Worker threads | Node.js v23.11.0 Documentation (original) (raw)

Source Code: lib/worker_threads.js

The node:worker_threads module enables the use of threads that execute JavaScript in parallel. To access it:

const worker = require('node:worker_threads');

Workers (threads) are useful for performing CPU-intensive JavaScript operations. They do not help much with I/O-intensive work. The Node.js built-in asynchronous I/O operations are more efficient than Workers can be.

Unlike child_process or cluster, worker_threads can share memory. They do so by transferring ArrayBuffer instances or sharing SharedArrayBufferinstances.

`` const { Worker, isMainThread, parentPort, workerData, } = require('node:worker_threads');

if (isMainThread) { module.exports = function parseJSAsync(script) { return new Promise((resolve, reject) => { const worker = new Worker(__filename, { workerData: script, }); worker.on('message', resolve); worker.on('error', reject); worker.on('exit', (code) => { if (code !== 0) reject(new Error(Worker stopped with exit code ${code})); }); }); }; } else { const { parse } = require('some-js-parsing-library'); const script = workerData; parentPort.postMessage(parse(script)); } ``

The above example spawns a Worker thread for each parseJSAsync() call. In practice, use a pool of Workers for these kinds of tasks. Otherwise, the overhead of creating Workers would likely exceed their benefit.

When implementing a worker pool, use the AsyncResource API to inform diagnostic tools (e.g. to provide asynchronous stack traces) about the correlation between tasks and their outcomes. See"Using AsyncResource for a Worker thread pool"in the async_hooks documentation for an example implementation.

Worker threads inherit non-process-specific options by default. Refer toWorker constructor options to know how to customize worker thread options, specifically argv and execArgv options.

worker.getEnvironmentData(key)#

• key Any arbitrary, cloneable JavaScript value that can be used as a key.
• Returns:

Within a worker thread, worker.getEnvironmentData() returns a clone of data passed to the spawning thread's worker.setEnvironmentData(). Every new Worker receives its own copy of the environment data automatically.

`const { Worker, isMainThread, setEnvironmentData, getEnvironmentData, } = require('node:worker_threads');

if (isMainThread) { setEnvironmentData('Hello', 'World!'); const worker = new Worker(__filename); } else { console.log(getEnvironmentData('Hello')); // Prints 'World!'. }`

worker.isInternalThread#

Added in: v23.7.0

Is true if this code is running inside of an internal Worker thread (e.g the loader thread).

node --experimental-loader ./loader.js main.js

// loader.js const { isInternalThread } = require('node:worker_threads'); console.log(isInternalThread); // true // loader.js import { isInternalThread } from 'node:worker_threads'; console.log(isInternalThread); // true

// main.js const { isInternalThread } = require('node:worker_threads'); console.log(isInternalThread); // false // main.js import { isInternalThread } from 'node:worker_threads'; console.log(isInternalThread); // false

worker.isMainThread#

Added in: v10.5.0

Is true if this code is not running inside of a Worker thread.

`const { Worker, isMainThread } = require('node:worker_threads');

if (isMainThread) { // This re-loads the current file inside a Worker instance. new Worker(__filename); } else { console.log('Inside Worker!'); console.log(isMainThread); // Prints 'false'. }`

worker.markAsUntransferable(object)#

Added in: v14.5.0, v12.19.0

• object Any arbitrary JavaScript value.

Mark an object as not transferable. If object occurs in the transfer list of a port.postMessage() call, an error is thrown. This is a no-op ifobject is a primitive value.

In particular, this makes sense for objects that can be cloned, rather than transferred, and which are used by other objects on the sending side. For example, Node.js marks the ArrayBuffers it uses for itsBuffer pool with this.

This operation cannot be undone.

`` const { MessageChannel, markAsUntransferable } = require('node:worker_threads');

const pooledBuffer = new ArrayBuffer(8); const typedArray1 = new Uint8Array(pooledBuffer); const typedArray2 = new Float64Array(pooledBuffer);

markAsUntransferable(pooledBuffer);

const { port1 } = new MessageChannel(); try { // This will throw an error, because pooledBuffer is not transferable. port1.postMessage(typedArray1, [ typedArray1.buffer ]); } catch (error) { // error.name === 'DataCloneError' }

// The following line prints the contents of typedArray1 -- it still owns // its memory and has not been transferred. Without // markAsUntransferable(), this would print an empty Uint8Array and the // postMessage call would have succeeded. // typedArray2 is intact as well. console.log(typedArray1); console.log(typedArray2); ``

There is no equivalent to this API in browsers.

worker.isMarkedAsUntransferable(object)#

Added in: v21.0.0

• object Any JavaScript value.
• Returns:

Check if an object is marked as not transferable withmarkAsUntransferable().

