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