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Introduction to Web Workers API in JavaScript

by Mozilla ContributorsMay 4th, 2020

Too Long; Didn't Read

Web Workers makes it possible to run a script operation in a background thread separate from the main execution thread of a web application. The advantage of this is that laborious processing can be performed in a separate thread, allowing the main (usually the UI) thread to run without being blocked/slowed down. You can run whatever code you like inside the worker thread, with some exceptions. Workers run in another global context that is represented by either a DedicatedWorkerGlobalScope object (in the case of dedicated workers - workers that are utilized by a single script), or a SharedWorker globalScope.

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Web Workers makes it possible to run a script operation in a background thread separate from the main execution thread of a web application. The advantage of this is that laborious processing can be performed in a separate thread, allowing the main (usually the UI) thread to run without being blocked/slowed down.

Web Workers concepts and usage

A worker is an object created using a constructor (e.g.

Worker()

) that runs a named JavaScript file — this file contains the code that will run in the worker thread; workers run in another global context that is different from the current

window

This context is represented by either a

DedicatedWorkerGlobalScope

object (in the case of dedicated workers - workers that are utilized by a single script), or a

SharedWorkerGlobalScope

(in the case of shared workers - workers that are shared between multiple scripts).

You can run whatever code you like inside the worker thread, with some exceptions. For example, you can't directly manipulate the DOM from inside a worker, or use some default methods and properties of the

window

object.

But you can use a large number of items available under

window

, including WebSockets, and data storage mechanisms like IndexedDB. See Functions and classes available to workers for more details.

Data is sent between workers and the main thread via a system of messages — both sides send their messages using the

postMessage()

method, and respond to messages via the

onmessage

event handler (the message is contained within the

Message

event's data property). The

data

is copied rather than shared.

Workers may in turn spawn new workers, as long as those workers are
hosted within the same origin as the parent page. In addition, workers
may use

XMLHttpRequest

for network I/O, with the exception that the

responseXML

and

channel

attributes on

XMLHttpRequest

always return

null

In addition to dedicated workers, there are other types of worker:

Shared workers are workers that can be utilized by multiple scripts running in different windows, IFrames, etc., as long as they are in the same domain as the worker. They are a little more complex than dedicated workers — scripts must communicate via an active port. See
```
SharedWorker
```
for more details.
ServiceWorkers essentially act as proxy servers that sit between web applications, the browser, and the network (when available). They are intended, among other things, to enable the creation of effective offline experiences, intercept network requests and take appropriate action based on whether the network is available, and update assets residing on the server. They will also allow access to push notifications and background sync APIs.
Chrome Workers are a Firefox-only type of worker that you can use if you are developing add-ons and want to use workers in extensions and have access to js-ctypes in your worker. See
```
ChromeWorker
```
for more details.
Audio Workers provide the ability for direct scripted audio processing to be done inside a web worker context.