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<h1>Node.js v16.16.0 documentation</h1>
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<li><span class="stability_2"><a href="#worker-threads">Worker threads</a></span>
<ul>
<li><a href="#workergetenvironmentdatakey"><code>worker.getEnvironmentData(key)</code></a></li>
<li><a href="#workerismainthread"><code>worker.isMainThread</code></a></li>
<li><a href="#workermarkasuntransferableobject"><code>worker.markAsUntransferable(object)</code></a></li>
<li><a href="#workermovemessageporttocontextport-contextifiedsandbox"><code>worker.moveMessagePortToContext(port, contextifiedSandbox)</code></a></li>
<li><a href="#workerparentport"><code>worker.parentPort</code></a></li>
<li><a href="#workerreceivemessageonportport"><code>worker.receiveMessageOnPort(port)</code></a></li>
<li><a href="#workerresourcelimits"><code>worker.resourceLimits</code></a></li>
<li><a href="#workershare_env"><code>worker.SHARE_ENV</code></a></li>
<li><a href="#workersetenvironmentdatakey-value"><code>worker.setEnvironmentData(key[, value])</code></a></li>
<li><a href="#workerthreadid"><code>worker.threadId</code></a></li>
<li><a href="#workerworkerdata"><code>worker.workerData</code></a></li>
<li><span class="stability_1"><a href="#class-broadcastchannel-extends-eventtarget">Class: <code>BroadcastChannel extends EventTarget</code></a></span>
<ul>
<li><a href="#new-broadcastchannelname"><code>new BroadcastChannel(name)</code></a></li>
<li><a href="#broadcastchannelclose"><code>broadcastChannel.close()</code></a></li>
<li><a href="#broadcastchannelonmessage"><code>broadcastChannel.onmessage</code></a></li>
<li><a href="#broadcastchannelonmessageerror"><code>broadcastChannel.onmessageerror</code></a></li>
<li><a href="#broadcastchannelpostmessagemessage"><code>broadcastChannel.postMessage(message)</code></a></li>
<li><a href="#broadcastchannelref"><code>broadcastChannel.ref()</code></a></li>
<li><a href="#broadcastchannelunref"><code>broadcastChannel.unref()</code></a></li>
</ul>
</li>
<li><a href="#class-messagechannel">Class: <code>MessageChannel</code></a></li>
<li><a href="#class-messageport">Class: <code>MessagePort</code></a>
<ul>
<li><a href="#event-close">Event: <code>'close'</code></a></li>
<li><a href="#event-message">Event: <code>'message'</code></a></li>
<li><a href="#event-messageerror">Event: <code>'messageerror'</code></a></li>
<li><a href="#portclose"><code>port.close()</code></a></li>
<li><a href="#portpostmessagevalue-transferlist"><code>port.postMessage(value[, transferList])</code></a>
<ul>
<li><a href="#considerations-when-transferring-typedarrays-and-buffers">Considerations when transferring TypedArrays and Buffers</a></li>
<li><a href="#considerations-when-cloning-objects-with-prototypes-classes-and-accessors">Considerations when cloning objects with prototypes, classes, and accessors</a></li>
</ul>
</li>
<li><a href="#portref"><code>port.ref()</code></a></li>
<li><a href="#portstart"><code>port.start()</code></a></li>
<li><a href="#portunref"><code>port.unref()</code></a></li>
</ul>
</li>
<li><a href="#class-worker">Class: <code>Worker</code></a>
<ul>
<li><a href="#new-workerfilename-options"><code>new Worker(filename[, options])</code></a></li>
<li><a href="#event-error">Event: <code>'error'</code></a></li>
<li><a href="#event-exit">Event: <code>'exit'</code></a></li>
<li><a href="#event-message_1">Event: <code>'message'</code></a></li>
<li><a href="#event-messageerror_1">Event: <code>'messageerror'</code></a></li>
<li><a href="#event-online">Event: <code>'online'</code></a></li>
<li><a href="#workergetheapsnapshot"><code>worker.getHeapSnapshot()</code></a></li>
<li><a href="#workerperformance"><code>worker.performance</code></a>
<ul>
<li><a href="#performanceeventlooputilizationutilization1-utilization2"><code>performance.eventLoopUtilization([utilization1[, utilization2]])</code></a></li>
</ul>
</li>
<li><a href="#workerpostmessagevalue-transferlist"><code>worker.postMessage(value[, transferList])</code></a></li>
<li><a href="#workerref"><code>worker.ref()</code></a></li>
<li><a href="#workerresourcelimits_1"><code>worker.resourceLimits</code></a></li>
