HTML

Living Standard — Last Updated 14 March 2025

1. 10 Web workers
   1. 10.1 Introduction
      1. 10.1.1 Scope
      2. 10.1.2 Examples
         1. 10.1.2.1 A background number-crunching worker
         2. 10.1.2.2 Using a JavaScript module as a worker
         3. 10.1.2.3 Shared workers introduction
         4. 10.1.2.4 Shared state using a shared worker
         5. 10.1.2.5 Delegation
         6. 10.1.2.6 Providing libraries
      3. 10.1.3 Tutorials
         1. 10.1.3.1 Creating a dedicated worker
         2. 10.1.3.2 Communicating with a dedicated worker
         3. 10.1.3.3 Shared workers
   2. 10.2 Infrastructure
      1. 10.2.1 The global scope
         1. 10.2.1.1 The WorkerGlobalScope common interface
         2. 10.2.1.2 Dedicated workers and the DedicatedWorkerGlobalScope interface
         3. 10.2.1.3 Shared workers and the SharedWorkerGlobalScope interface
      2. 10.2.2 The event loop
      3. 10.2.3 The worker's lifetime
      4. 10.2.4 Processing model
      5. 10.2.5 Runtime script errors
      6. 10.2.6 Creating workers
         1. 10.2.6.1 The AbstractWorker mixin
         2. 10.2.6.2 Script settings for workers
         3. 10.2.6.3 Dedicated workers and the Worker interface
         4. 10.2.6.4 Shared workers and the SharedWorker interface
      7. 10.2.7 Concurrent hardware capabilities
   3. 10.3 APIs available to workers
      1. 10.3.1 Importing scripts and libraries
      2. 10.3.2 The WorkerNavigator interface
      3. 10.3.3 The WorkerLocation interface

10 Web workers

10.1 Introduction

10.1.1 Scope

This section is non-normative.

This specification defines an API for running scripts in the background independently of any user interface scripts.

This allows for long-running scripts that are not interrupted by scripts that respond to clicks or other user interactions, and allows long tasks to be executed without yielding to keep the page responsive.

Workers (as these background scripts are called herein) are relatively heavy-weight, and are not intended to be used in large numbers. For example, it would be inappropriate to launch one worker for each pixel of a four megapixel image. The examples below show some appropriate uses of workers.

Generally, workers are expected to be long-lived, have a high start-up performance cost, and a high per-instance memory cost.

10.1.2 Examples

This section is non-normative.

There are a variety of uses that workers can be put to. The following subsections show various examples of this use.

10.1.2.1 A background number-crunching worker

This section is non-normative.

The simplest use of workers is for performing a computationally expensive task without interrupting the user interface.

In this example, the main document spawns a worker to (naïvely) compute prime numbers, and progressively displays the most recently found prime number.

The main page is as follows:

EXAMPLE
workers/primes/page.html

The Worker() constructor call creates a worker and returns a Worker object representing that worker, which is used to communicate with the worker. That object's onmessage event handler allows the code to receive messages from the worker.

The worker itself is as follows:

EXAMPLE
workers/primes/worker.js

The bulk of this code is simply an unoptimized search for a prime number. The postMessage() method is used to send a message back to the page when a prime is found.

View this example online .

10.1.2.2 Using a JavaScript module as a worker

This section is non-normative.

All of our examples so far show workers that run classic scripts . Workers can instead be instantiated using module scripts , which have the usual benefits: the ability to use the JavaScript import statement to import other modules; strict mode by default; and top-level declarations not polluting the worker's global scope.

As the import statement is available, the importScripts() method will automatically fail inside module workers.

In this example, the main document uses a worker to do off-main-thread image manipulation. It imports the filters used from another module.

The main page is as follows:

EXAMPLE
workers/modules/page.html

The worker file is then:

EXAMPLE
workers/modules/worker.js

Which imports the file filters.js :

EXAMPLE
workers/modules/filters.js

View this example online .

10.1.2.3 Shared workers introduction

This section is non-normative.

This section introduces shared workers using a Hello World example. Shared workers use slightly different APIs, since each worker can have multiple connections.

This first example shows how you connect to a worker and how a worker can send a message back to the page when it connects to it. Received messages are displayed in a log.

Here is the HTML page:

EXAMPLE
workers/shared/001/test.html

Here is the JavaScript worker:

EXAMPLE
workers/shared/001/test.js

View this example online .

This second example extends the first one by changing two things: first, messages are received using addEventListener() instead of an event handler IDL attribute , and second, a message is sent to the worker, causing the worker to send another message in return. Received messages are again displayed in a log.

Here is the HTML page:

EXAMPLE
workers/shared/002/test.html

Here is the JavaScript worker:

EXAMPLE
workers/shared/002/test.js

View this example online .

Finally, the example is extended to show how two pages can connect to the same worker; in this case, the second page is merely in an iframe on the first page, but the same principle would apply to an entirely separate page in a separate top-level traversable .

Here is the outer HTML page:

EXAMPLE
workers/shared/003/test.html

Here is the inner HTML page:

EXAMPLE
workers/shared/003/inner.html

Here is the JavaScript worker:

EXAMPLE
workers/shared/003/test.js

View this example online .

10.1.2.4 Shared state using a shared worker

This section is non-normative.

In this example, multiple windows (viewers) can be opened that are all viewing the same map. All the windows share the same map information, with a single worker coordinating all the viewers. Each viewer can move around independently, but if they set any data on the map, all the viewers are updated.

The main page isn't interesting, it merely provides a way to open the viewers:

EXAMPLE
workers/multiviewer/page.html

The viewer is more involved:

EXAMPLE
workers/multiviewer/viewer.html

There are several key things worth noting about the way the viewer is written.

Multiple listeners . Instead of a single message processing function, the code here attaches multiple event listeners, each one performing a quick check to see if it is relevant for the message. In this example it doesn't make much difference, but if multiple authors wanted to collaborate using a single port to communicate with a worker, it would allow for independent code instead of changes having to all be made to a single event handling function.