`const { markAsUntransferable, isMarkedAsUntransferable } = require('node:worker_threads');

const pooledBuffer = new ArrayBuffer(8); markAsUntransferable(pooledBuffer);

isMarkedAsUntransferable(pooledBuffer); // Returns true.`

There is no equivalent to this API in browsers.

worker.markAsUncloneable(object)#

Added in: v23.0.0

• object Any arbitrary JavaScript value.

Mark an object as not cloneable. If object is used as message in a port.postMessage() call, an error is thrown. This is a no-op if object is a primitive value.

This has no effect on ArrayBuffer, or any Buffer like objects.

This operation cannot be undone.

`const { markAsUncloneable } = require('node:worker_threads');

const anyObject = { foo: 'bar' }; markAsUncloneable(anyObject); const { port1 } = new MessageChannel(); try { // This will throw an error, because anyObject is not cloneable. port1.postMessage(anyObject); } catch (error) { // error.name === 'DataCloneError' }`

There is no equivalent to this API in browsers.

worker.moveMessagePortToContext(port, contextifiedSandbox)#

Added in: v11.13.0

• port The message port to transfer.
• contextifiedSandbox A contextified object as returned by thevm.createContext() method.
• Returns:

Transfer a MessagePort to a different vm Context. The original portobject is rendered unusable, and the returned MessagePort instance takes its place.

The returned MessagePort is an object in the target context and inherits from its global Object class. Objects passed to theport.onmessage() listener are also created in the target context and inherit from its global Object class.

However, the created MessagePort no longer inherits from, and only port.onmessage() can be used to receive events using it.

worker.parentPort#

Added in: v10.5.0

• |

If this thread is a Worker, this is a MessagePortallowing communication with the parent thread. Messages sent usingparentPort.postMessage() are available in the parent thread using worker.on('message'), and messages sent from the parent thread using worker.postMessage() are available in this thread usingparentPort.on('message').

`const { Worker, isMainThread, parentPort } = require('node:worker_threads');

if (isMainThread) { const worker = new Worker(__filename); worker.once('message', (message) => { console.log(message); // Prints 'Hello, world!'. }); worker.postMessage('Hello, world!'); } else { // When a message from the parent thread is received, send it back: parentPort.once('message', (message) => { parentPort.postMessage(message); }); }`

worker.postMessageToThread(threadId, value[, transferList][, timeout])#

Added in: v22.5.0

• threadId The target thread ID. If the thread ID is invalid, aERR_WORKER_MESSAGING_FAILED error will be thrown. If the target thread ID is the current thread ID, a ERR_WORKER_MESSAGING_SAME_THREAD error will be thrown.
• value The value to send.
• transferList <Object[]> If one or more MessagePort-like objects are passed in value, a transferList is required for those items or ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST is thrown. See port.postMessage() for more information.
• timeout Time to wait for the message to be delivered in milliseconds. By default it's undefined, which means wait forever. If the operation times out, a ERR_WORKER_MESSAGING_TIMEOUT error is thrown.
• Returns: A promise which is fulfilled if the message was successfully processed by destination thread.

Sends a value to another worker, identified by its thread ID.

If the target thread has no listener for the workerMessage event, then the operation will throw a ERR_WORKER_MESSAGING_FAILED error.

If the target thread threw an error while processing the workerMessage event, then the operation will throw a ERR_WORKER_MESSAGING_ERRORED error.

This method should be used when the target thread is not the direct parent or child of the current thread. If the two threads are parent-children, use the require('node:worker_threads').parentPort.postMessage()and the worker.postMessage() to let the threads communicate.

The example below shows the use of of postMessageToThread: it creates 10 nested threads, the last one will try to communicate with the main thread.

