worker_threads

• class Worker

  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 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: WorkerPerformance

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

  • readonly resourceLimits?: ResourceLimits

    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.

  • readonly stderr: Readable

    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.

  • readonly stdin: null | 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.

  • readonly stdout: Readable

    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.

  • readonly threadId: number

    An 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 | string

    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.

  • [Symbol.asyncDispose](): Promise<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.
    }
    
  • [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.
      }
    }
    
  • addListener<E 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

    ): Promise<CpuUsage>;

    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.
  • emit<E 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
    
  • 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(): Promise<Readable>;

    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.

    @returns

    A promise for a Readable Stream containing a V8 heap snapshot

  • getHeapStatistics(): Promise<HeapInfo>;

    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.

  • listenerCount<E 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 eventName

    The name of the event being listened for

    @param listener

    The 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 eventName

    The name of the event being listened for

    @param listener

    The event handler function

  • listeners<E 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] ]
    
  • off<E 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().
  • on<E 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 eventName

    The name of the event.

    @param listener

    The callback function

    on(

    eventName: string | symbol,

    listener: (...args: any[]) => 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 eventName

    The name of the event.

    @param listener

    The callback function

  • once<E extends keyof WorkerEventMap>(

    eventName: E,

    listener: (...args: WorkerEventMap[E]) => void

    ): this;

    Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

    server.once('connection', (stream) => {
      console.log('Ah, we have our first user!');
    });
    

    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.prependOnceListener() 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.once('foo', () => console.log('a'));
    myEE.prependOnceListener('foo', () => console.log('b'));
    myEE.emit('foo');
    // Prints:
    //   b
    //   a
    

    @param eventName

    The name of the event.

    @param listener

    The callback function

    once(

    eventName: string | symbol,

    listener: (...args: any[]) => void

    ): this;

    Adds a one-time listener function for the event named eventName. The next time eventName is triggered, this listener is removed and then invoked.

    server.once('connection', (stream) => {
      console.log('Ah, we have our first user!');
    });
    

    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.prependOnceListener() 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.once('foo', () => console.log('a'));
    myEE.prependOnceListener('foo', () => console.log('b'));
    myEE.emit('foo');
    // Prints:
    //   b
    //   a
    

    @param eventName

    The name of the event.

    @param listener

    The callback function

  • postMessage(

    value: any,

    transferList?: readonly Transferable[]

    ): void;

    Send a message to the worker that is received via require('node:worker_threads').parentPort.on('message'). See port.postMessage() for more details.
  • prependListener<E extends keyof WorkerEventMap>(

    eventName: E,

    listener: (...args: WorkerEventMap[E]) => void

    ): this;

    Adds the listener function to the beginning 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.prependListener('connection', (stream) => {
      console.log('someone connected!');
    });
    

    Returns a reference to the EventEmitter, so that calls can be chained.

    @param eventName

    The name of the event.

    @param listener

    The callback function

    prependListener(

    eventName: string | symbol,

    listener: (...args: any[]) => void

    ): this;

    Adds the listener function to the beginning 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.prependListener('connection', (stream) => {
      console.log('someone connected!');
    });
    

    Returns a reference to the EventEmitter, so that calls can be chained.

    @param eventName

    The name of the event.

    @param listener

    The callback function

  • prependOnceListener<E extends keyof WorkerEventMap>(

    eventName: E,

    listener: (...args: WorkerEventMap[E]) => void

    ): this;

    Adds a one-time listener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.

    server.prependOnceListener('connection', (stream) => {
      console.log('Ah, we have our first user!');
    });
    

    Returns a reference to the EventEmitter, so that calls can be chained.

    @param eventName

    The name of the event.

    @param listener

    The callback function

    prependOnceListener(

    eventName: string | symbol,

    listener: (...args: any[]) => void

    ): this;

    Adds a one-time listener function for the event named eventName to the beginning of the listeners array. The next time eventName is triggered, this listener is removed, and then invoked.

    server.prependOnceListener('connection', (stream) => {
      console.log('Ah, we have our first user!');
    });
    

    Returns a reference to the EventEmitter, so that calls can be chained.