Registering event listeners in this way also allows you to unregister specific listeners when you are done with them, as is done with the configure() method in this example.

Finally, the worker:

EXAMPLE
workers/multiviewer/worker.js

Connecting to multiple pages . The script uses the onconnect event listener to listen for multiple connections.

Direct channels . When the worker receives a "msg" message from one viewer naming another viewer, it sets up a direct connection between the two, so that the two viewers can communicate directly without the worker having to proxy all the messages.

View this example online .

10.1.2.5 Delegation

This section is non-normative.

With multicore CPUs becoming prevalent, one way to obtain better performance is to split computationally expensive tasks amongst multiple workers. In this example, a computationally expensive task that is to be performed for every number from 1 to 10,000,000 is farmed out to ten subworkers.

The main page is as follows, it just reports the result:

EXAMPLE
workers/multicore/page.html

The worker itself is as follows:

EXAMPLE
workers/multicore/worker.js

It consists of a loop to start the subworkers, and then a handler that waits for all the subworkers to respond.

The subworkers are implemented as follows:

EXAMPLE
workers/multicore/core.js

They receive two numbers in two events, perform the computation for the range of numbers thus specified, and then report the result back to the parent.

View this example online .

10.1.2.6 Providing libraries

This section is non-normative.

Suppose that a cryptography library is made available that provides three tasks:

Generate a public/private key pair

Takes a port, on which it will send two messages, first the public key and then the private key.

Given a plaintext and a public key, return the corresponding ciphertext

Takes a port, to which any number of messages can be sent, the first giving the public key, and the remainder giving the plaintext, each of which is encrypted and then sent on that same channel as the ciphertext. The user can close the port when it is done encrypting content.

Given a ciphertext and a private key, return the corresponding plaintext

Takes a port, to which any number of messages can be sent, the first giving the private key, and the remainder giving the ciphertext, each of which is decrypted and then sent on that same channel as the plaintext. The user can close the port when it is done decrypting content.

The library itself is as follows:

EXAMPLE
workers/crypto/libcrypto-v1.js

Note that the crypto functions here are just stubs and don't do real cryptography.

This library could be used as follows:

EXAMPLE
workers/crypto/page.html

A later version of the API, though, might want to offload all the crypto work onto subworkers. This could be done as follows:

EXAMPLE
workers/crypto/libcrypto-v2.js

The little subworkers would then be as follows.

For generating key pairs:

EXAMPLE
workers/crypto/libcrypto-v2-generator.js

For encrypting:

EXAMPLE
workers/crypto/libcrypto-v2-encryptor.js

For decrypting:

EXAMPLE
workers/crypto/libcrypto-v2-decryptor.js

Notice how the users of the API don't have to even know that this is happening — the API hasn't changed; the library can delegate to subworkers without changing its API, even though it is accepting data using message channels.

View this example online .

10.1.3 Tutorials

10.1.3.1 Creating a dedicated worker

This section is non-normative.

Creating a worker requires a URL to a JavaScript file. The Worker() constructor is invoked with the URL to that file as its only argument; a worker is then created and returned:

var
worker
=
new
Worker('helper.js');

If you want your worker script to be interpreted as a module script instead of the default classic script , you need to use a slightly different signature:

var
worker
=
new
Worker('helper.mjs',
{
type:
"module"
});

10.1.3.2 Communicating with a dedicated worker

This section is non-normative.

Dedicated workers use MessagePort objects behind the scenes, and thus support all the same features, such as sending structured data, transferring binary data, and transferring other ports.

To receive messages from a dedicated worker, use the onmessage event handler IDL attribute on the Worker object:

worker.onmessage
=
function
(event)
{
...
};

You can also use the addEventListener() method.

The implicit MessagePort used by dedicated workers has its port message queue implicitly enabled when it is created, so there is no equivalent to the MessagePort interface's start() method on the Worker interface.

To send data to a worker, use the postMessage() method. Structured data can be sent over this communication channel. To send ArrayBuffer objects efficiently (by transferring them rather than cloning them), list them in an array in the second argument.

worker.postMessage({
  operation: 'find-edges',
  input: buffer, // an ArrayBuffer object
  threshold: 0.6,
},
[buffer]);

To receive a message inside the worker, the onmessage event handler IDL attribute is used.

onmessage
=
function
(event)
{
...
};

You can again also use the addEventListener() method.

In either case, the data is provided in the event object's data attribute.

To send messages back, you again use postMessage() . It supports the structured data in the same manner.

postMessage(event.data.input,
[event.data.input]);
//
transfer
the
buffer
back

10.1.3.3 Shared workers

This section is non-normative.

Shared workers are identified by the URL of the script used to create it, optionally with an explicit name. The name allows multiple instances of a particular shared worker to be started.

Shared workers are scoped by origin . Two different sites using the same names will not collide. However, if a page tries to use the same shared worker name as another page on the same site, but with a different script URL, it will fail.

Creating shared workers is done using the SharedWorker() constructor. This constructor takes the URL to the script to use for its first argument, and the name of the worker, if any, as the second argument.

var
worker
=
new
SharedWorker('service.js');

Communicating with shared workers is done with explicit MessagePort objects. The object returned by the SharedWorker() constructor holds a reference to the port on its port attribute.

worker.port.onmessage = function (event) { ... };
worker.port.postMessage('some message');
worker.port.postMessage({
foo:
'structured',
bar:
['data',
'also',
'possible']});

Inside the shared worker, new clients of the worker are announced using the connect event. The port for the new client is given by the event object's source attribute.

onconnect = function (event) {
  var newPort = event.source;
  // set up a listener
  newPort.onmessage = function (event) { ... };
  // send a message back to the port
  newPort.postMessage('ready!'); // can also send structured data, of course
};

10.2 Infrastructure

This standard defines two kinds of workers: dedicated workers, and shared workers. Dedicated workers, once created, are linked to their creator, but message ports can be used to communicate from a dedicated worker to multiple other browsing contexts or workers. Shared workers, on the other hand, are named, and once created any script running in the same origin can obtain a reference to that worker and communicate with it. Service Workers defines a third kind. [SW]

10.2.1 The global scope

The global scope is the "inside" of a worker.

