } );
                                 *
                                 * Since MediaWiki 1.23 this also returns a promise.
                                 *
                                 * Since MediaWiki 1.28 the promise is resolved with a `require` function.
                                 *
                                 * @param {string|Array} dependencies Module name or array of modules names the
                                 *  callback depends on to be ready before executing
                                 * @param {Function} [ready] Callback to execute when all dependencies are ready
                                 * @param {Function} [error] Callback to execute if one or more dependencies failed
                                 * @return {jQuery.Promise} With a `require` function
                                 */
                                using: function ( dependencies, ready, error ) {
                                        var deferred = $.Deferred();

                                        // Allow calling with a single dependency as a string
                                        if ( typeof dependencies === 'string' ) {
                                                dependencies = [ dependencies ];
                                        }

                                        if ( ready ) {
                                                deferred.done( ready );
                                        }
                                        if ( error ) {
                                                deferred.fail( error );
                                        }

                                        try {
                                                // Resolve entire dependency map
                                                dependencies = resolve( dependencies );
                                        } catch ( e ) {
                                                return deferred.reject( e ).promise();
                                        }
                                        if ( allReady( dependencies ) ) {
                                                // Run ready immediately
                                                deferred.resolve( mw.loader.require );
                                        } else if ( anyFailed( dependencies ) ) {
                                                // Execute error immediately if any dependencies have errors
                                                deferred.reject(
                                                        new Error( 'One or more dependencies failed to load' ),
                                                        dependencies
                                                );
                                        } else {
                                                // Not all dependencies are ready, add to the load queue
                                                enqueue( dependencies, function () {
                                                        deferred.resolve( mw.loader.require );
                                                }, deferred.reject );
                                        }

                                        return deferred.promise();
                                },

                                /**
                                 * Load an external script or one or more modules.
                                 *
                                 * This method takes a list of unrelated modules. Use cases:
                                 *
                                 * - A web page will be composed of many different widgets. These widgets independently
                                 *   queue their ResourceLoader modules (`OutputPage::addModules()`). If any of them
                                 *   have problems, or are no longer known (e.g. cached HTML), the other modules
                                 *   should still be loaded.
                                 * - This method is used for preloading, which must not throw. Later code that
                                 *   calls #using() will handle the error.
                                 *
                                 * @param {string|Array} modules Either the name of a module, array of modules,
                                 *  or a URL of an external script or style
                                 * @param {string} [type='text/javascript'] MIME type to use if calling with a URL of an
                                 *  external script or style; acceptable values are "text/css" and
                                 *  "text/javascript"; if no type is provided, text/javascript is assumed.
                                 */
                                load: function ( modules, type ) {
                                        var filtered, l;

                                        // Allow calling with a url or single dependency as a string
                                        if ( typeof modules === 'string' ) {
                                                // "https://example.org/x.js", "http://example.org/x.js", "//example.org/x.js", "/x.js"
                                                if ( /^(https?:)?\/?\//.test( modules ) ) {
                                                        if ( type === 'text/css' ) {
                                                                l = document.createElement( 'link' );
                                                                l.rel = 'stylesheet';
                                                                l.href = modules;
                                                                document.head.appendChild( l );
                                                                return;
                                                        }
                                                        if ( type === 'text/javascript' || type === undefined ) {
                                                                addScript( modules );
                                                                return;
                                                        }
                                                        // Unknown type
                                                        throw new Error( 'invalid type for external url, must be text/css or text/javascript. not ' + type );
                                                }
                                                // Called with single module
                                                modules = [ modules ];
                                        }

                                        // Filter out top-level modules that are unknown or failed to load before.
                                        filtered = modules.filter( function ( module ) {
                                                var state = mw.loader.getState( module );
                                                return state !== 'error' && state !== 'missing';
                                        } );
                                        // Resolve remaining list using the known dependency tree.
                                        // This also filters out modules with unknown dependencies. (T36853)
                                        filtered = resolveStubbornly( filtered );
                                        // If all modules are ready, or if any modules have errors, nothing to be done.
                                        if ( allReady( filtered ) || anyFailed( filtered ) ) {
                                                return;
                                        }
                                        if ( filtered.length === 0 ) {
                                                return;
                                        }
                                        // Some modules are not yet ready, add to module load queue.
                                        enqueue( filtered, undefined, undefined );
                                },

