Worker

class Worker extends EventEmitter

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.stderr streams 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 (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 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 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 the MessagePorts 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(new URL(import.meta.url));
  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();
  });
}

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?): Worker

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

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 is 1.

This is the final event emitted by any Worker instance.

The 'message' event is emitted when the worker thread has invoked require('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.

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

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

worker.cpuUsage(prev?): Promise

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.

worker.getHeapSnapshot(options?): Promise

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.getHeapStatistics(): Promise

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.

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

performance.eventLoopUtilization(utilization1?, utilization2?): Object

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 example shows how the worker's entire lifetime never accumulates any idle time, but is still able to process messages.

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

if (isMainThread) {
  const worker = new Worker(new URL(import.meta.url));
  setInterval(() => {
    worker.postMessage('hi');
    console.log(worker.performance.eventLoopUtilization());
  }, 100).unref();
} else {
  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);
}

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();
} else {
  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?): void

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

worker.ref(): void

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.

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.startCpuProfile(options?): Promise

Starting a CPU profile then return a Promise that fulfills with an error or an CPUProfileHandle object. This API supports await using syntax.

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

const worker = new Worker(`
  const { parentPort } = require('worker_threads');
  parentPort.on('message', () => {});
  `, { eval: true });

worker.on('online', async () => {
  const handle = await worker.startCpuProfile({ sampleInterval: 1 });
  const profile = await handle.stop();
  console.log(profile);
  worker.terminate();
});

await using example.

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

const w = new Worker(`
  const { parentPort } = require('node:worker_threads');
  parentPort.on('message', () => {});
  `, { eval: true });

w.on('online', async () => {
  // Stop profile automatically when return and profile will be discarded
  await using handle = await w.startCpuProfile();
});

worker.startHeapProfile(options?): Promise

Starting a Heap profile then return a Promise that fulfills with an error or an HeapProfileHandle object. This API supports await using syntax.

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

const worker = new Worker(`
  const { parentPort } = require('worker_threads');
  parentPort.on('message', () => {});
  `, { eval: true });

worker.on('online', async () => {
  const handle = await worker.startHeapProfile();
  const profile = await handle.stop();
  console.log(profile);
  worker.terminate();
});

import { Worker } from 'node:worker_threads';

const worker = new Worker(`
  const { parentPort } = require('node:worker_threads');
  parentPort.on('message', () => {});
  `, { eval: true });

worker.on('online', async () => {
  const handle = await worker.startHeapProfile();
  const profile = await handle.stop();
  console.log(profile);
  worker.terminate();
});

await using example.

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

const w = new Worker(`
  const { parentPort } = require('node:worker_threads');
  parentPort.on('message', () => {});
  `, { eval: true });

w.on('online', async () => {
  // Stop profile automatically when return and profile will be discarded
  await using handle = await w.startHeapProfile();
});

import { Worker } from 'node:worker_threads';

const w = new Worker(`
  const { parentPort } = require('node:worker_threads');
  parentPort.on('message', () => {});
  `, { eval: true });

w.on('online', async () => {
  // Stop profile automatically when return and profile will be discarded
  await using handle = await w.startHeapProfile();
});

This 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.

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.

This 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.

worker.terminate(): Promise

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.

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.

A 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.

worker.unref(): void

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 calling unref() again has no effect.

worker[Symbol.asyncDispose](): void

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.
}