10.2.1.1 The WorkerGlobalScope common interface

[Exposed=Worker]
interface WorkerGlobalScope : EventTarget {
  readonly attribute WorkerGlobalScope self;
  readonly attribute WorkerLocation location;
  readonly attribute WorkerNavigator navigator;
  undefined importScripts((TrustedScriptURL or USVString)... urls);
  attribute OnErrorEventHandler onerror;
  attribute EventHandler onlanguagechange;
  attribute EventHandler onoffline;
  attribute EventHandler ononline;
  attribute EventHandler onrejectionhandled;
  attribute EventHandler onunhandledrejection;
};

WorkerGlobalScope serves as the base class for specific types of worker global scope objects, including DedicatedWorkerGlobalScope , SharedWorkerGlobalScope , and ServiceWorkerGlobalScope .

A WorkerGlobalScope object has an associated owner set (a set of Document and WorkerGlobalScope objects). It is initially empty and populated when the worker is created or obtained.

It is a set , instead of a single owner, to accommodate SharedWorkerGlobalScope objects.

A WorkerGlobalScope object has an associated type (" classic " or " module "). It is set during creation.

A WorkerGlobalScope object has an associated url (null or a URL ). It is initially null.

A WorkerGlobalScope object has an associated name (a string). It is set during creation.

The name can have different semantics for each subclass of WorkerGlobalScope . For DedicatedWorkerGlobalScope instances, it is simply a developer-supplied name, useful mostly for debugging purposes. For SharedWorkerGlobalScope instances, it allows obtaining a reference to a common shared worker via the SharedWorker() constructor. For ServiceWorkerGlobalScope objects, it doesn't make sense (and as such isn't exposed through the JavaScript API at all).

A WorkerGlobalScope object has an associated policy container (a policy container ). It is initially a new policy container .

A WorkerGlobalScope object has an associated embedder policy (an embedder policy ).

A WorkerGlobalScope object has an associated module map . It is a module map , initially empty.

A WorkerGlobalScope object has an associated cross-origin isolated capability boolean. It is initially false.

workerGlobal . self

✔ MDN

WorkerGlobalScope/self

Support in all current engines.

Firefox 3.5+ Safari 4+ Chrome 4+

---

Opera 11.5+ Edge 79+

---

Edge (Legacy) 12+ Internet Explorer 10+

---

Firefox Android 34+ Safari iOS 5+ Chrome Android ? WebView Android ? Samsung Internet ? Opera Android ?

Returns workerGlobal .

workerGlobal . location

✔ MDN

WorkerGlobalScope/location

Support in all current engines.

Firefox 3.5+ Safari 4+ Chrome 4+

---

Opera 11.5+ Edge 79+

---

Edge (Legacy) 12+ Internet Explorer 10+

---

Firefox Android ? Safari iOS 5+ Chrome Android ? WebView Android ? Samsung Internet ? Opera Android ?

Returns workerGlobal 's WorkerLocation object.

workerGlobal . navigator

✔ MDN

WorkerGlobalScope/navigator

Support in all current engines.

Firefox 3.5+ Safari 4+ Chrome 4+

---

Opera 11.5+ Edge 79+

---

Edge (Legacy) 17+ Internet Explorer 10+

---

Firefox Android ? Safari iOS 5+ Chrome Android ? WebView Android ? Samsung Internet ? Opera Android ?

Returns workerGlobal 's WorkerNavigator object.

workerGlobal . importScripts (... urls )

✔ MDN

WorkerGlobalScope/importScripts

Support in all current engines.

Firefox 4+ Safari 4+ Chrome 4+

---

Opera 10.6+ Edge 79+

---

Edge (Legacy) 12+ Internet Explorer 10+

---

Firefox Android ? Safari iOS 5+ Chrome Android ? WebView Android ? Samsung Internet ? Opera Android 11+

Fetches each URL in urls , executes them one-by-one in the order they are passed, and then returns (or throws if something went amiss).

The self attribute must return the WorkerGlobalScope object itself.

The location attribute must return the WorkerLocation object whose associated WorkerGlobalScope object is the WorkerGlobalScope object.

While the WorkerLocation object is created after the WorkerGlobalScope object, this is not problematic as it cannot be observed from script.

The following are the event handlers (and their corresponding event handler event types ) that must be supported, as event handler IDL attributes , by objects implementing the WorkerGlobalScope interface:

10.2.1.2 Dedicated workers and the DedicatedWorkerGlobalScope interface

[Global=(Worker,DedicatedWorker),Exposed=DedicatedWorker]
interface DedicatedWorkerGlobalScope : WorkerGlobalScope {
  [Replaceable] readonly attribute DOMString name;
  undefined postMessage(any message, sequence<object> transfer);
  undefined postMessage(any message, optional StructuredSerializeOptions options = {});
  undefined close();
};

DedicatedWorkerGlobalScope

includes

MessageEventTarget

;

DedicatedWorkerGlobalScope objects have an associated inside port (a MessagePort ). This port is part of a channel that is set up when the worker is created, but it is not exposed. This object must never be garbage collected before the DedicatedWorkerGlobalScope object.

dedicatedWorkerGlobal . name

✔ MDN

DedicatedWorkerGlobalScope/name

Support in all current engines.

Firefox 55+ Safari 12.1+ Chrome 70+

---

Opera ? Edge 79+

---

Edge (Legacy) 18 Internet Explorer No

---

Firefox Android ? Safari iOS ? Chrome Android ? WebView Android ? Samsung Internet ? Opera Android ?