                                /**
                                 * Change the state of one or more modules.
                                 *
                                 * @param {string|Object} module Module name or object of module name/state pairs
                                 * @param {string} state State name
                                 */
                                state: function ( module, state ) {
                                        var m;

                                        if ( typeof module === 'object' ) {
                                                for ( m in module ) {
                                                        mw.loader.state( m, module[ m ] );
                                                }
                                                return;
                                        }
                                        if ( !hasOwn.call( registry, module ) ) {
                                                mw.loader.register( module );
                                        }
                                        registry[ module ].state = state;
                                        if ( state === 'ready' || state === 'error' || state === 'missing' ) {
                                                // Make sure pending modules depending on this one get executed if their
                                                // dependencies are now fulfilled!
                                                handlePending( module );
                                        }
                                },

                                /**
                                 * Get the version of a module.
                                 *
                                 * @param {string} module Name of module
                                 * @return {string|null} The version, or null if the module (or its version) is not
                                 *  in the registry.
                                 */
                                getVersion: function ( module ) {
                                        return hasOwn.call( registry, module ) ? registry[ module ].version : null;
                                },

                                /**
                                 * Get the state of a module.
                                 *
                                 * @param {string} module Name of module
                                 * @return {string|null} The state, or null if the module (or its state) is not
                                 *  in the registry.
                                 */
                                getState: function ( module ) {
                                        return hasOwn.call( registry, module ) ? registry[ module ].state : null;
                                },

                                /**
                                 * Get the names of all registered modules.
                                 *
                                 * @return {Array}
                                 */
                                getModuleNames: function () {
                                        return Object.keys( registry );
                                },

                                /**
                                 * Get the exported value of a module.
                                 *
                                 * This static method is publicly exposed for debugging purposes
                                 * only and must not be used in production code. In production code,
                                 * please use the dynamically provided `require()` function instead.
                                 *
                                 * In case of lazy-loaded modules via mw.loader#using(), the returned
                                 * Promise provides the function, see #using() for examples.
                                 *
                                 * @private
                                 * @since 1.27
                                 * @param {string} moduleName Module name
                                 * @return {Mixed} Exported value
                                 */
                                require: function ( moduleName ) {
                                        var state = mw.loader.getState( moduleName );

                                        // Only ready modules can be required
                                        if ( state !== 'ready' ) {
                                                // Module may've forgotten to declare a dependency
                                                throw new Error( 'Module "' + moduleName + '" is not loaded.' );
                                        }

                                        return registry[ moduleName ].module.exports;
                                },

                                /**
                                 * On browsers that implement the localStorage API, the module store serves as a
                                 * smart complement to the browser cache. Unlike the browser cache, the module store
                                 * can slice a concatenated response from ResourceLoader into its constituent
                                 * modules and cache each of them separately, using each module's versioning scheme
                                 * to determine when the cache should be invalidated.
                                 *
                                 * @singleton
                                 * @class mw.loader.store
                                 */
                                store: {
                                        // Whether the store is in use on this page.
                                        enabled: null,

                                        // Modules whose string representation exceeds 100 kB are
                                        // ineligible for storage. See bug T66721.
                                        MODULE_SIZE_MAX: 100 * 1000,

                                        // The contents of the store, mapping '[name]@[version]' keys
                                        // to module implementations.
                                        items: {},

                                        // Cache hit stats
                                        stats: { hits: 0, misses: 0, expired: 0, failed: 0 },

                                        /**
                                         * Construct a JSON-serializable object representing the content of the store.
                                         *
                                         * @return {Object} Module store contents.
                                         */
                                        toJSON: function () {
                                                return { items: mw.loader.store.items, vary: mw.loader.store.getVary() };
                                        },

                                        /**
                                         * Get the localStorage key for the entire module store. The key references
                                         * $wgDBname to prevent clashes between wikis which share a common host.
                                         *
                                         * @return {string} localStorage item key
                                         */
                                        getStoreKey: function () {
                                                return 'MediaWikiModuleStore:' + mw.config.get( 'wgDBname' );
                                        },

                                        /**
                                         * Get a key on which to vary the module cache.
                                         *
                                         * @return {string} String of concatenated vary conditions.
                                         */
                                        getVary: function () {
                                                return [
                                                        mw.config.get( 'skin' ),
                                                        mw.config.get( 'wgResourceLoaderStorageVersion' ),
                                                        mw.config.get( 'wgUserLanguage' )
                                                ].join( ':' );
                                        },