    @param eventName

    The name of the event.

    @param listener

    The callback function

  • rawListeners<E extends keyof WorkerEventMap>(

    eventName: E

    ): (...args: WorkerEventMap[E]) => void[];

    Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).

    import { EventEmitter } from 'node:events';
    const emitter = new EventEmitter();
    emitter.once('log', () => console.log('log once'));
    
    // Returns a new Array with a function `onceWrapper` which has a property
    // `listener` which contains the original listener bound above
    const listeners = emitter.rawListeners('log');
    const logFnWrapper = listeners[0];
    
    // Logs "log once" to the console and does not unbind the `once` event
    logFnWrapper.listener();
    
    // Logs "log once" to the console and removes the listener
    logFnWrapper();
    
    emitter.on('log', () => console.log('log persistently'));
    // Will return a new Array with a single function bound by `.on()` above
    const newListeners = emitter.rawListeners('log');
    
    // Logs "log persistently" twice
    newListeners[0]();
    emitter.emit('log');
    

    rawListeners(

    eventName: string | symbol

    ): (...args: any[]) => void[];

    Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).

    import { EventEmitter } from 'node:events';
    const emitter = new EventEmitter();
    emitter.once('log', () => console.log('log once'));
    
    // Returns a new Array with a function `onceWrapper` which has a property
    // `listener` which contains the original listener bound above
    const listeners = emitter.rawListeners('log');
    const logFnWrapper = listeners[0];
    
    // Logs "log once" to the console and does not unbind the `once` event
    logFnWrapper.listener();
    
    // Logs "log once" to the console and removes the listener
    logFnWrapper();
    
    emitter.on('log', () => console.log('log persistently'));
    // Will return a new Array with a single function bound by `.on()` above
    const newListeners = emitter.rawListeners('log');
    
    // Logs "log persistently" twice
    newListeners[0]();
    emitter.emit('log');
    
  • 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.

  • removeAllListeners<E extends keyof WorkerEventMap>(

    eventName?: E

    ): this;

    Removes all listeners, or those of the specified eventName.

    It is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).

    Returns a reference to the EventEmitter, so that calls can be chained.

    removeAllListeners(

    eventName?: string | symbol

    ): this;

    Removes all listeners, or those of the specified eventName.

    It is bad practice to remove listeners added elsewhere in the code, particularly when the EventEmitter instance was created by some other component or module (e.g. sockets or file streams).

    Returns a reference to the EventEmitter, so that calls can be chained.
  • removeListener<E extends keyof WorkerEventMap>(

    eventName: E,

    listener: (...args: WorkerEventMap[E]) => void

    ): this;

    Removes the specified listener from the listener array for the event named eventName.

    const callback = (stream) => {
      console.log('someone connected!');
    };
    server.on('connection', callback);
    // ...
    server.removeListener('connection', callback);
    

    removeListener() will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener() must be called multiple times to remove each instance.

    Once an event is emitted, all listeners attached to it at the time of emitting are called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them from emit() in progress. Subsequent events behave as expected.

    import { EventEmitter } from 'node:events';
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();
    
    const callbackA = () => {
      console.log('A');
      myEmitter.removeListener('event', callbackB);
    };
    
    const callbackB = () => {
      console.log('B');
    };
    
    myEmitter.on('event', callbackA);
    
    myEmitter.on('event', callbackB);
    
    // callbackA removes listener callbackB but it will still be called.
    // Internal listener array at time of emit [callbackA, callbackB]
    myEmitter.emit('event');
    // Prints:
    //   A
    //   B
    
    // callbackB is now removed.
    // Internal listener array [callbackA]
    myEmitter.emit('event');
    // Prints:
    //   A
    

    Because listeners are managed using an internal array, calling this will change the position indexes of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.