Returns dedicatedWorkerGlobal 's name , i.e. the value given to the Worker constructor. Primarily useful for debugging.

dedicatedWorkerGlobal . postMessage ( message [, transfer ])

✔ MDN

DedicatedWorkerGlobalScope/postMessage

Support in all current engines.

Firefox 3.5+ Safari 4+ Chrome 4+

---

Opera 10.6+ Edge 79+

---

Edge (Legacy) 12+ Internet Explorer 10+

---

Firefox Android ? Safari iOS 5+ Chrome Android ? WebView Android ? Samsung Internet ? Opera Android 11+

dedicatedWorkerGlobal . postMessage ( message [, { transfer } ])

Clones message and transmits it to the Worker object associated with dedicatedWorkerGlobal . transfer can be passed as a list of objects that are to be transferred rather than cloned.

dedicatedWorkerGlobal . close ()

✔ MDN

DedicatedWorkerGlobalScope/close

Support in all current engines.

Firefox 3.5+ Safari 4+ Chrome 4+

---

Opera 10.6+ Edge 79+

---

Edge (Legacy) 12+ Internet Explorer 10+

---

Firefox Android ? Safari iOS 5+ Chrome Android ? WebView Android ? Samsung Internet ? Opera Android 11+

Aborts dedicatedWorkerGlobal .

The name getter steps are to return this 's name . Its value represents the name given to the worker using the Worker constructor, used primarily for debugging purposes.

The postMessage( message , transfer ) and postMessage( message , options ) methods on DedicatedWorkerGlobalScope objects act as if, when invoked, it immediately invoked the respective postMessage( message , transfer ) and postMessage( message , options ) on the port, with the same arguments, and returned the same return value.

To close a worker , given a workerGlobal , run these steps:

1. Discard any tasks that have been added to workerGlobal 's relevant agent 's event loop 's task queues .

2. Set workerGlobal 's closing flag to true. (This prevents any further tasks from being queued.)

The close() method steps are to close a worker given this .

10.2.1.3 Shared workers and the SharedWorkerGlobalScope interface

[Global=(Worker,SharedWorker),Exposed=SharedWorker]
interface SharedWorkerGlobalScope : WorkerGlobalScope {
  [Replaceable] readonly attribute DOMString name;
  undefined close();
  attribute EventHandler onconnect;
};

A SharedWorkerGlobalScope object has an associated constructor origin , constructor url , and credentials . They are initialized when the SharedWorkerGlobalScope object is created, in the run a worker algorithm.

Shared workers receive message ports through connect events on their SharedWorkerGlobalScope object for each connection.

sharedWorkerGlobal . name

✔ MDN

SharedWorkerGlobalScope/name

Support in all current engines.

Firefox 29+ Safari 16+ Chrome 4+

---

Opera 10.6+ Edge 79+

---

Edge (Legacy) ? Internet Explorer No

---

Firefox Android ? Safari iOS 16+ Chrome Android ? WebView Android ? Samsung Internet ? Opera Android 11+

Returns sharedWorkerGlobal 's name , i.e. the value given to the SharedWorker constructor. Multiple SharedWorker objects can correspond to the same shared worker (and SharedWorkerGlobalScope ), by reusing the same name.

sharedWorkerGlobal . close ()

✔ MDN

SharedWorkerGlobalScope/close

Support in all current engines.

Firefox 29+ Safari 16+ Chrome 4+

---

Opera 10.6+ Edge 79+

---

Edge (Legacy) ? Internet Explorer No

---

Firefox Android ? Safari iOS 16+ Chrome Android ? WebView Android ? Samsung Internet ? Opera Android 11+

Aborts sharedWorkerGlobal .

The name getter steps are to return this 's name . Its value represents the name that can be used to obtain a reference to the worker using the SharedWorker constructor.

The close() method steps are to close a worker given this .

The following are the event handlers (and their corresponding event handler event types ) that must be supported, as event handler IDL attributes , by objects implementing the SharedWorkerGlobalScope interface:

10.2.2 The event loop

A worker event loop 's task queues only have events, callbacks, and networking activity as tasks . These worker event loops are created by the run a worker algorithm.

Each WorkerGlobalScope object has a closing flag, which must be initially false, but which can get set to true by the algorithms in the processing model section below.

Once the WorkerGlobalScope 's closing flag is set to true, the event loop 's task queues must discard any further tasks that would be added to them (tasks already on the queue are unaffected except where otherwise specified). Effectively, once the closing flag is true, timers stop firing, notifications for all pending background operations are dropped, etc.

10.2.3 The worker's lifetime

Workers communicate with other workers and with Window s through message channels and their MessagePort objects.

Each WorkerGlobalScope object worker global scope has a list of the worker's ports , which consists of all the MessagePort objects that are entangled with another port and that have one (but only one) port owned by worker global scope . This list includes the implicit MessagePort in the case of dedicated workers .

Given an environment settings object o when creating or obtaining a worker, the relevant owner to add depends on the type of global object specified by o . If o 's global object is a WorkerGlobalScope object (i.e., if we are creating a nested dedicated worker), then the relevant owner is that global object. Otherwise, o 's global object is a Window object, and the relevant owner is that Window 's associated Document .

A worker is said to be a permissible worker if its WorkerGlobalScope 's owner set is not empty or:

• its owner set has been empty for no more than a short implementation-defined timeout value,
• its WorkerGlobalScope object is a SharedWorkerGlobalScope object (i.e., the worker is a shared worker), and
• the user agent has a navigable whose active document is not completely loaded .

The second part of this definition allows a shared worker to survive for a short time while a page is loading, in case that page is going to contact the shared worker again. This can be used by user agents as a way to avoid the cost of restarting a shared worker used by a site when the user is navigating from page to page within that site.

A worker is said to be an active needed worker if any of its owners are either Document objects that are fully active or active needed workers .

