WorkerDispatcher is a JavaScript class that adds events layer to Worker API

Usage no npm install needed!

<script type="module">
  import actualwaveWorkerDispatcher from 'https://cdn.skypack.dev/@actualwave/worker-dispatcher';



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This is extension of MessagePortDispatcher to work with Dedicated and Shared Workers. It makes possible two-way communication with Workers using custom events. So, instead of using postMessage() and catching message event all the time, you are free to send any type of events to and from worker.

Demo with dedicated and shared workers


Easy to install with npm package manager

npm install --save @actualwave/worker-dispatcher

with yarn package manager

yarn add @actualwave/worker-dispatcher


Note: WorkerDispatcher distribution package contains dist/ folder with package wrapped into UMD wrapper, so it can be used with any AMD module loader, nodejs require() or without any.

WorkerDispatcher should be used on HTML page and in Worker script to properly handle communication.

Dedicated Worker

WorkerDispatcher for Dedicated Worker can be created via WorkerDispatcher.create() factory function

var worker = new Worker('/workers/worker.js');
var dispatcher = WorkerDispatcher.create(worker);
/* or you can explicitly specify worker type */
var dispatcher = WorkerDispatcher.create(worker, WorkerDispatcher.DEDICATED_WORKER);

Within Worker script it can be created via WorkerDispatcher.createForSelf(), don't need to pass anything, it grabs Worker's global scope object to communicate.

var dispatcher = WorkerDispatcher.createForSelf();

WorkerDispatcher accepts Worker objects or string URL to JS file that should be launched in worker. Here Worker will be created from passed URL string:

var dispatcher = WorkerDispatcher.create('/workers/worker.js');

Shared Worker

To use WorkerDispatcher with Shared Worker, it should be created via WorkerDispatcher.create() factory method with specified Worker type.

 var worker = new SharedWorker('/workers/sharedworker.js');
 var dispatcher = WorkerDispatcher.create(worker, WorkerDispatcher.SHARED_WORKER);

Within SharedWorker it can be created via WorkerDispatcher.createForSelf(). WorkerDispatcher's for client connections will be created automatically.

var dispatcher = WorkerDispatcher.createForSelf();
dispatcher.addEventListener(WorkerDispatcher.WorkerEvent.CONNECT, function(event) {
  var client = event.client;
  client.addEventListener('data', function(event) {
    console.log('new data from client', event.data);

Sending and receiving messages

To send messages use dispatchEvent() event and to receive messages add event listeners. Sent events will not be fired for sender dispatcher, so you cannot listen for event you just sent

var dispatcher = WorkerDispatcher.create('/workers/worker.js');
dispatcher.addEventListener('anyEvent', function(){
    console.log('Event received');

In this case event listener will not be called, but if other side will send "anyEvent" event, this listener will be called. On HTML page:

var dispatcher = new WorkerDispatcher('/workers/worker.js');
dispatcher.addEventListener('anyEvent', function(event) {
    console.log('Event received');

Worker code:

var dispatcher = WorkerDispatcher.createForSelf();

Project contains example folder with examples for Dedicated and Shared workers communication built with WorkerDispatcher.


WorkerDispatcher constructor arguments

  • worker:Worker|MessagePort|String - Worker instance or URL string for worker script.
  • receiverEventPreprocessor:Function - Optional, allows pre-processing of events and their data before firing event.
  • senderEventPreprocessor:Function - Optional, allows pre-processing of events and their data before passing them to postMessage.
  • type?:String - argument used internally to generate type property in prototype.

WorkerDispatcher shared instance members

WorkerDispatcher is a base class and it shares functionality across all types of WorkerDispatcher's. When WorkerDispatcher instantiated directly, it actually creates DedicatedWorkerDispatcher.

