Node.js worker_threads module | API Reference | Bun

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Node.js worker_threads module | API Reference | BunBuild Docs Reference Guides Blog Discord/node:worker_threads FgetEnvironmentData VisInternalThread VisMainThread FisMarkedAsUntransferable Vlocks FmarkAsUncloneable FmarkAsUntransferable FmoveMessagePortToContext VparentPort FpostMessageToThread FreceiveMessageOnPort VresourceLimits FsetEnvironmentData VSHARE_ENV VthreadId VthreadName CWorker VworkerData

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Build Docs Reference Guides Blog Discord/node:worker_threads FgetEnvironmentData VisInternalThread VisMainThread FisMarkedAsUntransferable Vlocks FmarkAsUncloneable FmarkAsUntransferable FmoveMessagePortToContext VparentPort FpostMessageToThread FreceiveMessageOnPort VresourceLimits FsetEnvironmentData VSHARE_ENV VthreadId VthreadName CWorker VworkerData

Node.js module

worker_threadsThe 'node:worker_threads' module enables multithreaded execution by spawning Worker threads that run JavaScript in isolated contexts.

Features include message passing, transferable objects, SharedArrayBuffer support, and performance measurement, allowing compute-intensive tasks to be offloaded from the main event loop.

Works in Bun

Core worker creation and communication works. Several worker options (stdio, resourceLimits, etc.) and some advanced methods related to transferable objects and heap snapshots are not supported.

{ 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(); }); } ```" data-algolia-static="false" data-algolia-merged="false" data-type="Class">class WorkerThe 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.stderr streams may be redirected by the parent thread.The import { isMainThread } from 'node:worker_threads' variable is set to false.The import { parentPort } from 'node:worker_threads' 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 (unless worker.SHARE_ENV is passed as the env option to the Worker 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 fulfill certain 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 MessagePort s 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 through worker.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.

import assert from 'node:assert'; import { Worker, MessageChannel, MessagePort, isMainThread, parentPort, } from '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(); }); } readonly performance: WorkerPerformanceAn object that can be used to query performance information from a worker instance. Similar to perf_hooks.performance.

readonly resourceLimits?: ResourceLimitsProvides 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.

readonly stderr: ReadableThis is a readable stream which contains data written to process.stderr inside the worker thread. If stderr: true was not passed to the Worker constructor, then data is piped to the parent thread's process.stderr stream.

readonly stdin: null | WritableIf 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.

readonly stdout: ReadableThis is a readable stream which contains data written to process.stdout inside the worker thread. If stdout: true was not passed to the Worker constructor, then data is piped to the parent thread's process.stdout stream.

readonly threadId: numberAn integer identifier for the referenced thread. Inside the worker thread, it is available as import { threadId } from 'node:worker_threads'. This value is unique for each Worker instance inside a single process.

readonly threadName: null | stringA string identifier for the referenced thread or null if the thread is not running. Inside the worker thread, it is available as require('node:worker_threads').threadName.

[Symbol.asyncDispose](): Promisevoid>;Calls worker.terminate() when the dispose scope is exited.

async function example() { await using worker = new Worker('for (;;) {}', { eval: true }); // Worker is automatically terminate when the scope is exited. } [events.captureRejectionSymbol](error: Error,event: string | symbol,...args: any[]): void;The Symbol.for('nodejs.rejection') method is called in case a promise rejection happens when emitting an event and captureRejections is enabled on the emitter. It is possible to use events.captureRejectionSymbol in place of Symbol.for('nodejs.rejection').

import { EventEmitter, captureRejectionSymbol } from 'node:events'; class MyClass extends EventEmitter { constructor() { super({ captureRejections: true }); } [captureRejectionSymbol](err, event, ...args) { console.log('rejection happened for', event, 'with', err, ...args); this.destroy(err); } destroy(err) { // Tear the resource down here. } } addListenerE extends keyof WorkerEventMap>(eventName: E,listener: (...args: WorkerEventMap[E]) => void): this;Alias for emitter.on(eventName, listener).

addListener(eventName: string | symbol,listener: (...args: any[]) => void): this;Alias for emitter.on(eventName, listener).

cpuUsage(prev?: CpuUsage): PromiseCpuUsage>;This method returns a Promise that will resolve to an object identical to process.threadCpuUsage(), or reject with an ERR_WORKER_NOT_RUNNING error if the worker is no longer running. This methods allows the statistics to be observed from outside the actual thread.

emitE extends keyof WorkerEventMap>(eventName: E,...args: WorkerEventMap[E]): boolean;Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.

Returns true if the event had listeners, false otherwise.

import { EventEmitter } from 'node:events'; const myEmitter = new EventEmitter(); // First listener myEmitter.on('event', function firstListener() { console.log('Helloooo! first listener'); }); // Second listener myEmitter.on('event', function secondListener(arg1, arg2) { console.log(`event with parameters ${arg1}, ${arg2} in second listener`); }); // Third listener myEmitter.on('event', function thirdListener(...args) { const parameters = args.join(', '); console.log(`event with parameters ${parameters} in third listener`); }); console.log(myEmitter.listeners('event')); myEmitter.emit('event', 1, 2, 3, 4, 5); // Prints: // [ // [Function: firstListener], // [Function: secondListener], // [Function: thirdListener] // ] // Helloooo! first listener // event with parameters 1, 2 in second listener // event with parameters 1, 2, 3, 4, 5 in third listener emit(eventName: string | symbol,...args: any[]): boolean;Synchronously calls each of the listeners registered for the event named eventName, in the order they were registered, passing the supplied arguments to each.

