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, andprocess.stderrstreams may be redirected by the parent thread.The
require('node:worker_threads').isMainThreadproperty is set tofalse.The
require('node:worker_threads').parentPortmessage port is available.process.exit()does not stop the whole program, just the single thread, andprocess.abort()is not available.process.chdir()andprocessmethods that set group or user ids are not available.process.envis 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_ENVis passed as theenvoption to theWorkerconstructor). On Windows, unlike the main thread, a copy of the environment variables operates in a case-sensitive manner.process.titlecannot 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_eventsmodule 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.
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();
});
}Since
v10.5.0
Properties
An object that can be used to query performance information from a worker instance. Similar to perf_hooks.performance.
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.
Functions
Alias for emitter.on(eventName, listener).
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 an array listing the events for which the emitter has registered listeners. The values in the array are strings or Symbols.
Returns a readable stream for a V8 snapshot of the current state of the Worker. See v8.getHeapSnapshot() for more details.
Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to {@link defaultMaxListeners}.
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.
Returns a copy of the array of listeners for the event named eventName.
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.
Send a message to the worker that is received via require('node:worker_threads').parentPort.on('message'). See port.postMessage() for more details.
Sends a value to another worker, identified by its thread ID.
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.
Adds a one-timelistener 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.
Returns a copy of the array of listeners for the event named eventName, including any wrappers (such as those created by .once()).
Removes all listeners, or those of the specified eventName.
Removes the specified listener from the listener array for the event named eventName.
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.