                                        /**
                                         * Initialize the store.
                                         *
                                         * Retrieves store from localStorage and (if successfully retrieved) decoding
                                         * the stored JSON value to a plain object.
                                         *
                                         * The try / catch block is used for JSON & localStorage feature detection.
                                         * See the in-line documentation for Modernizr's localStorage feature detection
                                         * code for a full account of why we need a try / catch:
                                         * <https://github.com/Modernizr/Modernizr/blob/v2.7.1/modernizr.js#L771-L796>.
                                         */
                                        init: function () {
                                                var raw, data;

                                                if ( mw.loader.store.enabled !== null ) {
                                                        // Init already ran
                                                        return;
                                                }

                                                if (
                                                        // Disabled because localStorage quotas are tight and (in Firefox's case)
                                                        // shared by multiple origins.
                                                        // See T66721, and <https://bugzilla.mozilla.org/show_bug.cgi?id=1064466>.
                                                        /Firefox|Opera/.test( navigator.userAgent ) ||

                                                        // Disabled by configuration.
                                                        !mw.config.get( 'wgResourceLoaderStorageEnabled' )
                                                ) {
                                                        // Clear any previous store to free up space. (T66721)
                                                        mw.loader.store.clear();
                                                        mw.loader.store.enabled = false;
                                                        return;
                                                }
                                                if ( mw.config.get( 'debug' ) ) {
                                                        // Disable module store in debug mode
                                                        mw.loader.store.enabled = false;
                                                        return;
                                                }

                                                try {
                                                        raw = localStorage.getItem( mw.loader.store.getStoreKey() );
                                                        // If we get here, localStorage is available; mark enabled
                                                        mw.loader.store.enabled = true;
                                                        data = JSON.parse( raw );
                                                        if ( data && typeof data.items === 'object' && data.vary === mw.loader.store.getVary() ) {
                                                                mw.loader.store.items = data.items;
                                                                return;
                                                        }
                                                } catch ( e ) {
                                                        mw.track( 'resourceloader.exception', { exception: e, source: 'store-localstorage-init' } );
                                                }

                                                if ( raw === undefined ) {
                                                        // localStorage failed; disable store
                                                        mw.loader.store.enabled = false;
                                                } else {
                                                        mw.loader.store.update();
                                                }
                                        },

                                        /**
                                         * Retrieve a module from the store and update cache hit stats.
                                         *
                                         * @param {string} module Module name
                                         * @return {string|boolean} Module implementation or false if unavailable
                                         */
                                        get: function ( module ) {
                                                var key;

                                                if ( !mw.loader.store.enabled ) {
                                                        return false;
                                                }

                                                key = getModuleKey( module );
                                                if ( key in mw.loader.store.items ) {
                                                        mw.loader.store.stats.hits++;
                                                        return mw.loader.store.items[ key ];
                                                }
                                                mw.loader.store.stats.misses++;
                                                return false;
                                        },

                                        /**
                                         * Stringify a module and queue it for storage.
                                         *
                                         * @param {string} module Module name
                                         * @param {Object} descriptor The module's descriptor as set in the registry
                                         * @return {boolean} Module was set
                                         */
                                        set: function ( module, descriptor ) {
                                                var args, key, src;

                                                if ( !mw.loader.store.enabled ) {
                                                        return false;
                                                }

                                                key = getModuleKey( module );

                                                if (
                                                        // Already stored a copy of this exact version
                                                        key in mw.loader.store.items ||
                                                        // Module failed to load
                                                        descriptor.state !== 'ready' ||
                                                        // Unversioned, private, or site-/user-specific
                                                        !descriptor.version ||
                                                        descriptor.group === 'private' ||
                                                        descriptor.group === 'user' ||
                                                        // Partial descriptor
                                                        // (e.g. skipped module, or style module with state=ready)
                                                        [ descriptor.script, descriptor.style, descriptor.messages,
                                                                descriptor.templates ].indexOf( undefined ) !== -1
                                                ) {
                                                        // Decline to store
                                                        return false;
                                                }

