Broadcast Channel API polyfill

broadcast-channel-es6.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 0.0.6
 */

/*
  MIT License
  Copyright (c) 2021 Alessandro Piana
  Permission is hereby granted, free of charge, to any person obtaining a copy
  of this software and associated documentation files (the "Software"), to deal
  in the Software without restriction, including without limitation the rights
  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  copies of the Software, and to permit persons to whom the Software is
  furnished to do so, subject to the following conditions:
  The above copyright notice and this permission notice shall be included in all
  copies or substantial portions of the Software.
  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  SOFTWARE.
*/

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

  /**
   * Utils
   * @private
   */
  const getRandomString = (len = 5) => {
    let text = '';
    const possible = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
    for(let i=0; i < len; i++) {
      text += possible.charAt(Math.floor(Math.random() * possible.length));
    }
    return text;
  };

  const isEmpty = obj => !Object.keys(obj).length;

  const getTimestamp = () => (new Date()).getTime();

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

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

    // Actually checks if the messages if from us.
    if (key.indexOf(`${_prefix}message_`) > -1 && !isRemoved) {

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

      // 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] ) {

        const subscribers = _channels[obj.channelId];
        subscribers.forEach(sub => {
          if (!sub.closed) {
            const event = new CustomEvent('message', obj.message);
            sub.dispatchEvent(event);
          }
        });
        // Remove the item for safety.
        context.localStorage.removeItem(key);
      }
    }
  };

  /**
   * Creates a new BroadcastChannel
   * @param {String} channelName - the channel name.
   * return {BroadcastChannel}
   */
  class _BroadcastChannel extends EventTarget {

    channelId = '';
    channelName = '';
    name = '';
    closed = false;

    constructor(channelName = '') {
      super();

      this.channelName = channelName;

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

      // Add custom prefix to Channel Name.
      const _channelId = _prefix + channelName
      const 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);
      }
    }

    /**
     * Sends the message to different channels.
     * @param {Object} data - the data to be sent ( actually, it can be any JS type ).
     */
    postMessage(data) {
      if (!_channels) return;

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

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

      // SAME-TAB communication.
      const subscribers = _channels[this.channelId] || [];
      subscribers.forEach(sub => {
        // We don't send the message to ourselves.
        if (sub.closed || sub.name === this.name) return;
        const event = new CustomEvent('message', msgObj);
        sub.dispatchEvent(event);
      });

      // CROSS-TAB communication.
      // Adds some properties to communicate among the tabs.
      const editedObj = {
        channelId: this.channelId,
        bcId: this.name,
        tabId: _tabId,
        message: msgObj
      };

      try {
        const editedJSON = JSON.stringify(editedObj);
        const lsKey = `${_prefix}message_${getRandomString()}_${this.channelId}`;
        // Set localStorage item (and, after that, removes it).
        context.localStorage.setItem(lsKey, editedJSON);
        setTimeout(() => context.localStorage.removeItem(lsKey), 1000);
      } catch (ex) {
        throw 'Message conversion has resulted in an error.';
      }
    };

    /**
     * Closes a Broadcast channel.
     */
    close() {
      this.closed = true;
      const subscribers = _channels[this.channelId];
      const index = subscribers.indexOf(this);
      if (index > -1) {
        subscribers.splice(index, 1);
      }
      // If we have no channels, remove the listener.
      if (!subscribers.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);