A worker is said to be a protected worker if it is an active needed worker and either it has outstanding timers, database transactions, or network connections, or its list of the worker's ports is not empty, or its WorkerGlobalScope is actually a SharedWorkerGlobalScope object (i.e., the worker is a shared worker).

A worker is said to be a suspendable worker if it is not an active needed worker but it is a permissible worker .

10.2.4 Processing model

When a user agent is to run a worker for a script with Worker or SharedWorker object worker , URL url , environment settings object outside settings , MessagePort outside port , and a WorkerOptions dictionary options , it must run the following steps.

1. Let is shared be true if worker is a SharedWorker object, and false otherwise.

2. Let owner be the relevant owner to add given outside settings .

3. Let unsafeWorkerCreationTime be the unsafe shared current time .

4. Let agent be the result of obtaining a dedicated/shared worker agent given outside settings and is shared . Run the rest of these steps in that agent.

5. Let realm execution context be the result of creating a new realm given agent and the following customizations:

   • For the global object, if is shared is true, create a new SharedWorkerGlobalScope object. Otherwise, create a new DedicatedWorkerGlobalScope object.

6. Let worker global scope be the global object of realm execution context 's Realm component.

   This is the DedicatedWorkerGlobalScope or SharedWorkerGlobalScope object created in the previous step.

7. Set up a worker environment settings object with realm execution context , outside settings , and unsafeWorkerCreationTime , and let inside settings be the result.

8. Set worker global scope 's name to the value of options 's name member.

9. Append owner to worker global scope 's owner set .

10. If is shared is true, then:

    1. Set worker global scope 's constructor origin to outside settings 's origin .

    2. Set worker global scope 's constructor url to url .

    3. Set worker global scope 's type to the value of options 's type member.

    4. Set worker global scope 's credentials to the value of options 's credentials member.

11. Let destination be " sharedworker " if is shared is true, and " worker " otherwise.

12. Obtain script by switching on the value of options 's type member:

    " classic "

    Fetch a classic worker script given url , outside settings , destination , inside settings , and with onComplete and performFetch as defined below.

    " module "

    Fetch a module worker script graph given url , outside settings , destination , the value of the credentials member of options , inside settings , and with onComplete and performFetch as defined below.

    In both cases, let performFetch be the following perform the fetch hook given request , isTopLevel and processCustomFetchResponse :

    1. If isTopLevel is false, fetch request with processResponseConsumeBody set to processCustomFetchResponse , and abort these steps.

    2. Set request 's reserved client to inside settings .

    3. Fetch request with processResponseConsumeBody set to the following steps given response response and null, failure, or a byte sequence bodyBytes :

       1. Set worker global scope 's url to response 's url .

       2. Initialize worker global scope's policy container given worker global scope , response , and inside settings .

       3. If the Run CSP initialization for a global object algorithm returns " Blocked " when executed upon worker global scope , set response to a network error . [CSP]

       4. If worker global scope 's embedder policy 's value is compatible with cross-origin isolation and is shared is true, then set agent 's agent cluster 's cross-origin isolation mode to " logical " or " concrete ". The one chosen is implementation-defined .

          This really ought to be set when the agent cluster is created, which requires a redesign of this section.

       5. If the result of checking a global object's embedder policy with worker global scope , outside settings , and response is false, then set response to a network error .

       6. Set worker global scope 's cross-origin isolated capability to true if agent 's agent cluster 's cross-origin isolation mode is " concrete ".

       7. If is shared is false and owner 's cross-origin isolated capability is false, then set worker global scope 's cross-origin isolated capability to false.

       8. If is shared is false and response 's url 's scheme is " data ", then set worker global scope 's cross-origin isolated capability to false.

          This is a conservative default for now, while we figure out how workers in general, and data: URL workers in particular (which are cross-origin from their owner), will be treated in the context of permissions policies. See w3c/webappsec-permissions-policy issue #207 for more details.

       9. Run processCustomFetchResponse with response and bodyBytes .

    In both cases, let onComplete given script be the following steps:

    1. If script is null or if script 's error to rethrow is non-null, then:

       1. Queue a global task on the DOM manipulation task source given worker 's relevant global object to fire an event named error at worker .

       2. Run the environment discarding steps for inside settings .

       3. Abort these steps.

    2. Associate worker with worker global scope .

    3. Let inside port be a new MessagePort object in inside settings 's realm .

    4. If is shared is false, then:

       1. Set inside port 's message event target to worker global scope .

       2. Set worker global scope 's inside port to inside port .

    5. Entangle outside port and inside port .

    6. Create a new WorkerLocation object and associate it with worker global scope .

    7. Closing orphan workers : Start monitoring the worker such that no sooner than it stops being a protected worker , and no later than it stops being a permissible worker , worker global scope 's closing flag is set to true.

    8. Suspending workers : Start monitoring the worker, such that whenever worker global scope 's closing flag is false and the worker is a suspendable worker , the user agent suspends execution of script in that worker until such time as either the closing flag switches to true or the worker stops being a suspendable worker .

    9. Set inside settings 's execution ready flag .

    10. If script is a classic script , then run the classic script script . Otherwise, it is a module script ; run the module script script .

        In addition to the usual possibilities of returning a value or failing due to an exception, this could be prematurely aborted by the terminate a worker algorithm defined below.

    11. Enable outside port 's port message queue .

    12. If is shared is false, enable the port message queue of the worker's implicit port.

    13. If is shared is true, then queue a global task on DOM manipulation task source given worker global scope to fire an event named connect at worker global scope , using MessageEvent , with the data attribute initialized to the empty string, the ports attribute initialized to a new frozen array containing inside port , and the source attribute initialized to inside port .

    14. Enable the client message queue of the ServiceWorkerContainer object whose associated service worker client is worker global scope 's relevant settings object .

    15. Event loop : Run the responsible event loop specified by inside settings until it is destroyed.

        The handling of events or the execution of callbacks by tasks run by the event loop might get prematurely aborted by the terminate a worker algorithm defined below.