`` import { fileURLToPath } from 'node:url'; import process from 'node:process'; import { postMessageToThread, threadId, workerData, Worker, } from 'node:worker_threads';

const channel = new BroadcastChannel('sync'); const level = workerData?.level ?? 0;

if (level < 10) { const worker = new Worker(fileURLToPath(import.meta.url), { workerData: { level: level + 1 }, }); }

if (level === 0) { process.on('workerMessage', (value, source) => { console.log(${source} -> ${threadId}:, value); postMessageToThread(source, { message: 'pong' }); }); } else if (level === 10) { process.on('workerMessage', (value, source) => { console.log(${source} -> ${threadId}:, value); channel.postMessage('done'); channel.close(); });

await postMessageToThread(0, { message: 'ping' }); }

channel.onmessage = channel.close; const { postMessageToThread, threadId, workerData, Worker, } = require('node:worker_threads');

const channel = new BroadcastChannel('sync'); const level = workerData?.level ?? 0;

if (level < 10) { const worker = new Worker(__filename, { workerData: { level: level + 1 }, }); }

if (level === 0) { process.on('workerMessage', (value, source) => { console.log(${source} -> ${threadId}:, value); postMessageToThread(source, { message: 'pong' }); }); } else if (level === 10) { process.on('workerMessage', (value, source) => { console.log(${source} -> ${threadId}:, value); channel.postMessage('done'); channel.close(); });

postMessageToThread(0, { message: 'ping' }); }

channel.onmessage = channel.close; ``

worker.receiveMessageOnPort(port)#

• port |
• Returns: |

Receive a single message from a given MessagePort. If no message is available,undefined is returned, otherwise an object with a single message property that contains the message payload, corresponding to the oldest message in theMessagePort's queue.

`const { MessageChannel, receiveMessageOnPort } = require('node:worker_threads'); const { port1, port2 } = new MessageChannel(); port1.postMessage({ hello: 'world' });

console.log(receiveMessageOnPort(port2)); // Prints: { message: { hello: 'world' } } console.log(receiveMessageOnPort(port2)); // Prints: undefined`

When this function is used, no 'message' event is emitted and theonmessage listener is not invoked.

worker.resourceLimits#

Added in: v13.2.0, v12.16.0

• • maxYoungGenerationSizeMb
  • maxOldGenerationSizeMb
  • codeRangeSizeMb
  • stackSizeMb

Provides the set of JS engine resource constraints inside this Worker thread. If the resourceLimits option was passed to the Worker constructor, this matches its values.

If this is used in the main thread, its value is an empty object.

worker.SHARE_ENV#

Added in: v11.14.0

A special value that can be passed as the env option of the Workerconstructor, to indicate that the current thread and the Worker thread should share read and write access to the same set of environment variables.

const { Worker, SHARE_ENV } = require('node:worker_threads'); new Worker('process.env.SET_IN_WORKER = "foo"', { eval: true, env: SHARE_ENV }) .on('exit', () => { console.log(process.env.SET_IN_WORKER); // Prints 'foo'. });

worker.setEnvironmentData(key[, value])#

• key Any arbitrary, cloneable JavaScript value that can be used as a key.
• value Any arbitrary, cloneable JavaScript value that will be cloned and passed automatically to all new Worker instances. If value is passed as undefined, any previously set value for the key will be deleted.

The worker.setEnvironmentData() API sets the content ofworker.getEnvironmentData() in the current thread and all new Workerinstances spawned from the current context.

worker.threadId#

Added in: v10.5.0

An integer identifier for the current thread. On the corresponding worker object (if there is any), it is available as worker.threadId. This value is unique for each Worker instance inside a single process.

worker.workerData#

Added in: v10.5.0

An arbitrary JavaScript value that contains a clone of the data passed to this thread's Worker constructor.

The data is cloned as if using postMessage(), according to the HTML structured clone algorithm.

`const { Worker, isMainThread, workerData } = require('node:worker_threads');

if (isMainThread) { const worker = new Worker(__filename, { workerData: 'Hello, world!' }); } else { console.log(workerData); // Prints 'Hello, world!'. }`

Class: BroadcastChannel extends EventTarget#

Instances of BroadcastChannel allow asynchronous one-to-many communication with all other BroadcastChannel instances bound to the same channel name.

