Broadcast Channel API polyfill

## gistfile1.js
/**
  @class BroadcastChannel

  A simple BroadcastChannel polyfill that works with all major browsers.
  Please refer to the official MDN documentation of the Broadcast Channel API.
  ======

  @see <a href="https://developer.mozilla.org/en-US/docs/Web/API/Broadcast_Channel_API">Broadcast Channel API on MDN</a>
  @author Alessandro Piana
  @version 1.0.0
  @copyright Alessandro Piana 2015
*/

(function( context ) {
    // Internal variables
    var _channels = null, // List of channels
        _tabId = null, // Current window browser tab identifier (see IE problem, later)
        _prefix = 'polyBC_'; // prefix to identify localStorage keys.

    /**
     * Internal function, generates pseudo-random strings.
     * @see http://stackoverflow.com/a/1349426/2187738
     * @private
     */
    function getRandomString( length ) {
        var text = "",
            possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
        for( var i=0; i < (length || 5); i++ ) {
            text += possible.charAt(Math.floor(Math.random() * possible.length));
        }
        return text;
    };

    /**
     * Check if an object is empty.
     * @see http://stackoverflow.com/a/679937/2187738
     * @private
     */
    function isEmpty(obj) {
        for( var prop in obj) {
            if(obj.hasOwnProperty(prop))
                return false;
        }
        return true;
        // Also this is good.
        // returns 0 if empty or an integer > 0 if non-empty
        //return Object.keys(obj).length;
    };

    /**
     * Gets the current timestamp
     * @private
     */
    function getTimestamp() {
        return (new Date().getTime());
    };

    /**
     * Build a "similar" response as done in the real BroadcastChannel API
     */
    function buildResponse( data ) {
        return {
            timestamp: getTimestamp(),
            isTrusted: true,
            target: null, // Since we are using JSON stringify, we cannot pass references.
            currentTarget: null,
            data: data,
            bubbles: false,
            cancelable: false,
            defaultPrevented: false,
            lastEventId: '',
            origin: context.location.origin
        };
    };

    /**
     * Creates a new BroadcastChannel
     * @param {String} channelName - the channel name.
     * return {BroadcastChannel}
     */
    function BroadcastChannel( channelName ) {

        // Check if localStorage is available.
        if (!context.localStorage) {
            throw new 'localStorage not available';
            return;
        }

        // Add custom prefix to Channel Name.
        var _channelId = _prefix + channelName,
            isFirstChannel = (_channels === null);

        this.channelId = _channelId;

        _tabId = _tabId || getRandomString(); // Creates a new tab identifier, if necessary.
        _channels = _channels || {}; // Initializes channels, if necessary.
        _channels[ _channelId ] = _channels[ _channelId ] || [];

        // Adds the current Broadcast Channel.
        _channels[ _channelId ].push( this );

        // Creates a sufficiently random name for the current instance of BC.
        this.name = _channelId + '::::' + getRandomString() + getTimestamp();

        // If it is the first instance of Channel created, also creates the storage listener.
        if (isFirstChannel) {
            // addEventListener.
            context.addEventListener('storage', _onmsg.bind(this), false);
        }

        return this;
    };

    /**
     * Empty function to prevent errors when calling onmessage.
     */
    BroadcastChannel.prototype.onmessage = function( ev ){};

    /**
     * Sends the message to different channels.
     * @param {Object} data - the data to be sent ( actually, it can be any JS type ).
     */
    BroadcastChannel.prototype.postMessage = function( data ) {
        // Gets all the 'Same tab' channels available.
        if (!_channels) return;

        if (this.closed) {
            throw new 'This BroadcastChannel is closed.';
            return;
        }

        // Build the event-like response.
        var msgObj = buildResponse( data );

        // SAME-TAB communication.
        var subscribers = _channels[ this.channelId ] || [];
        for (var j in subscribers) {
            // We don't send the message to ourselves.
            if (subscribers[j].closed || subscribers[j].name === this.name) continue;

            if (subscribers[j].onmessage ) {
                subscribers[j].onmessage( msgObj );
            }
        }

        // CROSS-TAB communication.
        // Adds some properties to communicate among the tabs.
        var editedObj = {
            channelId: this.channelId,
            bcId: this.name,
            tabId: _tabId,
            message: msgObj
        };
        try {
            var editedJSON = JSON.stringify( editedObj ),
                lsKey = 'eomBCmessage_' + getRandomString() + '_' + this.channelId;
            // Set localStorage item (and, after that, removes it).
            context.localStorage.setItem( lsKey, editedJSON );
        } catch (ex) {
            throw new 'Message conversion has resulted in an error.';
            return;
        }

        setTimeout(function(){ context.localStorage.removeItem( lsKey ) }, 1000);

    };

    /**
     * Handler of the 'storage' function.
     * Called when another window has sent a message.
     * @param {Object} ev - the message.
     * @private
     */
    function _onmsg( ev ) {
        var key = ev.key,
            newValue = ev.newValue,
            isRemoved = !newValue,
            obj = null;

        // Actually checks if the messages if from us.
        if ( key.indexOf('eomBCmessage_') > -1 && !isRemoved) {

            try {
                obj = JSON.parse( newValue );
            } catch( ex ) {
                throw new 'Message conversion has resulted in an error.';
                return;
            }

            // NOTE: Check on tab is done to prevent IE error
            // (localStorage event is called even in the same tab :( )

            if ( (obj.tabId !== _tabId) &&
                 obj.channelId &&
                 _channels &&
                 _channels[ obj.channelId ] ) {

                var subscribers = _channels[ obj.channelId ];
                for (var j in subscribers) {
                    if (!subscribers[j].closed && subscribers[j].onmessage ) {
                        subscribers[j].onmessage( obj.message );
                    }
                }
                // Remove the item for safety.
                context.localStorage.removeItem( key );
            }
        }
    };

    /**
     * Closes a Broadcast channel.
     */
    BroadcastChannel.prototype.close = function() {

        this.closed = true;

        var index = _channels[ this.channelId ].indexOf(this);
        if (index > -1)
            _channels[ this.channelId ].splice( index, 1 );

        // If we have no channels, remove the listener.
        if (!_channels[ this.channelId ].length) {
            delete _channels[ this.channelId ];
        }
        if ( isEmpty( _channels ) ) {
            context.removeEventListener( 'storage', _onmsg.bind(this) );
        }
    };

    // Sets BroadcastChannel, if not available.
    context.BroadcastChannel = context.BroadcastChannel || BroadcastChannel;

})( window.top );