  • type:String - type of the worker Including all members of MessagePortDispatcher, some most important:
  • addEventListener(eventType:String, listener:Function):void - add listener for incoming events. This method copied from receiver.
  • hasEventListener(eventType:String):Boolean - check if incoming event has listeners. This method copied from receiver.
  • removeEventListener(eventType:String, listener:Function):void - remove event listener for incoming event. This method copied from receiver.
  • dispatchEvent(event:Object):void - does not fire event, it sends event to postMessage(). Can be used with two arguments:
  • dispatchEvent(eventType:String, data?:Object):void

WorkerDispatcher static members

  • CONNECT_EVENT:String - Short of WorkerEvent.CONNECT. Event fired in Shared Worker script when new client is available.


  • SHARED_WORKER:String - Short of WorkerType.SHARED_WORKER

  • create(target:String|Worker|SharedWorker, type?:String, receiverEventPreprocessor?:Function, senderEventPreprocessor?:Function):WorkerDispatcher - Creates WorkerDispatcher instance based on type. Currently supported types are WorkerDispatcher.DEDICATED_WORKER and WorkerDispatcher.SHARED_WORKER. By default will create dispatcher for Dedicated Worker.

  • self(receiverEventPreprocessor?:Function, senderEventPreprocessor?:Function):WorkerDispatcher - Can be used in Worker script, it checks what kind of worker is used and returns proper dispatcher object for WorkerGlobalScope. For Dedicated Worker returns instance of DedicatedWorkerDispatcher and for Shared Worker -- ServerEventDispatcher.

  • WorkerEvent:Object - Worker event types

    • CONNECT:String - Mirroring connect event fired from WorkerGlobalScope, fired when new client connected. Event object contains field client with ClientEventDispatcher instance, to communicate with client.
    • ERROR:String - Mirroring error event fired from WorkerGlobalScope
    • LANGUAGECHANGE:String - Mirroring languagechange event fired from WorkerGlobalScope
    • ONLINE:String - Mirroring online event fired from WorkerGlobalScope
    • OFFLINE:String - Mirroring offline event fired from WorkerGlobalScope
  • WorkerType:Object - Possible dispatcher types, used with WorkerDispatcher.create()

    • DEDICATED_WORKER:String - Default type, will create DedicatedWorkerDispatcher
    • SHARED_WORKER:String - Will create SharedWorkerDispatcher
    • SHARED_WORKER_SERVER:String - For internal usage, will create ServerEventDispatcher
    • SHARED_WORKER_CLIENT:String - For internal usage, will create ClientEventDispatcher
  • DedicatedWorker:Function - Constructor of DedicatedWorkerDispatcher

  • SharedWorker:Function - Constructor of SharedWorkerDispatcher

  • Server:Function - Constructor of ServerEventDispatcher

  • Client:Function - Constructor of ClientEventDispatcher


Created when WorkerDispatcher.DEDICATED_WORKER used, when WorkerDispatcher.createForSelf() called in Dedicated Worker or when WorkerDispatcher called with new operator.

  • terminate():void - close connection to worker, i.e. destroy worker.


Created when WorkerDispatcher.SHARED_WORKER used. When created using WorkerDispatcher.create(), worker's name will default to null, if you need to specify name, you can instantiate it with constructor.

var dispatcher = new WorkerDispatcher.SharedWorkerDispatcher('/workers/sharedworker.js', 'worker-name');


Created when WorkerDispatcher.createForSelf() called in Shared Worker. It differs from other types of WorkerDispatcher's because does not have dispatchEvent() method, so it can only listen for events, like WorkerEvent.CONNECT to accept connections. Since it cannot send data, it does not have sender EventDispatcher either, only receiver available.


Created when Shared Worker gets new connection. to capture new connections, you shuld listen to WorkerEvent.CONNECT event.

  • start():void - Start communication with client
  • close():void - Close connection to client
var _clients = [];
// Create ServerEventDispatcher
var dispatcher = WorkerDispatcher.createForSelf();
// Listen to incoming connections
dispatcher.addEventListener(WorkerDispatcher.WorkerEvent.CONNECT, function(event) {
  // Get ClientEventDispatcher of new connection from event, save and start it
  var client = event.client;


MDN - Using web workers https://www.w3.org/TR/workers/