    When a single function has been added as a handler multiple times for a single event (as in the example below), removeListener() will remove the most recently added instance. In the example the once('ping') listener is removed:

    import { EventEmitter } from 'node:events';
    const ee = new EventEmitter();
    
    function pong() {
      console.log('pong');
    }
    
    ee.on('ping', pong);
    ee.once('ping', pong);
    ee.removeListener('ping', pong);
    
    ee.emit('ping');
    ee.emit('ping');
    

    Returns a reference to the EventEmitter, so that calls can be chained.

    removeListener(

    eventName: string | symbol,

    listener: (...args: any[]) => void

    ): this;

    Removes the specified listener from the listener array for the event named eventName.

    const callback = (stream) => {
      console.log('someone connected!');
    };
    server.on('connection', callback);
    // ...
    server.removeListener('connection', callback);
    

    removeListener() will remove, at most, one instance of a listener from the listener array. If any single listener has been added multiple times to the listener array for the specified eventName, then removeListener() must be called multiple times to remove each instance.

    Once an event is emitted, all listeners attached to it at the time of emitting are called in order. This implies that any removeListener() or removeAllListeners() calls after emitting and before the last listener finishes execution will not remove them from emit() in progress. Subsequent events behave as expected.

    import { EventEmitter } from 'node:events';
    class MyEmitter extends EventEmitter {}
    const myEmitter = new MyEmitter();
    
    const callbackA = () => {
      console.log('A');
      myEmitter.removeListener('event', callbackB);
    };
    
    const callbackB = () => {
      console.log('B');
    };
    
    myEmitter.on('event', callbackA);
    
    myEmitter.on('event', callbackB);
    
    // callbackA removes listener callbackB but it will still be called.
    // Internal listener array at time of emit [callbackA, callbackB]
    myEmitter.emit('event');
    // Prints:
    //   A
    //   B
    
    // callbackB is now removed.
    // Internal listener array [callbackA]
    myEmitter.emit('event');
    // Prints:
    //   A
    

    Because listeners are managed using an internal array, calling this will change the position indexes of any listener registered after the listener being removed. This will not impact the order in which listeners are called, but it means that any copies of the listener array as returned by the emitter.listeners() method will need to be recreated.

    When a single function has been added as a handler multiple times for a single event (as in the example below), removeListener() will remove the most recently added instance. In the example the once('ping') listener is removed:

    import { EventEmitter } from 'node:events';
    const ee = new EventEmitter();
    
    function pong() {
      console.log('pong');
    }
    
    ee.on('ping', pong);
    ee.once('ping', pong);
    ee.removeListener('ping', pong);
    
    ee.emit('ping');
    ee.emit('ping');
    

    Returns a reference to the EventEmitter, so that calls can be chained.
  • setMaxListeners(

    n: number

    ): this;

    By default EventEmitters will print a warning if more than 10 listeners are added for a particular event. This is a useful default that helps finding memory leaks. The emitter.setMaxListeners() method allows the limit to be modified for this specific EventEmitter instance. The value can be set to Infinity (or 0) to indicate an unlimited number of listeners.

    Returns a reference to the EventEmitter, so that calls can be chained.
  • startCpuProfile(): Promise<CPUProfileHandle>;

    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();
      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();
    });
    
  • startHeapProfile(): Promise<HeapProfileHandle>;

    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();
    });
    

    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();
    });
    
  • terminate(): Promise<number>;

    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.

  • 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.

• const BroadcastChannel: new (name: string) => BroadcastChannel
• const isInternalThread: boolean
• const isMainThread: boolean
• const locks: LockManager
• const MessageChannel: new () => MessageChannel
• const MessagePort: new () => MessagePort
• const parentPort: null | MessagePort
• const resourceLimits: ResourceLimits
• const SHARE_ENV: unique symbol
• const threadId: number
• const threadName: string | null
• const workerData: any
• function getEnvironmentData(

  key: Serializable

  ): Serializable;

  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.

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

  @param key

  Any arbitrary, cloneable JavaScript value that can be used as a {Map} key.

• function isMarkedAsUntransferable(

  object: object

  ): boolean;

  Check if an object is marked as not transferable with markAsUntransferable.
• function markAsUncloneable(

  object: object

  ): void;

  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.
• function markAsUntransferable(

  object: object

  ): void;

  Mark an object as not transferable. If object occurs in the transfer list of a port.postMessage() call, it is ignored.