`'use strict';

const { isMainThread, BroadcastChannel, Worker, } = require('node:worker_threads');

const bc = new BroadcastChannel('hello');

if (isMainThread) { let c = 0; bc.onmessage = (event) => { console.log(event.data); if (++c === 10) bc.close(); }; for (let n = 0; n < 10; n++) new Worker(__filename); } else { bc.postMessage('hello from every worker'); bc.close(); }`

new BroadcastChannel(name)#

Added in: v15.4.0

• name The name of the channel to connect to. Any JavaScript value that can be converted to a string using `${name}` is permitted.

broadcastChannel.close()#

Added in: v15.4.0

Closes the BroadcastChannel connection.

broadcastChannel.onmessage#

Added in: v15.4.0

• Type: Invoked with a single MessageEvent argument when a message is received.

broadcastChannel.onmessageerror#

Added in: v15.4.0

• Type: Invoked with a received message cannot be deserialized.

broadcastChannel.postMessage(message)#

Added in: v15.4.0

• message Any cloneable JavaScript value.

broadcastChannel.ref()#

Added in: v15.4.0

Opposite of unref(). Calling ref() on a previously unref()ed BroadcastChannel does not let the program exit if it's the only active handle left (the default behavior). If the port is ref()ed, calling ref() again has no effect.

broadcastChannel.unref()#

Added in: v15.4.0

Calling unref() on a BroadcastChannel allows the thread to exit if this is the only active handle in the event system. If the BroadcastChannel is already unref()ed calling unref() again has no effect.

Class: MessageChannel#

Added in: v10.5.0

Instances of the worker.MessageChannel class represent an asynchronous, two-way communications channel. The MessageChannel has no methods of its own. new MessageChannel()yields an object with port1 and port2 properties, which refer to linkedMessagePort instances.

`` const { MessageChannel } = require('node:worker_threads');

const { port1, port2 } = new MessageChannel(); port1.on('message', (message) => console.log('received', message)); port2.postMessage({ foo: 'bar' }); // Prints: received { foo: 'bar' } from the port1.on('message') listener ``

Class: MessagePort#

• Extends:

Instances of the worker.MessagePort class represent one end of an asynchronous, two-way communications channel. It can be used to transfer structured data, memory regions and other MessagePorts between differentWorkers.

This implementation matches browser MessagePorts.

Event: 'close'#

Added in: v10.5.0

The 'close' event is emitted once either side of the channel has been disconnected.

`const { MessageChannel } = require('node:worker_threads'); const { port1, port2 } = new MessageChannel();

// Prints: // foobar // closed! port2.on('message', (message) => console.log(message)); port2.on('close', () => console.log('closed!'));

port1.postMessage('foobar'); port1.close();`

Event: 'message'#

Added in: v10.5.0

• value The transmitted value

The 'message' event is emitted for any incoming message, containing the cloned input of port.postMessage().

Listeners on this event receive a clone of the value parameter as passed to postMessage() and no further arguments.

Event: 'messageerror'#

Added in: v14.5.0, v12.19.0

• error An Error object

The 'messageerror' event is emitted when deserializing a message failed.

Currently, this event is emitted when there is an error occurring while instantiating the posted JS object on the receiving end. Such situations are rare, but can happen, for instance, when certain Node.js API objects are received in a vm.Context (where Node.js APIs are currently unavailable).

port.close()#

Added in: v10.5.0

Disables further sending of messages on either side of the connection. This method can be called when no further communication will happen over thisMessagePort.

The 'close' event is emitted on both MessagePort instances that are part of the channel.

port.postMessage(value[, transferList])#

• value
• transferList <Object[]>

Sends a JavaScript value to the receiving side of this channel.value is transferred in a way which is compatible with the HTML structured clone algorithm.

In particular, the significant differences to JSON are:

• value may contain circular references.
• value may contain instances of builtin JS types such as RegExps,BigInts, Maps, Sets, etc.
• value may contain typed arrays, both using ArrayBuffers and SharedArrayBuffers.
• value may contain WebAssembly.Module instances.
• value may not contain native (C++-backed) objects other than:
  • s,
  • s,
  • s,
  • s,
  • s,
  • <net.BlockList>s,
  • <net.SocketAddress>es,
  • s.

`const { MessageChannel } = require('node:worker_threads'); const { port1, port2 } = new MessageChannel();

port1.on('message', (message) => console.log(message));

const circularData = {}; circularData.foo = circularData; // Prints: { foo: [Circular] } port2.postMessage(circularData);`

transferList may be a list of , MessagePort, andFileHandle objects. After transferring, they are not usable on the sending side of the channel anymore (even if they are not contained in value). Unlike withchild processes, transferring handles such as network sockets is currently not supported.

