Worker threads#
Source Code: lib/worker_threads.js
The node:worker_threads module enables the use of threads that execute
JavaScript in parallel. To access it:
import worker from 'node:worker_threads';'use strict';
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 SharedArrayBuffer
instances.
import {
Worker,
isMainThread,
parentPort,
workerData,
} from 'node:worker_threads';
if (!isMainThread) {
const { parse } = await import('some-js-parsing-library');
const script = workerData;
parentPort.postMessage(parse(script));
}
export default function parseJSAsync(script) {
return new Promise((resolve, reject) => {
const worker = new Worker(new URL(import.meta.url), {
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}`));
});
});
};'use strict';
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 to
Worker constructor options to know how to customize worker thread options,
specifically argv and execArgv options.
worker.getEnvironmentData(key)#
key <any> Any arbitrary, cloneable JavaScript value that can be used as a
<Map> key.
- Returns: <any>
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(new URL(import.meta.url));
} else {
console.log(getEnvironmentData('Hello'));
}'use strict';
const {
Worker,
isMainThread,
setEnvironmentData,
getEnvironmentData,
} = require('node:worker_threads');
if (isMainThread) {
setEnvironmentData('Hello', 'World!');
const worker = new Worker(__filename);
} else {
console.log(getEnvironmentData('Hello'));
}
worker.markAsUntransferable(object)#
Added in: v14.5.0, v12.19.0
object <any> 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 if
object 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 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();
try {
port1.postMessage(typedArray1, [ typedArray1.buffer ]);
} catch (error) {
}
console.log(typedArray1);
console.log(typedArray2);'use strict';
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 {
port1.postMessage(typedArray1, [ typedArray1.buffer ]);
} catch (error) {
}
console.log(typedArray1);
console.log(typedArray2);
There is no equivalent to this API in browsers.
worker.isMarkedAsUntransferable(object)#
Added in: v21.0.0
Check if an object is marked as not transferable with
markAsUntransferable().
import { markAsUntransferable, isMarkedAsUntransferable } from 'node:worker_threads';
const pooledBuffer = new ArrayBuffer(8);
markAsUntransferable(pooledBuffer);
isMarkedAsUntransferable(pooledBuffer); 'use strict';
const { markAsUntransferable, isMarkedAsUntransferable } = require('node:worker_threads');
const pooledBuffer = new ArrayBuffer(8);
markAsUntransferable(pooledBuffer);
isMarkedAsUntransferable(pooledBuffer);
There is no equivalent to this API in browsers.
worker.markAsUncloneable(object)#
Added in: v22.10.0
object <any> 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.
import { markAsUncloneable } from 'node:worker_threads';
const anyObject = { foo: 'bar' };
markAsUncloneable(anyObject);
const { port1 } = new MessageChannel();
try {
port1.postMessage(anyObject);
} catch (error) {
}'use strict';
const { markAsUncloneable } = require('node:worker_threads');
const anyObject = { foo: 'bar' };
markAsUncloneable(anyObject);
const { port1 } = new MessageChannel();
try {
port1.postMessage(anyObject);
} catch (error) {
}
There is no equivalent to this API in browsers.
worker.moveMessagePortToContext(port, contextifiedSandbox)#
Added in: v11.13.0
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.
worker.parentPort#
Added in: v10.5.0
If this thread is a Worker, this is a MessagePort
allowing communication with the parent thread. Messages sent using
parentPort.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 using
parentPort.on('message').