  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 its Buffer pool with this.

  This operation cannot be undone.

  import { MessageChannel, markAsUntransferable } from 'node:worker_threads';
  
  const pooledBuffer = new ArrayBuffer(8);
  const typedArray1 = new Uint8Array(pooledBuffer);
  const typedArray2 = new Float64Array(pooledBuffer);
  
  markAsUntransferable(pooledBuffer);
  
  const { port1 } = new MessageChannel();
  port1.postMessage(typedArray1, [ typedArray1.buffer ]);
  
  // The following line prints the contents of typedArray1 -- it still owns
  // its memory and has been cloned, not transferred. Without
  // `markAsUntransferable()`, this would print an empty Uint8Array.
  // typedArray2 is intact as well.
  console.log(typedArray1);
  console.log(typedArray2);
  

  There is no equivalent to this API in browsers.
• function moveMessagePortToContext(

  port: MessagePort,

  contextifiedSandbox: Context

  ): MessagePort;

  Transfer a MessagePort to a different vm Context. The original port object 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 the port.onmessage() listener are also created in the target context and inherit from its global Object class.

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

  @param port

  The message port to transfer.

  @param contextifiedSandbox

  A contextified object as returned by the vm.createContext() method.

• function postMessageToThread(

  threadId: number,

  value: any,

  timeout?: number

  ): Promise<void>;

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

  @param threadId

  The target thread ID. If the thread ID is invalid, a ERR_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.

  @param value

  The value to send.

  @param 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.

  function postMessageToThread(

  threadId: number,

  value: any,

  transferList: readonly Transferable[],

  timeout?: number

  ): Promise<void>;

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

  @param threadId

  The target thread ID. If the thread ID is invalid, a ERR_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.

  @param value

  The value to send.

  @param transferList

  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.

  @param 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.

• function receiveMessageOnPort(

  port: MessagePort

  ): undefined | { message: any };

  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 the MessagePort's queue.

  import { MessageChannel, receiveMessageOnPort } from '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 the onmessage listener is not invoked.
• function setEnvironmentData(

  key: Serializable,

  value?: Serializable

  ): void;

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

  @param key

  Any arbitrary, cloneable JavaScript value that can be used as a {Map} key.

  @param 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.

Type definitions

• interface BroadcastChannel

  EventTarget is a DOM interface implemented by objects that can receive events and may have listeners for them.

  MDN Reference

  • readonly name: string
  • onmessage: null | (ev: MessageEvent<any>) => void
  • onmessageerror: null | (ev: MessageEvent<any>) => void
  • addEventListener<K extends keyof BroadcastChannelEventMap>(

    type: K,

    listener: (ev: BroadcastChannelEventMap[K]) => void,

    options?: boolean | AddEventListenerOptions

    ): void;

    Appends an event listener for events whose type attribute value is type. The callback argument sets the callback that will be invoked when the event is dispatched.

    The options argument sets listener-specific options. For compatibility this can be a boolean, in which case the method behaves exactly as if the value was specified as options's capture.

    When set to true, options's capture prevents callback from being invoked when the event's eventPhase attribute value is BUBBLING_PHASE. When false (or not present), callback will not be invoked when event's eventPhase attribute value is CAPTURING_PHASE. Either way, callback will be invoked if event's eventPhase attribute value is AT_TARGET.

    When set to true, options's passive indicates that the callback will not cancel the event by invoking preventDefault(). This is used to enable performance optimizations described in § 2.8 Observing event listeners.

    When set to true, options's once indicates that the callback will only be invoked once after which the event listener will be removed.

    If an AbortSignal is passed for options's signal, then the event listener will be removed when signal is aborted.

    The event listener is appended to target's event listener list and is not appended if it has the same type, callback, and capture.