        The worker processing model remains on this step until the event loop is destroyed, which happens after the closing flag is set to true, as described in the event loop processing model.

    16. Clear the worker global scope 's map of active timers .

    17. Disentangle all the ports in the list of the worker's ports .

    18. Empty worker global scope 's owner set .

When a user agent is to terminate a worker it must run the following steps in parallel with the worker's main loop (the " run a worker " processing model defined above):

1. Set the worker's WorkerGlobalScope object's closing flag to true.

2. If there are any tasks queued in the WorkerGlobalScope object's relevant agent 's event loop 's task queues , discard them without processing them.

3. Abort the script currently running in the worker.

4. If the worker's WorkerGlobalScope object is actually a DedicatedWorkerGlobalScope object (i.e. the worker is a dedicated worker), then empty the port message queue of the port that the worker's implicit port is entangled with.

User agents may invoke the terminate a worker algorithm when a worker stops being an active needed worker and the worker continues executing even after its closing flag was set to true.

10.2.5 Runtime script errors

Whenever an uncaught runtime script error occurs in one of the worker's scripts, if the error did not occur while handling a previous script error, the user agent will report it for the worker's WorkerGlobalScope object.

10.2.6 Creating workers

10.2.6.1 The AbstractWorker mixin

interface mixin AbstractWorker {
  attribute EventHandler onerror;
};

The following are the event handlers (and their corresponding event handler event types ) that must be supported, as event handler IDL attributes , by objects implementing the AbstractWorker interface:

10.2.6.2 Script settings for workers

To set up a worker environment settings object , given a JavaScript execution context execution context , an environment settings object outside settings , and a number unsafeWorkerCreationTime :

1. Let inherited origin be outside settings 's origin .

2. Let realm be the value of execution context 's Realm component.

3. Let worker global scope be realm 's global object .

4. Let settings object be a new environment settings object whose algorithms are defined as follows:

   The realm execution context

   Return execution context .

   The module map

   Return worker global scope 's module map .

   The API base URL

   Return worker global scope 's url .

   The origin

   Return a unique opaque origin if worker global scope 's url 's scheme is " data ", and inherited origin otherwise.

   The policy container

   Return worker global scope 's policy container .

   The cross-origin isolated capability

   Return worker global scope 's cross-origin isolated capability .

   The time origin

   Return the result of coarsening unsafeWorkerCreationTime with worker global scope 's cross-origin isolated capability .

5. Set settings object 's id to a new unique opaque string, creation URL to worker global scope 's url , top-level creation URL to null, target browsing context to null, and active service worker to null. null, and cross-site ancestor flag to outside settings 's cross-site ancestor flag .

6. If worker global scope is a DedicatedWorkerGlobalScope object, then set settings object 's top-level origin to outside settings 's top-level origin .

7. Otherwise, set settings object 's top-level origin to an implementation-defined value.

   See Client-Side Storage Partitioning for the latest on properly defining this.

8. Set realm 's [[HostDefined]] field to settings object .

9. Return settings object .

10.2.6.3 Dedicated workers and the Worker interface

[Exposed=(Window,DedicatedWorker,SharedWorker)]
interface Worker : EventTarget {
  constructor((TrustedScriptURL or USVString) scriptURL, optional WorkerOptions options = {});
  undefined terminate();
  undefined postMessage(any message, sequence<object> transfer);
  undefined postMessage(any message, optional StructuredSerializeOptions options = {});
};
dictionary WorkerOptions {
  WorkerType type = "classic";
  RequestCredentials credentials = "same-origin"; // credentials is only used if type is "module"
  DOMString name = "";
};
enum WorkerType { "classic", "module" };
Worker includes AbstractWorker;

Worker

includes

MessageEventTarget

;

worker = new Worker ( scriptURL [, options ])

✔ MDN

Worker/Worker

Support in all current engines.

Firefox 3.5+ Safari 4+ Chrome 4+

---

Opera 10.6+ Edge 79+

---

Edge (Legacy) 12+ Internet Explorer 10+

---

Firefox Android ? Safari iOS 5+ Chrome Android ? WebView Android ? Samsung Internet ? Opera Android 11+

Returns a new Worker object. scriptURL will be fetched and executed in the background, creating a new global environment for which worker represents the communication channel. options can be used to define the name of that global environment via the name option, primarily for debugging purposes. It can also ensure this new global environment supports JavaScript modules (specify type: "module" ), and if that is specified, can also be used to specify how scriptURL is fetched through the credentials option.

worker . terminate ()

✔ MDN

Worker/terminate

Support in all current engines.

Firefox 3.5+ Safari 4+ Chrome 2+

---

Opera 10.6+ Edge 79+

---

Edge (Legacy) 12+ Internet Explorer 10+

---

Firefox Android ? Safari iOS 5+ Chrome Android ? WebView Android ? Samsung Internet ? Opera Android 11+

Aborts worker 's associated global environment.

worker . postMessage ( message [, transfer ])

✔ MDN

Worker/postMessage

Support in all current engines.

Firefox 3.5+ Safari 4+ Chrome 2+

---

Opera 10.6+ Edge 79+

---

Edge (Legacy) 12+ Internet Explorer 10+

---

Firefox Android ? Safari iOS 5+ Chrome Android ? WebView Android ? Samsung Internet ? Opera Android 11+

worker . postMessage ( message [, { transfer } ])

Clones message and transmits it to worker 's global environment. transfer can be passed as a list of objects that are to be transferred rather than cloned.

Each Worker object has an associated outside port (a MessagePort ). This port is part of a channel that is set up when the worker is created, but it is not exposed. This object must never be garbage collected before the Worker object.

The terminate() method, when invoked, must cause the terminate a worker algorithm to be run on the worker with which the object is associated.