If value contains instances, those are accessible from either thread. They cannot be listed in transferList.

value may still contain ArrayBuffer instances that are not intransferList; in that case, the underlying memory is copied rather than moved.

`` const { MessageChannel } = require('node:worker_threads'); const { port1, port2 } = new MessageChannel();

port1.on('message', (message) => console.log(message));

const uint8Array = new Uint8Array([ 1, 2, 3, 4 ]); // This posts a copy of uint8Array: port2.postMessage(uint8Array); // This does not copy data, but renders uint8Array unusable: port2.postMessage(uint8Array, [ uint8Array.buffer ]);

// The memory for the sharedUint8Array is accessible from both the // original and the copy received by .on('message'): const sharedUint8Array = new Uint8Array(new SharedArrayBuffer(4)); port2.postMessage(sharedUint8Array);

// This transfers a freshly created message port to the receiver. // This can be used, for example, to create communication channels between // multiple Worker threads that are children of the same parent thread. const otherChannel = new MessageChannel(); port2.postMessage({ port: otherChannel.port1 }, [ otherChannel.port1 ]); ``

The message object is cloned immediately, and can be modified after posting without having side effects.

For more information on the serialization and deserialization mechanisms behind this API, see the serialization API of the node:v8 module.

Considerations when transferring TypedArrays and Buffers#

All | instances are views over an underlying. That is, it is the ArrayBuffer that actually stores the raw data while the TypedArray and Buffer objects provide a way of viewing and manipulating the data. It is possible and common for multiple views to be created over the same ArrayBuffer instance. Great care must be taken when using a transfer list to transfer anArrayBuffer as doing so causes all TypedArray and Bufferinstances that share that same ArrayBuffer to become unusable.

`const ab = new ArrayBuffer(10);

const u1 = new Uint8Array(ab); const u2 = new Uint16Array(ab);

console.log(u2.length); // prints 5

port.postMessage(u1, [u1.buffer]);

console.log(u2.length); // prints 0`

For Buffer instances, specifically, whether the underlyingArrayBuffer can be transferred or cloned depends entirely on how instances were created, which often cannot be reliably determined.

An ArrayBuffer can be marked with markAsUntransferable() to indicate that it should always be cloned and never transferred.

Depending on how a Buffer instance was created, it may or may not own its underlying ArrayBuffer. An ArrayBuffer must not be transferred unless it is known that the Buffer instance owns it. In particular, for Buffers created from the internalBuffer pool (using, for instance Buffer.from() or Buffer.allocUnsafe()), transferring them is not possible and they are always cloned, which sends a copy of the entire Buffer pool. This behavior may come with unintended higher memory usage and possible security concerns.

See Buffer.allocUnsafe() for more details on Buffer pooling.

The ArrayBuffers for Buffer instances created usingBuffer.alloc() or Buffer.allocUnsafeSlow() can always be transferred but doing so renders all other existing views of those ArrayBuffers unusable.

Considerations when cloning objects with prototypes, classes, and accessors#

Because object cloning uses the HTML structured clone algorithm, non-enumerable properties, property accessors, and object prototypes are not preserved. In particular, objects will be read as plain s on the receiving side, and instances of JavaScript classes will be cloned as plain JavaScript objects.

`const b = Symbol('b');

class Foo { #a = 1; constructor() { this[b] = 2; this.c = 3; }

get d() { return 4; } }

const { port1, port2 } = new MessageChannel();

port1.onmessage = ({ data }) => console.log(data);

port2.postMessage(new Foo());

// Prints: { c: 3 }`

This limitation extends to many built-in objects, such as the global URLobject:

`const { port1, port2 } = new MessageChannel();

port1.onmessage = ({ data }) => console.log(data);

port2.postMessage(new URL('https://example.org'));

// Prints: { }`

port.hasRef()#

Added in: v18.1.0, v16.17.0

• Returns:

If true, the MessagePort object will keep the Node.js event loop active.

port.ref()#

Added in: v10.5.0

Opposite of unref(). Calling ref() on a previously unref()ed port does_not_ let the program exit if it's the only active handle left (the default behavior). If the port is ref()ed, calling ref() again has no effect.

If listeners are attached or removed using .on('message'), the port is ref()ed and unref()ed automatically depending on whether listeners for the event exist.

port.start()#

Added in: v10.5.0

Starts receiving messages on this MessagePort. When using this port as an event emitter, this is called automatically once 'message'listeners are attached.