    MDN Reference

    addEventListener(

    type: string,

    listener: EventListener | EventListenerObject,

    options?: boolean | AddEventListenerOptions

    ): void;

    Appends an event listener for events whose type attribute value is type. The callback argument sets the callback that will be invoked when the event is dispatched.

    The options argument sets listener-specific options. For compatibility this can be a boolean, in which case the method behaves exactly as if the value was specified as options's capture.

    When set to true, options's capture prevents callback from being invoked when the event's eventPhase attribute value is BUBBLING_PHASE. When false (or not present), callback will not be invoked when event's eventPhase attribute value is CAPTURING_PHASE. Either way, callback will be invoked if event's eventPhase attribute value is AT_TARGET.

    When set to true, options's passive indicates that the callback will not cancel the event by invoking preventDefault(). This is used to enable performance optimizations described in § 2.8 Observing event listeners.

    When set to true, options's once indicates that the callback will only be invoked once after which the event listener will be removed.

    If an AbortSignal is passed for options's signal, then the event listener will be removed when signal is aborted.

    The event listener is appended to target's event listener list and is not appended if it has the same type, callback, and capture.

    MDN Reference
  • close(): void;
  • dispatchEvent(

    event: Event

    ): boolean;

    Dispatches a synthetic event event to target and returns true if either event's cancelable attribute value is false or its preventDefault() method was not invoked, and false otherwise.
  • postMessage(

    message: any

    ): void;
  • ref(): this;
  • removeEventListener<K extends keyof BroadcastChannelEventMap>(

    type: K,

    listener: (ev: BroadcastChannelEventMap[K]) => void,

    options?: boolean | EventListenerOptions

    ): void;

    Removes the event listener in target's event listener list with the same type, callback, and options.

    MDN Reference

    removeEventListener(

    type: string,

    listener: EventListener | EventListenerObject,

    options?: boolean | EventListenerOptions

    ): void;

    Removes the event listener in target's event listener list with the same type, callback, and options.

    MDN Reference
  • unref(): this;

• interface BroadcastChannelEventMap

  • message: MessageEvent
  • messageerror: MessageEvent

• interface Lock

  • readonly mode: LockMode
  • readonly name: string

• interface LockGrantedCallback<T>

• interface LockInfo

  • clientId: string
  • mode: LockMode
  • name: string

• interface LockManager

  • query(): Promise<LockManagerSnapshot>;
  • request<T>(

    name: string,

    callback: LockGrantedCallback<T>

    ): Promise<Awaited<T>>;

    request<T>(

    name: string,

    options: LockOptions,

    callback: LockGrantedCallback<T>

    ): Promise<Awaited<T>>;

• interface LockManagerSnapshot

  • held: LockInfo[]
  • pending: LockInfo[]

• interface LockOptions

  • ifAvailable?: boolean
  • mode?: LockMode
  • signal?: AbortSignal
  • steal?: boolean

• interface MessageChannel

  • readonly port1: MessagePort
  • readonly port2: MessagePort

• interface MessagePort

  The NodeEventTarget is a Node.js-specific extension to EventTarget that emulates a subset of the EventEmitter API.

  • onclose: null | (ev: Event) => void
  • onmessage: null | (ev: MessageEvent<any>) => void
  • onmessageerror: null | (ev: MessageEvent<any>) => void
  • addEventListener<K extends keyof MessagePortEventMap>(

    type: K,

    listener: (ev: MessagePortEventMap[K]) => void,

    options?: boolean | AddEventListenerOptions

    ): void;

    Appends an event listener for events whose type attribute value is type. The callback argument sets the callback that will be invoked when the event is dispatched.

    The options argument sets listener-specific options. For compatibility this can be a boolean, in which case the method behaves exactly as if the value was specified as options's capture.

    When set to true, options's capture prevents callback from being invoked when the event's eventPhase attribute value is BUBBLING_PHASE. When false (or not present), callback will not be invoked when event's eventPhase attribute value is CAPTURING_PHASE. Either way, callback will be invoked if event's eventPhase attribute value is AT_TARGET.

    When set to true, options's passive indicates that the callback will not cancel the event by invoking preventDefault(). This is used to enable performance optimizations described in § 2.8 Observing event listeners.

