BroadcastChannel

A BroadcastChannel to send data across multiple browser-tabs or nodejs-processes
+ LeaderElection over the channels

A BroadcastChannel that allows you to send data between different browser-tabs or nodejs-processes.

• It works completely client-side and offline,
• Tested on old browsers, new browsers, WebWorkers, Iframes and NodeJs.

This behaves similar to the BroadcastChannel-API which is currently only featured in some browsers.

Sponsored by

The JavaScript Database

Using the BroadcastChannel

npm install --save broadcast-channel

Create a channel in one tab/process and send a message

import { BroadcastChannel } from 'broadcast-channel';
const channel = new BroadcastChannel('foobar');
channel.postMessage('I am not alone');

Create a channel with the same name in another tab/process and recieve messages

import { BroadcastChannel } from 'broadcast-channel';
const channel = new BroadcastChannel('foobar');
channel.onmessage = msg => console.dir(msg);
// > 'I am not alone'

Add and remove multiple eventlisteners

import { BroadcastChannel } from 'broadcast-channel';
const channel = new BroadcastChannel('foobar');

const handler = msg => console.log(msg);
channel.addEventListener('message', handler);

// remove it
channel.removeEventListener('message', handler);

Close the channel if you do not need it anymore

Returns a Promise which is resolved when everything is processed.

await channel.close();

Set options when creating a channel (optional)

const options = {
    type: 'localstorage', // (optional) enforce a type, oneOf['native', 'idb', 'localstorage', 'node']
    webWorkerSupport: true; // (optional) set this to false if you know that your channel will never be used in a WebWorker (increases performance)
};
const channel = new BroadcastChannel('foobar', options);

Create a typed channel in typescript

import { BroadcastChannel } from 'broadcast-channel';
declare type Message = {
  foo: string;
};
const channel: BroadcastChannel<Message> = new BroadcastChannel('foobar');
channel.postMessage({
  foo: 'bar'
});

Enforce options globally

When you use this module in a test-suite, it is recommended to enforce the fast simulate method on all channels so your tests run faster. You can do this with enforceOptions(). If you set this, all channels have the enforced options, no mather what options are given in the constructor.

import { enforceOptions } from 'broadcast-channel';

// enforce this config for all channels
enforceOptions({
  type: 'simulate'
});

// reset the enforcement
enforceOptions(null);

Clear tmp-folder

When used in NodeJs, the BroadcastChannel will communicate with other processes over filesystem sockets. When you create a huge amount of channels, like you would do when running unit tests, you might get problems because there are too many folders in the tmp-directory. Calling BroadcastChannel.clearNodeFolder() will clear the tmp-folder. It is recommended to run this at the beginning of your test-suite.

import { clearNodeFolder } from 'broadcast-channel';
// jest
beforeAll(async () => {
  const hasRun = await clearNodeFolder();
  console.log(hasRun); // > true on NodeJs, false on Browsers
})

import { clearNodeFolder } from 'broadcast-channel';
// mocha
before(async () => {
  const hasRun = await clearNodeFolder();
  console.log(hasRun); // > true on NodeJs, false on Browsers
})

Handling IndexedDB onclose events

IndexedDB databases can close unexpectedly for various reasons. This could happen, for example, if the underlying storage is removed or if a user clears the database in the browser's history preferences. Most often we have seen this happen in Mobile Safari. By default, we let the connection close and stop polling for changes. If you would like to continue listening you should close BroadcastChannel and create a new one.

Example of how you might do this:

import { BroadcastChannel } from 'broadcast-channel';

let channel;

const createChannel = () => {
  channel = new BroadcastChannel(CHANNEL_NAME, {
    idb: {
      onclose: () => {
        // the onclose event is just the IndexedDB closing.
        // you should also close the channel before creating
        // a new one.
        channel.close();
        createChannel();
      },
    },
  });

  channel.onmessage = message => {
    // handle message
  };
};

Methods

Depending in which environment this is used, a proper method is automatically selected to ensure it always works.

Using the LeaderElection

This module also comes with a leader-election which can be used to elect a leader between different BroadcastChannels. For example if you have a stable connection from the frontend to your server, you can use the LeaderElection to save server-side performance by only connecting once, even if the user has opened your website in multiple tabs.

In the background it will use the Web Locks API if possible and fall back to a message-based election algorithm if WebLocks is not available.

In this example the leader is marked with the crown ♛:

Create a channel and an elector:

import {
  BroadcastChannel,
  createLeaderElection
} from 'broadcast-channel';
const channel = new BroadcastChannel('foobar');
const elector = createLeaderElection(channel);

Wait until the elector becomes leader:

import { createLeaderElection } from 'broadcast-channel';
const elector = createLeaderElection(channel);
elector.awaitLeadership().then(()=> {
  console.log('this tab is now leader');
});

Check if there is a leader at this point in time. hasLeader() returns true when there is a leader. It returns false, if the leader is dead. Then it returns true again when a new leader is elected.

const elector = createLeaderElection(channel);
const hasLeader = await elector.hasLeader();
console.log('leader exists: ' + hasLeader);

If more than one tab is becoming leader adjust LeaderElectionOptions configuration.

import { createLeaderElection } from 'broadcast-channel';
const elector = createLeaderElection(channel, {
  fallbackInterval: 2000, // optional configuration for how often will renegotiation for leader occur
  responseTime: 1000, // optional configuration for how long will instances have to respond
});
elector.awaitLeadership().then(()=> {
  console.log('this tab is now leader');
})

Let the leader die. (automatically happens if a tab is closed or the process exits).

const elector = createLeaderElection(channel);
await elector.die();

Handle duplicate leaders

Duplicate leadership can happen on rare occurences like when the CPU is on 100% for longer time, or the browser has throttled the javascript timers.

const elector = createLeaderElection(channel);
elector.onduplicate = () => {
  alert('have duplicate leaders!');
}

What this is

This module is optimised for:

• low latency: When you postMessage to a channel, it will be delivered to other channels as soon as possible,
• lossless: When you send a message, it should be impossible that the message is lost before other channels recieved it,
• low idle workload: During the time when no messages are send, there should be a low processor footprint.

What this is not

• This is not a polyfill. Do not set this module to window.BroadcastChannel. This implementation behaves similiar to the BroadcastChannel-Standard with these limitations:
  • You can only send data that can be JSON.stringify-ed,
  • While the offical API emits onmessage-events, this module directly emitts the data which was posted.
• This is not a replacement for a message queue. If you use this in NodeJs and want send more than 50 messages per second, you should use proper IPC-Tooling.

Browser Support

I have tested this in all browsers that I could find. For ie8 and ie9 you must transpile the code before you can use this. If you want to know if this works with your browser, open the demo page.

Thanks

Thanks to Hemanth.HM for the module name.