This method exists for parity with the Web MessagePort API. In Node.js, it is only useful for ignoring messages when no event listener is present. Node.js also diverges in its handling of .onmessage. Setting it automatically calls .start(), but unsetting it lets messages queue up until a new handler is set or the port is discarded.

port.unref()#

Added in: v10.5.0

Calling unref() on a port allows the thread to exit if this is the only active handle in the event system. If the port is already unref()ed callingunref() again has no effect.

If listeners are attached or removed using .on('message'), the port isref()ed and unref()ed automatically depending on whether listeners for the event exist.

Class: Worker#

Added in: v10.5.0

• Extends:

The Worker class represents an independent JavaScript execution thread. Most Node.js APIs are available inside of it.

Notable differences inside a Worker environment are:

• The process.stdin, process.stdout, and process.stderrstreams may be redirected by the parent thread.
• The require('node:worker_threads').isMainThread property is set to false.
• The require('node:worker_threads').parentPort message port is available.
• process.exit() does not stop the whole program, just the single thread, and process.abort() is not available.
• process.chdir() and process methods that set group or user ids are not available.
• process.env is a copy of the parent thread's environment variables, unless otherwise specified. Changes to one copy are not visible in other threads, and are not visible to native add-ons (unlessworker.SHARE_ENV is passed as the env option to theWorker constructor). On Windows, unlike the main thread, a copy of the environment variables operates in a case-sensitive manner.
• process.title cannot be modified.
• Signals are not delivered through process.on('...').
• Execution may stop at any point as a result of worker.terminate()being invoked.
• IPC channels from parent processes are not accessible.
• The trace_events module is not supported.
• Native add-ons can only be loaded from multiple threads if they fulfillcertain conditions.

Creating Worker instances inside of other Workers is possible.

Like Web Workers and the node:cluster module, two-way communication can be achieved through inter-thread message passing. Internally, a Worker has a built-in pair of MessagePorts that are already associated with each other when the Worker is created. While the MessagePort object on the parent side is not directly exposed, its functionalities are exposed throughworker.postMessage() and the worker.on('message') event on the Worker object for the parent thread.

To create custom messaging channels (which is encouraged over using the default global channel because it facilitates separation of concerns), users can create a MessageChannel object on either thread and pass one of theMessagePorts on that MessageChannel to the other thread through a pre-existing channel, such as the global one.

See port.postMessage() for more information on how messages are passed, and what kind of JavaScript values can be successfully transported through the thread barrier.

const assert = require('node:assert'); const { Worker, MessageChannel, MessagePort, isMainThread, parentPort, } = require('node:worker_threads'); if (isMainThread) { const worker = new Worker(__filename); const subChannel = new MessageChannel(); worker.postMessage({ hereIsYourPort: subChannel.port1 }, [subChannel.port1]); subChannel.port2.on('message', (value) => { console.log('received:', value); }); } else { parentPort.once('message', (value) => { assert(value.hereIsYourPort instanceof MessagePort); value.hereIsYourPort.postMessage('the worker is sending this'); value.hereIsYourPort.close(); }); }