Returns true if the event had listeners, false otherwise.

import { EventEmitter } from 'node:events'; const myEmitter = new EventEmitter(); // First listener myEmitter.on('event', function firstListener() { console.log('Helloooo! first listener'); }); // Second listener myEmitter.on('event', function secondListener(arg1, arg2) { console.log(`event with parameters ${arg1}, ${arg2} in second listener`); }); // Third listener myEmitter.on('event', function thirdListener(...args) { const parameters = args.join(', '); console.log(`event with parameters ${parameters} in third listener`); }); console.log(myEmitter.listeners('event')); myEmitter.emit('event', 1, 2, 3, 4, 5); // Prints: // [ // [Function: firstListener], // [Function: secondListener], // [Function: thirdListener] // ] // Helloooo! first listener // event with parameters 1, 2 in second listener // event with parameters 1, 2, 3, 4, 5 in third listener {}); myEE.on('bar', () => {}); const sym = Symbol('symbol'); myEE.on(sym, () => {}); console.log(myEE.eventNames()); // Prints: [ 'foo', 'bar', Symbol(symbol) ] ```" data-algolia-static="false" data-algolia-merged="false" data-type="Method">eventNames(): string | symbol[];Returns an array listing the events for which the emitter has registered listeners.

import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.on('foo', () => {}); myEE.on('bar', () => {}); const sym = Symbol('symbol'); myEE.on(sym, () => {}); console.log(myEE.eventNames()); // Prints: [ 'foo', 'bar', Symbol(symbol) ] getHeapSnapshot(): PromiseReadable>;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.

@returnsA promise for a Readable Stream containing a V8 heap snapshot

getHeapStatistics(): PromiseHeapInfo>;This method returns a Promise that will resolve to an object identical to v8.getHeapStatistics(), or reject with an ERR_WORKER_NOT_RUNNING error if the worker is no longer running. This methods allows the statistics to be observed from outside the actual thread.

getMaxListeners(): number;Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to events.defaultMaxListeners.

listenerCountE extends keyof WorkerEventMap>(eventName: E,listener?: (...args: WorkerEventMap[E]) => void): number;Returns the number of listeners listening for the event named eventName. If listener is provided, it will return how many times the listener is found in the list of the listeners of the event.

@param eventNameThe name of the event being listened for

@param listenerThe event handler function

listenerCount(eventName: string | symbol,listener?: (...args: any[]) => void): number;Returns the number of listeners listening for the event named eventName. If listener is provided, it will return how many times the listener is found in the list of the listeners of the event.

@param eventNameThe name of the event being listened for

@param listenerThe event handler function

{ console.log('someone connected!'); }); console.log(util.inspect(server.listeners('connection'))); // Prints: [ [Function] ] ```" data-algolia-static="false" data-algolia-merged="false" data-type="Method">listenersE extends keyof WorkerEventMap>(eventName: E): (...args: WorkerEventMap[E]) => void[];Returns a copy of the array of listeners for the event named eventName.

server.on('connection', (stream) => { console.log('someone connected!'); }); console.log(util.inspect(server.listeners('connection'))); // Prints: [ [Function] ] listeners(eventName: string | symbol): (...args: any[]) => void[];Returns a copy of the array of listeners for the event named eventName.

server.on('connection', (stream) => { console.log('someone connected!'); }); console.log(util.inspect(server.listeners('connection'))); // Prints: [ [Function] ] offE extends keyof WorkerEventMap>(eventName: E,listener: (...args: WorkerEventMap[E]) => void): this;Alias for emitter.removeListener().

off(eventName: string | symbol,listener: (...args: any[]) => void): this;Alias for emitter.removeListener().

{ console.log('someone connected!'); }); ``` Returns a reference to the `EventEmitter`, so that calls can be chained. By default, event listeners are invoked in the order they are added. The `emitter.prependListener()` method can be used as an alternative to add the event listener to the beginning of the listeners array. ```js import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.on('foo', () => console.log('a')); myEE.prependListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a ```" data-algolia-static="false" data-algolia-merged="false" data-type="Method">onE extends keyof WorkerEventMap>(eventName: E,listener: (...args: WorkerEventMap[E]) => void): this;Adds the listener function to the end of the listeners array for the event named eventName. No checks are made to see if the listener has already been added. Multiple calls passing the same combination of eventName and listener will result in the listener being added, and called, multiple times.

server.on('connection', (stream) => { console.log('someone connected!'); }); Returns a reference to the EventEmitter, so that calls can be chained.

By default, event listeners are invoked in the order they are added. The emitter.prependListener() method can be used as an alternative to add the event listener to the beginning of the listeners array.

import { EventEmitter } from 'node:events'; const myEE = new EventEmitter(); myEE.on('foo', () => console.log('a')); myEE.prependListener('foo', () => console.log('b')); myEE.emit('foo'); // Prints: // b // a @param eventNameThe name of the event.