    When set to true, options's once indicates that the callback will only be invoked once after which the event listener will be removed.

    If an AbortSignal is passed for options's signal, then the event listener will be removed when signal is aborted.

    The event listener is appended to target's event listener list and is not appended if it has the same type, callback, and capture.

    MDN Reference

    addEventListener(

    type: string,

    listener: EventListener | EventListenerObject,

    options?: boolean | AddEventListenerOptions

    ): void;

    Appends an event listener for events whose type attribute value is type. The callback argument sets the callback that will be invoked when the event is dispatched.

    The options argument sets listener-specific options. For compatibility this can be a boolean, in which case the method behaves exactly as if the value was specified as options's capture.

    When set to true, options's capture prevents callback from being invoked when the event's eventPhase attribute value is BUBBLING_PHASE. When false (or not present), callback will not be invoked when event's eventPhase attribute value is CAPTURING_PHASE. Either way, callback will be invoked if event's eventPhase attribute value is AT_TARGET.

    When set to true, options's passive indicates that the callback will not cancel the event by invoking preventDefault(). This is used to enable performance optimizations described in § 2.8 Observing event listeners.

    When set to true, options's once indicates that the callback will only be invoked once after which the event listener will be removed.

    If an AbortSignal is passed for options's signal, then the event listener will be removed when signal is aborted.

    The event listener is appended to target's event listener list and is not appended if it has the same type, callback, and capture.

    MDN Reference
  • addListener(

    event: 'close',

    listener: (ev: Event) => void

    ): this;

    Node.js-specific extension to the EventTarget class that emulates the equivalent EventEmitter API. The only difference between addListener() and addEventListener() is that addListener() will return a reference to the EventTarget.

    addListener(

    event: 'message',

    listener: (value: any) => void

    ): this;

    Node.js-specific extension to the EventTarget class that emulates the equivalent EventEmitter API. The only difference between addListener() and addEventListener() is that addListener() will return a reference to the EventTarget.

    addListener(

    event: 'messageerror',

    listener: (error: Error) => void

    ): this;

    Node.js-specific extension to the EventTarget class that emulates the equivalent EventEmitter API. The only difference between addListener() and addEventListener() is that addListener() will return a reference to the EventTarget.

    addListener(

    event: string,

    listener: (arg: any) => void

    ): this;

    Node.js-specific extension to the EventTarget class that emulates the equivalent EventEmitter API. The only difference between addListener() and addEventListener() is that addListener() will return a reference to the EventTarget.
  • close(): void;
  • dispatchEvent(

    event: Event

    ): boolean;

    Dispatches a synthetic event event to target and returns true if either event's cancelable attribute value is false or its preventDefault() method was not invoked, and false otherwise.
  • emit(

    event: 'close',

    ev: Event

    ): boolean;

    Node.js-specific extension to the EventTarget class that dispatches the arg to the list of handlers for type.

    @returns

    true if event listeners registered for the type exist, otherwise false.

    emit(

    event: 'message',

    value: any

    ): boolean;

    Node.js-specific extension to the EventTarget class that dispatches the arg to the list of handlers for type.

    @returns

    true if event listeners registered for the type exist, otherwise false.

    emit(

    event: 'messageerror',

    error: Error

    ): boolean;

    Node.js-specific extension to the EventTarget class that dispatches the arg to the list of handlers for type.

    @returns

    true if event listeners registered for the type exist, otherwise false.

    emit(

    event: string,

    arg: any

    ): boolean;

    Node.js-specific extension to the EventTarget class that dispatches the arg to the list of handlers for type.

    @returns

    true if event listeners registered for the type exist, otherwise false.

  • eventNames(): string[];

    Node.js-specific extension to the EventTarget class that returns an array of event type names for which event listeners are registered.

  • getMaxListeners(): number;

    Node.js-specific extension to the EventTarget class that returns the number of max event listeners.