new Worker(filename[, options])#

• filename | The path to the Worker's main script or module. Must be either an absolute path or a relative path (i.e. relative to the current working directory) starting with ./ or ../, or a WHATWG URLobject using file: or data: protocol. When using a data: URL, the data is interpreted based on MIME type using the ECMAScript module loader. If options.eval is true, this is a string containing JavaScript code rather than a path.
• options
  • argv <any[]> List of arguments which would be stringified and appended toprocess.argv in the worker. This is mostly similar to the workerDatabut the values are available on the global process.argv as if they were passed as CLI options to the script.
  • env If set, specifies the initial value of process.env inside the Worker thread. As a special value, worker.SHARE_ENV may be used to specify that the parent thread and the child thread should share their environment variables; in that case, changes to one thread's process.envobject affect the other thread as well. Default: process.env.
  • eval If true and the first argument is a string, interpret the first argument to the constructor as a script that is executed once the worker is online.
  • execArgv <string[]> List of node CLI options passed to the worker. V8 options (such as --max-old-space-size) and options that affect the process (such as --title) are not supported. If set, this is provided as process.execArgv inside the worker. By default, options are inherited from the parent thread.
  • stdin If this is set to true, then worker.stdinprovides a writable stream whose contents appear as process.stdininside the Worker. By default, no data is provided.
  • stdout If this is set to true, then worker.stdout is not automatically piped through to process.stdout in the parent.
  • stderr If this is set to true, then worker.stderr is not automatically piped through to process.stderr in the parent.
  • workerData Any JavaScript value that is cloned and made available as require('node:worker_threads').workerData. The cloning occurs as described in the HTML structured clone algorithm, and an error is thrown if the object cannot be cloned (e.g. because it containsfunctions).
  • trackUnmanagedFds If this is set to true, then the Worker tracks raw file descriptors managed through fs.open() andfs.close(), and closes them when the Worker exits, similar to other resources like network sockets or file descriptors managed through the FileHandle API. This option is automatically inherited by all nested Workers. Default: true.
  • transferList <Object[]> If one or more MessagePort-like objects are passed in workerData, a transferList is required for those items or ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST is thrown. See port.postMessage() for more information.
  • resourceLimits An optional set of resource limits for the new JS engine instance. Reaching these limits leads to termination of the Workerinstance. These limits only affect the JS engine, and no external data, including no ArrayBuffers. Even if these limits are set, the process may still abort if it encounters a global out-of-memory situation.
    * maxOldGenerationSizeMb The maximum size of the main heap in MB. If the command-line argument --max-old-space-size is set, it overrides this setting.
    * maxYoungGenerationSizeMb The maximum size of a heap space for recently created objects. If the command-line argument--max-semi-space-size is set, it overrides this setting.
    * codeRangeSizeMb The size of a pre-allocated memory range used for generated code.
    * stackSizeMb The default maximum stack size for the thread. Small values may lead to unusable Worker instances. Default: 4.
  • name An optional name to be replaced in the thread name and to the worker title for debugging/identification purposes, making the final title as [worker <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mrow><mi>i</mi><mi>d</mi></mrow><mo stretchy="false">]</mo></mrow><annotation encoding="application/x-tex">{id}] </annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord"><span class="mord mathnormal">i</span><span class="mord mathnormal">d</span></span><span class="mclose">]</span></span></span></span>{name}. This parameter has a maximum allowed size, depending on the operating system. If the provided name exceeds the limit, it will be truncated
    * Maximum sizes:
    * Windows: 32,767 characters
    * macOS: 64 characters
    * Linux: 16 characters
    * NetBSD: limited to PTHREAD_MAX_NAMELEN_NP
    * FreeBSD and OpenBSD: limited to MAXCOMLEN Default: 'WorkerThread'.

Event: 'error'#

Added in: v10.5.0

• err

The 'error' event is emitted if the worker thread throws an uncaught exception. In that case, the worker is terminated.

Event: 'exit'#

Added in: v10.5.0

• exitCode

The 'exit' event is emitted once the worker has stopped. If the worker exited by calling process.exit(), the exitCode parameter is the passed exit code. If the worker was terminated, the exitCode parameter is1.

This is the final event emitted by any Worker instance.

Event: 'message'#

Added in: v10.5.0

• value The transmitted value

The 'message' event is emitted when the worker thread has invokedrequire('node:worker_threads').parentPort.postMessage(). See the port.on('message') event for more details.

All messages sent from the worker thread are emitted before the'exit' event is emitted on the Worker object.

Event: 'messageerror'#

Added in: v14.5.0, v12.19.0

• error An Error object

The 'messageerror' event is emitted when deserializing a message failed.

Event: 'online'#

Added in: v10.5.0

The 'online' event is emitted when the worker thread has started executing JavaScript code.

worker.getHeapSnapshot([options])#

• options
  • exposeInternals If true, expose internals in the heap snapshot.Default: false.
  • exposeNumericValues If true, expose numeric values in artificial fields. Default: false.
• Returns: A promise for a Readable Stream containing a V8 heap snapshot

Returns a readable stream for a V8 snapshot of the current state of the Worker. See v8.getHeapSnapshot() for more details.

If the Worker thread is no longer running, which may occur before the'exit' event is emitted, the returned Promise is rejected immediately with an ERR_WORKER_NOT_RUNNING error.

worker.performance#

Added in: v15.1.0, v14.17.0, v12.22.0

An object that can be used to query performance information from a worker instance. Similar to perf_hooks.performance.

performance.eventLoopUtilization([utilization1[, utilization2]])#

Added in: v15.1.0, v14.17.0, v12.22.0

• utilization1 The result of a previous call toeventLoopUtilization().
• utilization2 The result of a previous call toeventLoopUtilization() prior to utilization1.
• Returns:
  • idle
  • active
  • utilization

The same call as perf_hooks eventLoopUtilization(), except the values of the worker instance are returned.