import { Worker, isMainThread, parentPort } from 'node:worker_threads';
if (isMainThread) {
const worker = new Worker(new URL(import.meta.url));
worker.once('message', (message) => {
console.log(message);
});
worker.postMessage('Hello, world!');
} else {
parentPort.once('message', (message) => {
parentPort.postMessage(message);
});
}'use strict';
const { Worker, isMainThread, parentPort } = require('node:worker_threads');
if (isMainThread) {
const worker = new Worker(__filename);
worker.once('message', (message) => {
console.log(message);
});
worker.postMessage('Hello, world!');
} else {
parentPort.once('message', (message) => {
parentPort.postMessage(message);
});
}
worker.postMessageToThread(threadId, value[, transferList][, timeout])#
Added in: v22.5.0
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 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(new URL(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} -> ${threadId}:`, value);
channel.postMessage('done');
channel.close();
});
await postMessageToThread(0, { message: 'ping' });
}
channel.onmessage = channel.close;'use strict';
const process = require('node:process');
const {
postMessageToThread,
threadId,
workerData,
Worker,
} = require('node:worker_threads');
const channel = new BroadcastChannel('sync');
const level = workerData?.level ?? 0;
if (level < 10) {
const worker = new Worker(__filename, {
workerData: { level: level + 1 },
});
}
if (level === 0) {
process.on('workerMessage', (value, source) => {
console.log(`${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.resourceLimits#
Added in: v13.2.0, v12.16.0
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.setEnvironmentData(key[, value])#
key <any> Any arbitrary, cloneable JavaScript value that can be used as a
<Map> key.
value <any> 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 of
worker.getEnvironmentData() in the current thread and all new Worker
instances 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.threadName#
Added in: v22.20.0
A string identifier for the current thread or null if the thread is not running.
On the corresponding worker object (if there is any), it is available as worker.threadName.
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.
import { Worker, isMainThread, workerData } from 'node:worker_threads';
if (isMainThread) {
const worker = new Worker(new URL(import.meta.url), { workerData: 'Hello, world!' });
} else {
console.log(workerData);
}'use strict';
const { Worker, isMainThread, workerData } = require('node:worker_threads');
if (isMainThread) {
const worker = new Worker(__filename, { workerData: 'Hello, world!' });
} else {
console.log(workerData);
}
Class: BroadcastChannel extends EventTarget#
Instances of BroadcastChannel allow asynchronous one-to-many communication
with all other BroadcastChannel instances bound to the same channel name.
import {
isMainThread,
BroadcastChannel,
Worker,
} from '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(new URL(import.meta.url));
} else {
bc.postMessage('hello from every worker');
bc.close();
}'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 <any> 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: <Function> Invoked with a single
MessageEvent argument
when a message is received.
broadcastChannel.onmessageerror#
Added in: v15.4.0
- Type: <Function> Invoked with a received message cannot be
deserialized.
broadcastChannel.postMessage(message)#
Added in: v15.4.0
message <any> 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: MessagePort#
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 different
Workers.
This implementation matches browser MessagePorts.
Event: 'message'#
Added in: v10.5.0
value <any> 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
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 this
MessagePort.
The 'close' event is emitted on both MessagePort instances that
are part of the channel.
port.postMessage(value[, transferList])#
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:
import { MessageChannel } from 'node:worker_threads';
const { port1, port2 } = new MessageChannel();
port1.on('message', (message) => console.log(message));
const circularData = {};
circularData.foo = circularData;
port2.postMessage(circularData);'use strict';
const { MessageChannel } = require('node:worker_threads');
const { port1, port2 } = new MessageChannel();
port1.on('message', (message) => console.log(message));
const circularData = {};
circularData.foo = circularData;
port2.postMessage(circularData);
transferList may be a list of <ArrayBuffer>, MessagePort, and
FileHandle objects.
After transferring, they are not usable on the sending side of the channel
anymore (even if they are not contained in value). Unlike with
child processes, transferring handles such as network sockets is currently
not supported.