The postMessage( message , transfer ) and postMessage( message , options ) methods on Worker objects act as if, when invoked, they immediately invoked the respective postMessage( message , transfer ) and postMessage( message , options ) on this 's outside port , with the same arguments, and returned the same return value.

worker.postMessage({opcode:
'activate',
device:
1938,
parameters:
[23,
102]});

When the Worker( scriptURL , options ) constructor is invoked, the user agent must run the following steps:

1. Let compliantScriptURL be the result of invoking the Get Trusted Type compliant string algorithm with TrustedScriptURL , this 's relevant global object , scriptURL , " Worker constructor ", and " script ".

2. Let outside settings be the current settings object .

3. Let worker URL be the result of encoding-parsing a URL given compliantScriptURL , relative to outside settings .

   Any same-origin URL (including blob: URLs) can be used. data: URLs can also be used, but they create a worker with an opaque origin .

4. If worker URL is failure, then throw a " SyntaxError " DOMException .

5. Let worker be a new Worker object.

6. Let outside port be a new MessagePort in outside settings 's realm .

7. Set outside port 's message event target to worker .

8. Set worker 's outside port to outside port .

9. Run this step in parallel :

   1. Run a worker given worker , worker URL , outside settings , outside port , and options .

10. Return worker .

10.2.6.4 Shared workers and the SharedWorker interface

[Exposed=Window]
interface SharedWorker : EventTarget {
  constructor((TrustedScriptURL or USVString) scriptURL, optional (DOMString or WorkerOptions) options = {});
  readonly attribute MessagePort port;
};

SharedWorker

includes

AbstractWorker

;

sharedWorker = new SharedWorker ( scriptURL [, name ])

✔ MDN

SharedWorker/SharedWorker

Support in all current engines.

Firefox 29+ Safari 16+ Chrome 5+

---

Opera 10.6+ Edge 79+

---

Edge (Legacy) ? Internet Explorer No

---

Firefox Android 33+ Safari iOS 16+ Chrome Android No WebView Android ? Samsung Internet 4.0–5.0 Opera Android 11–14

Returns a new SharedWorker object. scriptURL will be fetched and executed in the background, creating a new global environment for which sharedWorker represents the communication channel. name can be used to define the name of that global environment.

sharedWorker = new SharedWorker ( scriptURL [, options ])

Returns a new SharedWorker object. scriptURL will be fetched and executed in the background, creating a new global environment for which sharedWorker represents the communication channel. options can be used to define the name of that global environment via the name option. It can also ensure this new global environment supports JavaScript modules (specify type: "module" ), and if that is specified, can also be used to specify how scriptURL is fetched through the credentials option. Note that attempting to construct a shared worker with options whose type or credentials values mismatch an existing shared worker will cause the returned sharedWorker to fire an error event and not connect to the existing shared worker.

sharedWorker . port

✔ MDN

SharedWorker/port

Support in all current engines.

Firefox 29+ Safari 16+ Chrome 5+

---

Opera 10.6+ Edge 79+

---

Edge (Legacy) ? Internet Explorer No

---

Firefox Android 33+ Safari iOS 16+ Chrome Android No WebView Android ? Samsung Internet 4.0–5.0 Opera Android 11–14

Returns sharedWorker 's MessagePort object which can be used to communicate with the global environment.

The port attribute must return the value it was assigned by the object's constructor. It represents the MessagePort for communicating with the shared worker.

A user agent has an associated shared worker manager which is the result of starting a new parallel queue .

Each user agent has a single shared worker manager for simplicity. Implementations could use one per origin ; that would not be observably different and enables more concurrency.

When the SharedWorker( scriptURL , options ) constructor is invoked:

1. Let compliantScriptURL be the result of invoking the Get Trusted Type compliant string algorithm with TrustedScriptURL , this 's relevant global object , scriptURL , " SharedWorker constructor ", and " script ".

2. If options is a DOMString , set options to a new WorkerOptions dictionary whose name member is set to the value of options and whose other members are set to their default values.

3. Let outside settings be the current settings object .

4. Let urlRecord be the result of encoding-parsing a URL given compliantScriptURL , relative to outside settings .

   Any same-origin URL (including blob: URLs) can be used. data: URLs can also be used, but they create a worker with an opaque origin .

5. If urlRecord is failure, then throw a " SyntaxError " DOMException .

6. Let worker be a new SharedWorker object.

7. Let outside port be a new MessagePort in outside settings 's realm .

8. Assign outside port to the port attribute of worker .

9. Let callerIsSecureContext be true if outside settings is a secure context ; otherwise, false.

10. Let outside storage key be the result of running obtain a storage key for non-storage purposes given outside settings .

11. Enqueue the following steps to the shared worker manager :

    1. Let worker global scope be null.

    2. For each scope in the list of all SharedWorkerGlobalScope objects:

       1. Let worker storage key be the result of running obtain a storage key for non-storage purposes given scope 's relevant settings object .

       2. If all of the following are true:

          • worker storage key equals outside storage key ;
          • scope 's closing flag is false;
          • scope 's constructor url equals urlRecord ; and
          • scope 's name equals the value of options 's name member,

          then:

          1. Set worker global scope to scope .

          2. Break .

       data: URLs create a worker with an opaque origin . Both the constructor origin and constructor url are compared so the same data: URL can be used within an origin to get to the same SharedWorkerGlobalScope object, but cannot be used to bypass the same origin restriction.

    3. If worker global scope is not null, but the user agent has been configured to disallow communication between the worker represented by the worker global scope and the scripts whose settings object is outside settings , then set worker global scope to null.

       For example, a user agent could have a development mode that isolates a particular top-level traversable from all other pages, and scripts in that development mode could be blocked from connecting to shared workers running in the normal browser mode.

    4. If worker global scope is not null, then check if worker global scope 's type and credentials match the options values. If not, queue a task to fire an event named error and abort these steps.