One difference is that, unlike the main thread, bootstrapping within a worker is done within the event loop. So the event loop utilization is immediately available once the worker's script begins execution.

An idle time that does not increase does not indicate that the worker is stuck in bootstrap. The following examples shows how the worker's entire lifetime never accumulates any idle time, but is still be able to process messages.

`const { Worker, isMainThread, parentPort } = require('node:worker_threads');

if (isMainThread) { const worker = new Worker(__filename); setInterval(() => { worker.postMessage('hi'); console.log(worker.performance.eventLoopUtilization()); }, 100).unref(); return; }

parentPort.on('message', () => console.log('msg')).unref(); (function r(n) { if (--n < 0) return; const t = Date.now(); while (Date.now() - t < 300); setImmediate(r, n); })(10);`

The event loop utilization of a worker is available only after the 'online'event emitted, and if called before this, or after the 'exit'event, then all properties have the value of 0.

worker.postMessage(value[, transferList])#

Added in: v10.5.0

• value
• transferList <Object[]>

Send a message to the worker that is received viarequire('node:worker_threads').parentPort.on('message'). See port.postMessage() for more details.

worker.ref()#

Added in: v10.5.0

Opposite of unref(), calling ref() on a previously unref()ed worker does_not_ let the program exit if it's the only active handle left (the default behavior). If the worker is ref()ed, calling ref() again has no effect.

worker.resourceLimits#

Added in: v13.2.0, v12.16.0

• • maxYoungGenerationSizeMb
  • maxOldGenerationSizeMb
  • codeRangeSizeMb
  • stackSizeMb

Provides the set of JS engine resource constraints for this Worker thread. If the resourceLimits option was passed to the Worker constructor, this matches its values.

If the worker has stopped, the return value is an empty object.

worker.stderr#

Added in: v10.5.0

• <stream.Readable>

This is a readable stream which contains data written to process.stderrinside the worker thread. If stderr: true was not passed to theWorker constructor, then data is piped to the parent thread'sprocess.stderr stream.

worker.stdin#

Added in: v10.5.0

• | <stream.Writable>

If stdin: true was passed to the Worker constructor, this is a writable stream. The data written to this stream will be made available in the worker thread as process.stdin.

worker.stdout#

Added in: v10.5.0

• <stream.Readable>

This is a readable stream which contains data written to process.stdoutinside the worker thread. If stdout: true was not passed to theWorker constructor, then data is piped to the parent thread'sprocess.stdout stream.

worker.terminate()#

• Returns:

Stop all JavaScript execution in the worker thread as soon as possible. Returns a Promise for the exit code that is fulfilled when the'exit' event is emitted.

worker.threadId#

Added in: v10.5.0

An integer identifier for the referenced thread. Inside the worker thread, it is available as require('node:worker_threads').threadId. This value is unique for each Worker instance inside a single process.

worker.unref()#

Added in: v10.5.0

Calling unref() on a worker allows the thread to exit if this is the only active handle in the event system. If the worker is already unref()ed callingunref() again has no effect.

Notes#

Synchronous blocking of stdio#

Workers utilize message passing via to implement interactions with stdio. This means that stdio output originating from a Worker can get blocked by synchronous code on the receiving end that is blocking the Node.js event loop.

`import { Worker, isMainThread, } from 'node:worker_threads';

if (isMainThread) { new Worker(new URL(import.meta.url)); for (let n = 0; n < 1e10; n++) { // Looping to simulate work. } } else { // This output will be blocked by the for loop in the main thread. console.log('foo'); } 'use strict';

const { Worker, isMainThread, } = require('node:worker_threads');

if (isMainThread) { new Worker(__filename); for (let n = 0; n < 1e10; n++) { // Looping to simulate work. } } else { // This output will be blocked by the for loop in the main thread. console.log('foo'); }`

Launching worker threads from preload scripts#

Take care when launching worker threads from preload scripts (scripts loaded and run using the -r command line flag). Unless the execArgv option is explicitly set, new Worker threads automatically inherit the command line flags from the running process and will preload the same preload scripts as the main thread. If the preload script unconditionally launches a worker thread, every thread spawned will spawn another until the application crashes.