  • listenerCount(

    type: string

    ): number;

    Node.js-specific extension to the EventTarget class that returns the number of event listeners registered for the type.
  • off(

    event: 'close',

    listener: (ev: Event) => void,

    options?: EventListenerOptions

    ): this;

    Node.js-specific alias for eventTarget.removeEventListener().

    off(

    event: 'message',

    listener: (value: any) => void,

    options?: EventListenerOptions

    ): this;

    Node.js-specific alias for eventTarget.removeEventListener().

    off(

    event: 'messageerror',

    listener: (error: Error) => void,

    options?: EventListenerOptions

    ): this;

    Node.js-specific alias for eventTarget.removeEventListener().

    off(

    event: string,

    listener: (arg: any) => void,

    options?: EventListenerOptions

    ): this;

    Node.js-specific alias for eventTarget.removeEventListener().
  • on(

    event: 'close',

    listener: (ev: Event) => void

    ): this;

    Node.js-specific alias for eventTarget.addEventListener().

    on(

    event: 'message',

    listener: (value: any) => void

    ): this;

    Node.js-specific alias for eventTarget.addEventListener().

    on(

    event: 'messageerror',

    listener: (error: Error) => void

    ): this;

    Node.js-specific alias for eventTarget.addEventListener().

    on(

    event: string,

    listener: (arg: any) => void

    ): this;

    Node.js-specific alias for eventTarget.addEventListener().
  • once(

    event: 'close',

    listener: (ev: Event) => void

    ): this;

    Node.js-specific extension to the EventTarget class that adds a once listener for the given event type. This is equivalent to calling on with the once option set to true.

    once(

    event: 'message',

    listener: (value: any) => void

    ): this;

    Node.js-specific extension to the EventTarget class that adds a once listener for the given event type. This is equivalent to calling on with the once option set to true.

    once(

    event: 'messageerror',

    listener: (error: Error) => void

    ): this;

    Node.js-specific extension to the EventTarget class that adds a once listener for the given event type. This is equivalent to calling on with the once option set to true.

    once(

    event: string,

    listener: (arg: any) => void

    ): this;

    Node.js-specific extension to the EventTarget class that adds a once listener for the given event type. This is equivalent to calling on with the once option set to true.
  • postMessage(

    message: any,

    transfer: Transferable[]

    ): void;

    postMessage(

    message: any,

    options?: StructuredSerializeOptions

    ): void;
  • removeAllListeners(

    type?: string

    ): this;

    Node.js-specific extension to the EventTarget class. If type is specified, removes all registered listeners for type, otherwise removes all registered listeners.
  • removeEventListener<K extends keyof MessagePortEventMap>(

    type: K,

    listener: (ev: MessagePortEventMap[K]) => void,

    options?: boolean | EventListenerOptions

    ): void;

    Removes the event listener in target's event listener list with the same type, callback, and options.

    MDN Reference

    removeEventListener(

    type: string,

    listener: EventListener | EventListenerObject,

    options?: boolean | EventListenerOptions

    ): void;

    Removes the event listener in target's event listener list with the same type, callback, and options.

    MDN Reference
  • removeListener(

    event: 'close',

    listener: (ev: Event) => void,

    options?: EventListenerOptions

    ): this;

    Node.js-specific extension to the EventTarget class that removes the listener for the given type. The only difference between removeListener() and removeEventListener() is that removeListener() will return a reference to the EventTarget.

    removeListener(

    event: 'message',

    listener: (value: any) => void,

    options?: EventListenerOptions

    ): this;

    Node.js-specific extension to the EventTarget class that removes the listener for the given type. The only difference between removeListener() and removeEventListener() is that removeListener() will return a reference to the EventTarget.

    removeListener(

    event: 'messageerror',

    listener: (error: Error) => void,

    options?: EventListenerOptions

    ): this;

    Node.js-specific extension to the EventTarget class that removes the listener for the given type. The only difference between removeListener() and removeEventListener() is that removeListener() will return a reference to the EventTarget.

    removeListener(

    event: string,

    listener: (arg: any) => void,

    options?: EventListenerOptions

    ): this;

    Node.js-specific extension to the EventTarget class that removes the listener for the given type. The only difference between removeListener() and removeEventListener() is that removeListener() will return a reference to the EventTarget.
  • setMaxListeners(

    n: number

    ): void;

    Node.js-specific extension to the EventTarget class that sets the number of max event listeners as n.
  • start(): void;

• interface MessagePortEventMap

  • close: Event
  • message: MessageEvent
  • messageerror: MessageEvent

• interface ResourceLimits

  • codeRangeSizeMb?: number

    The size of a pre-allocated memory range used for generated code.