                                                try {
                                                        args = [
                                                                JSON.stringify( key ),
                                                                typeof descriptor.script === 'function' ?
                                                                        String( descriptor.script ) :
                                                                        JSON.stringify( descriptor.script ),
                                                                JSON.stringify( descriptor.style ),
                                                                JSON.stringify( descriptor.messages ),
                                                                JSON.stringify( descriptor.templates )
                                                        ];
                                                        // Attempted workaround for a possible Opera bug (bug T59567).
                                                        // This regex should never match under sane conditions.
                                                        if ( /^\s*\(/.test( args[ 1 ] ) ) {
                                                                args[ 1 ] = 'function' + args[ 1 ];
                                                                mw.track( 'resourceloader.assert', { source: 'bug-T59567' } );
                                                        }
                                                } catch ( e ) {
                                                        mw.track( 'resourceloader.exception', { exception: e, source: 'store-localstorage-json' } );
                                                        return false;
                                                }

                                                src = 'mw.loader.implement(' + args.join( ',' ) + ');';
                                                if ( src.length > mw.loader.store.MODULE_SIZE_MAX ) {
                                                        return false;
                                                }
                                                mw.loader.store.items[ key ] = src;
                                                mw.loader.store.update();
                                                return true;
                                        },

                                        /**
                                         * Iterate through the module store, removing any item that does not correspond
                                         * (in name and version) to an item in the module registry.
                                         *
                                         * @return {boolean} Store was pruned
                                         */
                                        prune: function () {
                                                var key, module;

                                                if ( !mw.loader.store.enabled ) {
                                                        return false;
                                                }

                                                for ( key in mw.loader.store.items ) {
                                                        module = key.slice( 0, key.indexOf( '@' ) );
                                                        if ( getModuleKey( module ) !== key ) {
                                                                mw.loader.store.stats.expired++;
                                                                delete mw.loader.store.items[ key ];
                                                        } else if ( mw.loader.store.items[ key ].length > mw.loader.store.MODULE_SIZE_MAX ) {
                                                                // This value predates the enforcement of a size limit on cached modules.
                                                                delete mw.loader.store.items[ key ];
                                                        }
                                                }
                                                return true;
                                        },

                                        /**
                                         * Clear the entire module store right now.
                                         */
                                        clear: function () {
                                                mw.loader.store.items = {};
                                                try {
                                                        localStorage.removeItem( mw.loader.store.getStoreKey() );
                                                } catch ( ignored ) {}
                                        },

                                        /**
                                         * Sync in-memory store back to localStorage.
                                         *
                                         * This function debounces updates. When called with a flush already pending,
                                         * the call is coalesced into the pending update. The call to
                                         * localStorage.setItem will be naturally deferred until the page is quiescent.
                                         *
                                         * Because localStorage is shared by all pages from the same origin, if multiple
                                         * pages are loaded with different module sets, the possibility exists that
                                         * modules saved by one page will be clobbered by another. But the impact would
                                         * be minor and the problem would be corrected by subsequent page views.
                                         *
                                         * @method
                                         */
                                        update: ( function () {
                                                var hasPendingWrite = false;

                                                function flushWrites() {
                                                        var data, key;
                                                        if ( !hasPendingWrite || !mw.loader.store.enabled ) {
                                                                return;
                                                        }

                                                        mw.loader.store.prune();
                                                        key = mw.loader.store.getStoreKey();
                                                        try {
                                                                // Replacing the content of the module store might fail if the new
                                                                // contents would exceed the browser's localStorage size limit. To
                                                                // avoid clogging the browser with stale data, always remove the old
                                                                // value before attempting to set the new one.
                                                                localStorage.removeItem( key );
                                                                data = JSON.stringify( mw.loader.store );
                                                                localStorage.setItem( key, data );
                                                        } catch ( e ) {
                                                                mw.track( 'resourceloader.exception', { exception: e, source: 'store-localstorage-update' } );
                                                        }

                                                        hasPendingWrite = false;
                                                }

                                                return function () {
                                                        if ( !hasPendingWrite ) {
                                                                hasPendingWrite = true;
                                                                mw.requestIdleCallback( flushWrites );
                                                        }
                                                };
                                        }() )
                                }
                        };
                }() ),