If value contains <SharedArrayBuffer> instances, those are accessible
from either thread. They cannot be listed in transferList.
value may still contain ArrayBuffer instances that are not in
transferList; in that case, the underlying memory is copied rather than moved.
import { MessageChannel } from 'node:worker_threads';
const { port1, port2 } = new MessageChannel();
port1.on('message', (message) => console.log(message));
const uint8Array = new Uint8Array([ 1, 2, 3, 4 ]);
port2.postMessage(uint8Array);
port2.postMessage(uint8Array, [ uint8Array.buffer ]);
const sharedUint8Array = new Uint8Array(new SharedArrayBuffer(4));
port2.postMessage(sharedUint8Array);
const otherChannel = new MessageChannel();
port2.postMessage({ port: otherChannel.port1 }, [ otherChannel.port1 ]);'use strict';
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 ]);
port2.postMessage(uint8Array);
port2.postMessage(uint8Array, [ uint8Array.buffer ]);
const sharedUint8Array = new Uint8Array(new SharedArrayBuffer(4));
port2.postMessage(sharedUint8Array);
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 <TypedArray> | <Buffer> instances are views over an underlying
<ArrayBuffer>. 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 an
ArrayBuffer as doing so causes all TypedArray and Buffer
instances 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);
port.postMessage(u1, [u1.buffer]);
console.log(u2.length);
For Buffer instances, specifically, whether the underlying
ArrayBuffer 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 internal
Buffer 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 using
Buffer.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, <Buffer> objects will be read as
plain <Uint8Array>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());
This limitation extends to many built-in objects, such as the global URL
object:
const { port1, port2 } = new MessageChannel();
port1.onmessage = ({ data }) => console.log(data);
port2.postMessage(new URL('https://example.org'));
port.hasRef()#
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 calling
unref() 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.
Class: Worker#
Added in: v10.5.0
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:
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();
});
}'use strict';
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 <string> | <URL> 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 URL
object 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 <Object>
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 are available on the global process.argv as if they
were passed as CLI options to the script.
env <Object> 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.env
object affect the other thread as well. Default: process.env.
eval <boolean> 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 <boolean> If this is set to true, then worker.stdin
provides a writable stream whose contents appear as process.stdin
inside the Worker. By default, no data is provided.
stdout <boolean> If this is set to true, then worker.stdout is
not automatically piped through to process.stdout in the parent.
stderr <boolean> If this is set to true, then worker.stderr is
not automatically piped through to process.stderr in the parent.
workerData <any> 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 contains
functions).
trackUnmanagedFds <boolean> If this is set to true, then the Worker
tracks raw file descriptors managed through fs.open() and
fs.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 <Object> An optional set of resource limits for the new JS
engine instance. Reaching these limits leads to termination of the Worker
instance. 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 <number> 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 <number> 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 <number> The size of a pre-allocated memory range
used for generated code.
stackSizeMb <number> The default maximum stack size for the thread.
Small values may lead to unusable Worker instances. Default: 4.
name <string> An optional name to be appended to the worker title
for debugging/identification purposes, making the final title as
[worker ${id}] ${name}. Default: ''.
Event: 'error'#
Added in: v10.5.0
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
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.
Event: 'messageerror'#
Added in: v14.5.0, v12.19.0
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.cpuUsage([prev])#
Added in: v22.19.0
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])#
options <Object>
exposeInternals <boolean> If true, expose internals in the heap snapshot.
Default: false.
exposeNumericValues <boolean> If true, expose numeric values in
artificial fields. Default: false.
- Returns: <Promise> 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.getHeapStatistics()#
Added in: v22.16.0
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.
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 <Object> The result of a previous call to
eventLoopUtilization().
utilization2 <Object> The result of a previous call to
eventLoopUtilization() prior to utilization1.
- Returns: <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 examples shows how the worker's entire
lifetime never accumulates any idle time, but is still be 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);
}'use strict';
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.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
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(name)#
Added in: v22.20.0
Starting a CPU profile with the given name, then return a Promise that fulfills
with an error or an object which has a stop method. Calling the stop method will
stop collecting the profile, then return a Promise that fulfills with an error or the
profile data.
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('demo');
const profile = await handle.stop();
console.log(profile);
worker.terminate();
});
worker.stderr#
Added in: v10.5.0
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.
worker.stdin#
Added in: v10.5.0
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
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()#
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 calling
unref() again has no effect.
Notes#
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.