  • maxOldGenerationSizeMb?: number

    The maximum size of the main heap in MB.

  • maxYoungGenerationSizeMb?: number

    The maximum size of a heap space for recently created objects.

  • stackSizeMb?: number

    The default maximum stack size for the thread. Small values may lead to unusable Worker instances.

• interface StructuredSerializeOptions

  • transfer?: Transferable[]

• interface WorkerEventMap

  • error: [err: unknown]
  • exit: [exitCode: number]
  • message: [value: any]
  • messageerror: [error: Error]
  • online: []

• interface WorkerOptions

  • argv?: any[]

    List of arguments which would be stringified and appended to process.argv in the worker. This is mostly similar to the workerData but the values will be available on the global process.argv as if they were passed as CLI options to the script.

  • env?: Dict<string> | typeof SHARE_ENV
  • eval?: boolean
  • execArgv?: string[]
  • name?: string

    An optional name to be appended to the worker title for debugging/identification purposes, making the final title as [worker ${id}] ${name}.

  • resourceLimits?: ResourceLimits
  • stderr?: boolean
  • stdin?: boolean
  • stdout?: boolean
  • trackUnmanagedFds?: boolean
  • transferList?: Transferable[]

    Additional data to send in the first worker message.

  • workerData?: any

• interface WorkerPerformance

  • eventLoopUtilization(

    utilization1?: EventLoopUtilization,

    utilization2?: EventLoopUtilization

    ): EventLoopUtilization;

    The eventLoopUtilization() method returns an object that contains the cumulative duration of time the event loop has been both idle and active as a high resolution milliseconds timer. The utilization value is the calculated Event Loop Utilization (ELU).

    If bootstrapping has not yet finished on the main thread the properties have the value of 0. The ELU is immediately available on Worker threads since bootstrap happens within the event loop.

    Both utilization1 and utilization2 are optional parameters.

    If utilization1 is passed, then the delta between the current call's active and idle times, as well as the corresponding utilization value are calculated and returned (similar to process.hrtime()).

    If utilization1 and utilization2 are both passed, then the delta is calculated between the two arguments. This is a convenience option because, unlike process.hrtime(), calculating the ELU is more complex than a single subtraction.

    ELU is similar to CPU utilization, except that it only measures event loop statistics and not CPU usage. It represents the percentage of time the event loop has spent outside the event loop's event provider (e.g. epoll_wait). No other CPU idle time is taken into consideration. The following is an example of how a mostly idle process will have a high ELU.

    import { eventLoopUtilization } from 'node:perf_hooks';
    import { spawnSync } from 'node:child_process';
    
    setImmediate(() => {
      const elu = eventLoopUtilization();
      spawnSync('sleep', ['5']);
      console.log(eventLoopUtilization(elu).utilization);
    });
    

    Although the CPU is mostly idle while running this script, the value of utilization is 1. This is because the call to child_process.spawnSync() blocks the event loop from proceeding.

    Passing in a user-defined object instead of the result of a previous call to eventLoopUtilization() will lead to undefined behavior. The return values are not guaranteed to reflect any correct state of the event loop.

    @param utilization1

    The result of a previous call to eventLoopUtilization().

    @param utilization2

    The result of a previous call to eventLoopUtilization() prior to utilization1.

• type LockMode = 'exclusive' | 'shared'
• type Serializable = string | object | number | boolean | bigint
• type Transferable = ArrayBuffer | MessagePort | AbortSignal | FileHandle | ReadableStream | WritableStream | TransformStream