                /**
                 * HTML construction helper functions
                 *
                 *     @example
                 *
                 *     var Html, output;
                 *
                 *     Html = mw.html;
                 *     output = Html.element( 'div', {}, new Html.Raw(
                 *         Html.element( 'img', { src: '<' } )
                 *     ) );
                 *     mw.log( output ); // <div><img src="&lt;"/></div>
                 *
                 * @class mw.html
                 * @singleton
                 */
                html: ( function () {
                        function escapeCallback( s ) {
                                switch ( s ) {
                                        case '\'':
                                                return '&#039;';
                                        case '"':
                                                return '&quot;';
                                        case '<':
                                                return '&lt;';
                                        case '>':
                                                return '&gt;';
                                        case '&':
                                                return '&amp;';
                                }
                        }

                        return {
                                /**
                                 * Escape a string for HTML.
                                 *
                                 * Converts special characters to HTML entities.
                                 *
                                 *     mw.html.escape( '< > \' & "' );
                                 *     // Returns &lt; &gt; &#039; &amp; &quot;
                                 *
                                 * @param {string} s The string to escape
                                 * @return {string} HTML
                                 */
                                escape: function ( s ) {
                                        return s.replace( /['"<>&]/g, escapeCallback );
                                },

                                /**
                                 * Create an HTML element string, with safe escaping.
                                 *
                                 * @param {string} name The tag name.
                                 * @param {Object} [attrs] An object with members mapping element names to values
                                 * @param {string|mw.html.Raw|mw.html.Cdata|null} [contents=null] The contents of the element.
                                 *
                                 *  - string: Text to be escaped.
                                 *  - null: The element is treated as void with short closing form, e.g. `<br/>`.
                                 *  - this.Raw: The raw value is directly included.
                                 *  - this.Cdata: The raw value is directly included. An exception is
                                 *    thrown if it contains any illegal ETAGO delimiter.
                                 *    See <https://www.w3.org/TR/html401/appendix/notes.html#h-B.3.2>.
                                 * @return {string} HTML
                                 */
                                element: function ( name, attrs, contents ) {
                                        var v, attrName, s = '<' + name;

                                        if ( attrs ) {
                                                for ( attrName in attrs ) {
                                                        v = attrs[ attrName ];
                                                        // Convert name=true, to name=name
                                                        if ( v === true ) {
                                                                v = attrName;
                                                        // Skip name=false
                                                        } else if ( v === false ) {
                                                                continue;
                                                        }
                                                        s += ' ' + attrName + '="' + this.escape( String( v ) ) + '"';
                                                }
                                        }
                                        if ( contents === undefined || contents === null ) {
                                                // Self close tag
                                                s += '/>';
                                                return s;
                                        }
                                        // Regular open tag
                                        s += '>';
                                        switch ( typeof contents ) {
                                                case 'string':
                                                        // Escaped
                                                        s += this.escape( contents );
                                                        break;
                                                case 'number':
                                                case 'boolean':
                                                        // Convert to string
                                                        s += String( contents );
                                                        break;
                                                default:
                                                        if ( contents instanceof this.Raw ) {
                                                                // Raw HTML inclusion
                                                                s += contents.value;
                                                        } else if ( contents instanceof this.Cdata ) {
                                                                // CDATA
                                                                if ( /<\/[a-zA-z]/.test( contents.value ) ) {
                                                                        throw new Error( 'mw.html.element: Illegal end tag found in CDATA' );
                                                                }
                                                                s += contents.value;
                                                        } else {
                                                                throw new Error( 'mw.html.element: Invalid type of contents' );
                                                        }
                                        }
                                        s += '</' + name + '>';
                                        return s;
                                },

                                /**
                                 * Wrapper object for raw HTML passed to mw.html.element().
                                 *
                                 * @class mw.html.Raw
                                 * @constructor
                                 * @param {string} value
                                 */
                                Raw: function ( value ) {
                                        this.value = value;
                                },

                                /**
                                 * Wrapper object for CDATA element contents passed to mw.html.element()
                                 *
                                 * @class mw.html.Cdata
                                 * @constructor
                                 * @param {string} value
                                 */
                                Cdata: function ( value ) {
                                        this.value = value;
                                }
                        };
                }() ),

                // Skeleton user object, extended by the 'mediawiki.user' module.
                /**
                 * @class mw.user
                 * @singleton
                 */
                user: {
                        /**
                         * @property {mw.Map}
                         */
                        options: new Map(),
                        /**
                         * @property {mw.Map}
                         */
                        tokens: new Map()
                },