    5. If worker global scope is not null, then run these subsubsteps:

       1. Let settings object be the relevant settings object for worker global scope .

       2. Let workerIsSecureContext be true if settings object is a secure context ; otherwise, false.

       3. If workerIsSecureContext is not callerIsSecureContext , then queue a task to fire an event named error at worker and abort these steps. [SECURE-CONTEXTS]

       4. Associate worker with worker global scope .

       5. Let inside port be a new MessagePort in settings object 's realm .

       6. Entangle outside port and inside port .

       7. Queue a task , using the DOM manipulation task source , to fire an event named connect at worker global scope , using MessageEvent , with the data attribute initialized to the empty string, the ports attribute initialized to a new frozen array containing only inside port , and the source attribute initialized to inside port .

       8. Append the relevant owner to add given outside settings to worker global scope 's owner set .

    6. Otherwise, in parallel , run a worker given worker , urlRecord , outside settings , outside port , and options .

12. Return worker .

10.2.7 Concurrent hardware capabilities

interface mixin NavigatorConcurrentHardware {
  readonly attribute unsigned long long hardwareConcurrency;
};

self . navigator . hardwareConcurrency

MDN

Navigator/hardwareConcurrency

Firefox 48+ Safari 10.1–11 Chrome 37+

---

Opera ? Edge 79+

---

Edge (Legacy) 15+ Internet Explorer No

---

Firefox Android ? Safari iOS ? Chrome Android ? WebView Android ? Samsung Internet ? Opera Android ?

Navigator/hardwareConcurrency

Firefox 48+ Safari 10.1–11 Chrome 37+

---

Opera ? Edge 79+

---

Edge (Legacy) 15+ Internet Explorer No

---

Firefox Android ? Safari iOS ? Chrome Android ? WebView Android ? Samsung Internet ? Opera Android ?

Returns the number of logical processors potentially available to the user agent.

The navigator.hardwareConcurrency attribute's getter must return a number between 1 and the number of logical processors potentially available to the user agent. If this cannot be determined, the getter must return 1.

User agents should err toward exposing the number of logical processors available, using lower values only in cases where there are user-agent specific limits in place (such as a limitation on the number of workers that can be created) or when the user agent desires to limit fingerprinting possibilities.

10.3 APIs available to workers

10.3.1 Importing scripts and libraries

The importScripts(... urls ) method steps are:

1. Let urlStrings be « ».

2. For each url of urls :

   1. Append the result of invoking the Get Trusted Type compliant string algorithm with TrustedScriptURL , this 's relevant global object , url , " WorkerGlobalScope importScripts ", and " script " to urlStrings .

3. Import scripts into worker global scope given this and urlStrings .

To import scripts into worker global scope , given a WorkerGlobalScope object worker global scope , a list of scalar value strings urls , and an optional perform the fetch hook performFetch :

1. If worker global scope 's type is " module ", throw a TypeError exception.

2. Let settings object be the current settings object .

3. If urls is empty, return.

4. Let urlRecords be « ».

5. For each url of urls :

   1. Let urlRecord be the result of encoding-parsing a URL given url , relative to settings object .

   2. If urlRecord is failure, then throw a " SyntaxError " DOMException .

   3. Append urlRecord to urlRecords .

6. For each urlRecord of urlRecords :

   1. Fetch a classic worker-imported script given urlRecord and settings object , passing along performFetch if provided. If this succeeds, let script be the result. Otherwise, rethrow the exception.

   2. Run the classic script script , with the rethrow errors argument set to true.

      script will run until it either returns, fails to parse, fails to catch an exception, or gets prematurely aborted by the terminate a worker algorithm defined above.

      If an exception was thrown or if the script was prematurely aborted , then abort all these steps, letting the exception or aborting continue to be processed by the calling script .

Service Workers is an example of a specification that runs this algorithm with its own perform the fetch hook . [SW]

10.3.2 The WorkerNavigator interface

The navigator attribute of the WorkerGlobalScope interface must return an instance of the WorkerNavigator interface, which represents the identity and state of the user agent (the client):

[Exposed=Worker]
interface WorkerNavigator {};
WorkerNavigator includes NavigatorID;
WorkerNavigator includes NavigatorLanguage;
WorkerNavigator includes NavigatorOnLine;

WorkerNavigator

includes

NavigatorConcurrentHardware

;

10.3.3 The WorkerLocation interface

[Exposed=Worker]
interface WorkerLocation {
  stringifier readonly attribute USVString href;
  readonly attribute USVString origin;
  readonly attribute USVString protocol;
  readonly attribute USVString host;
  readonly attribute USVString hostname;
  readonly attribute USVString port;
  readonly attribute USVString pathname;
  readonly attribute USVString search;
  readonly attribute USVString hash;
};

A WorkerLocation object has an associated WorkerGlobalScope object (a WorkerGlobalScope object).

The href getter steps are to return this 's WorkerGlobalScope object 's url , serialized .

The origin getter steps are to return the serialization of this 's WorkerGlobalScope object 's url 's origin .

The protocol getter steps are to return this 's WorkerGlobalScope object 's url 's scheme , followed by " : ".

The host getter steps are:

1. Let url be this 's WorkerGlobalScope object 's url .

2. If url 's host is null, return the empty string.

3. If url 's port is null, return url 's host , serialized .

4. Return url 's host , serialized , followed by " : " and url 's port , serialized .

The hostname getter steps are:

1. Let host be this 's WorkerGlobalScope object 's url 's host .

2. If host is null, return the empty string.

3. Return host , serialized .

The port getter steps are:

1. Let port be this 's WorkerGlobalScope object 's url 's port .

2. If port is null, return the empty string.

3. Return port , serialized .

The pathname getter steps are to return the result of URL path serializing this 's WorkerGlobalScope object 's url .

The search getter steps are:

1. Let query be this 's WorkerGlobalScope object 's url 's query .

2. If query is either null or the empty string, return the empty string.

3. Return " ? ", followed by query .

The hash getter steps are:

1. Let fragment be this 's WorkerGlobalScope object 's url 's fragment .

2. If fragment is either null or the empty string, return the empty string.

3. Return " # ", followed by fragment .