                // OOUI widgets specific to MediaWiki
                widgets: {},

                /**
                 * Registry and firing of events.
                 *
                 * MediaWiki has various interface components that are extended, enhanced
                 * or manipulated in some other way by extensions, gadgets and even
                 * in core itself.
                 *
                 * This framework helps streamlining the timing of when these other
                 * code paths fire their plugins (instead of using document-ready,
                 * which can and should be limited to firing only once).
                 *
                 * Features like navigating to other wiki pages, previewing an edit
                 * and editing itself – without a refresh – can then retrigger these
                 * hooks accordingly to ensure everything still works as expected.
                 *
                 * Example usage:
                 *
                 *     mw.hook( 'wikipage.content' ).add( fn ).remove( fn );
                 *     mw.hook( 'wikipage.content' ).fire( $content );
                 *
                 * Handlers can be added and fired for arbitrary event names at any time. The same
                 * event can be fired multiple times. The last run of an event is memorized
                 * (similar to `$(document).ready` and `$.Deferred().done`).
                 * This means if an event is fired, and a handler added afterwards, the added
                 * function will be fired right away with the last given event data.
                 *
                 * Like Deferreds and Promises, the mw.hook object is both detachable and chainable.
                 * Thus allowing flexible use and optimal maintainability and authority control.
                 * You can pass around the `add` and/or `fire` method to another piece of code
                 * without it having to know the event name (or `mw.hook` for that matter).
                 *
                 *     var h = mw.hook( 'bar.ready' );
                 *     new mw.Foo( .. ).fetch( { callback: h.fire } );
                 *
                 * Note: Events are documented with an underscore instead of a dot in the event
                 * name due to jsduck not supporting dots in that position.
                 *
                 * @class mw.hook
                 */
                hook: ( function () {
                        var lists = {};

                        /**
                         * Create an instance of mw.hook.
                         *
                         * @method hook
                         * @member mw
                         * @param {string} name Name of hook.
                         * @return {mw.hook}
                         */
                        return function ( name ) {
                                var list = hasOwn.call( lists, name ) ?
                                        lists[ name ] :
                                        lists[ name ] = $.Callbacks( 'memory' );

                                return {
                                        /**
                                         * Register a hook handler
                                         *
                                         * @param {...Function} handler Function to bind.
                                         * @chainable
                                         */
                                        add: list.add,

                                        /**
                                         * Unregister a hook handler
                                         *
                                         * @param {...Function} handler Function to unbind.
                                         * @chainable
                                         */
                                        remove: list.remove,

                                        /**
                                         * Run a hook.
                                         *
                                         * @param {...Mixed} data
                                         * @return {mw.hook}
                                         * @chainable
                                         */
                                        fire: function () {
                                                return list.fireWith.call( this, null, slice.call( arguments ) );
                                        }
                                };
                        };
                }() )
        };

        // Alias $j to jQuery for backwards compatibility
        // @deprecated since 1.23 Use $ or jQuery instead
        mw.log.deprecate( window, '$j', $, 'Use $ or jQuery instead.' );

        /**
         * Log a message to window.console, if possible.
         *
         * Useful to force logging of some errors that are otherwise hard to detect (i.e., this logs
         * also in production mode). Gets console references in each invocation instead of caching the
         * reference, so that debugging tools loaded later are supported (e.g. Firebug Lite in IE).
         *
         * @private
         * @param {string} topic Stream name passed by mw.track
         * @param {Object} data Data passed by mw.track
         * @param {Error} [data.exception]
         * @param {string} data.source Error source
         * @param {string} [data.module] Name of module which caused the error
         */
        function logError( topic, data ) {
                /* eslint-disable no-console */
                var msg,
                        e = data.exception,
                        source = data.source,
                        module = data.module,
                        console = window.console;

                if ( console && console.log ) {
                        msg = ( e ? 'Exception' : 'Error' ) + ' in ' + source;
                        if ( module ) {
                                msg += ' in module ' + module;
                        }
                        msg += ( e ? ':' : '.' );
                        console.log( msg );

                        // If we have an exception object, log it to the warning channel to trigger
                        // proper stacktraces in browsers that support it.
                        if ( e && console.warn ) {
                                console.warn( e );
                        }
                }
                /* eslint-enable no-console */
        }

        // Subscribe to error streams
        mw.trackSubscribe( 'resourceloader.exception', logError );
        mw.trackSubscribe( 'resourceloader.assert', logError );

        // Attach to window and globally alias
        window.mw = window.mediaWiki = mw;
}( jQuery ) );