<li><a href="#workerstderr"><code>worker.stderr</code></a></li>
<li><a href="#workerstdin"><code>worker.stdin</code></a></li>
<li><a href="#workerstdout"><code>worker.stdout</code></a></li>
<li><a href="#workerterminate"><code>worker.terminate()</code></a></li>
<li><a href="#workerthreadid_1"><code>worker.threadId</code></a></li>
<li><a href="#workerunref"><code>worker.unref()</code></a></li>
</ul>
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<li><a href="#notes">Notes</a>
<ul>
<li><a href="#synchronous-blocking-of-stdio">Synchronous blocking of stdio</a></li>
<li><a href="#launching-worker-threads-from-preload-scripts">Launching worker threads from preload scripts</a></li>
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<details id="toc" open><summary>Table of contents</summary><ul>
<li><span class="stability_2"><a href="#worker-threads">Worker threads</a></span>
<ul>
<li><a href="#workergetenvironmentdatakey"><code>worker.getEnvironmentData(key)</code></a></li>
<li><a href="#workerismainthread"><code>worker.isMainThread</code></a></li>
<li><a href="#workermarkasuntransferableobject"><code>worker.markAsUntransferable(object)</code></a></li>
<li><a href="#workermovemessageporttocontextport-contextifiedsandbox"><code>worker.moveMessagePortToContext(port, contextifiedSandbox)</code></a></li>
<li><a href="#workerparentport"><code>worker.parentPort</code></a></li>
<li><a href="#workerreceivemessageonportport"><code>worker.receiveMessageOnPort(port)</code></a></li>
<li><a href="#workerresourcelimits"><code>worker.resourceLimits</code></a></li>
<li><a href="#workershare_env"><code>worker.SHARE_ENV</code></a></li>
<li><a href="#workersetenvironmentdatakey-value"><code>worker.setEnvironmentData(key[, value])</code></a></li>
<li><a href="#workerthreadid"><code>worker.threadId</code></a></li>
<li><a href="#workerworkerdata"><code>worker.workerData</code></a></li>
<li><span class="stability_1"><a href="#class-broadcastchannel-extends-eventtarget">Class: <code>BroadcastChannel extends EventTarget</code></a></span>
<ul>
<li><a href="#new-broadcastchannelname"><code>new BroadcastChannel(name)</code></a></li>
<li><a href="#broadcastchannelclose"><code>broadcastChannel.close()</code></a></li>
<li><a href="#broadcastchannelonmessage"><code>broadcastChannel.onmessage</code></a></li>
<li><a href="#broadcastchannelonmessageerror"><code>broadcastChannel.onmessageerror</code></a></li>
<li><a href="#broadcastchannelpostmessagemessage"><code>broadcastChannel.postMessage(message)</code></a></li>
<li><a href="#broadcastchannelref"><code>broadcastChannel.ref()</code></a></li>
<li><a href="#broadcastchannelunref"><code>broadcastChannel.unref()</code></a></li>
</ul>
</li>
<li><a href="#class-messagechannel">Class: <code>MessageChannel</code></a></li>
<li><a href="#class-messageport">Class: <code>MessagePort</code></a>
<ul>
<li><a href="#event-close">Event: <code>'close'</code></a></li>
<li><a href="#event-message">Event: <code>'message'</code></a></li>
<li><a href="#event-messageerror">Event: <code>'messageerror'</code></a></li>
<li><a href="#portclose"><code>port.close()</code></a></li>
<li><a href="#portpostmessagevalue-transferlist"><code>port.postMessage(value[, transferList])</code></a>
<ul>
<li><a href="#considerations-when-transferring-typedarrays-and-buffers">Considerations when transferring TypedArrays and Buffers</a></li>
<li><a href="#considerations-when-cloning-objects-with-prototypes-classes-and-accessors">Considerations when cloning objects with prototypes, classes, and accessors</a></li>
</ul>
</li>
<li><a href="#portref"><code>port.ref()</code></a></li>
<li><a href="#portstart"><code>port.start()</code></a></li>
<li><a href="#portunref"><code>port.unref()</code></a></li>
</ul>
</li>
<li><a href="#class-worker">Class: <code>Worker</code></a>
<ul>
<li><a href="#new-workerfilename-options"><code>new Worker(filename[, options])</code></a></li>
<li><a href="#event-error">Event: <code>'error'</code></a></li>
<li><a href="#event-exit">Event: <code>'exit'</code></a></li>
<li><a href="#event-message_1">Event: <code>'message'</code></a></li>
<li><a href="#event-messageerror_1">Event: <code>'messageerror'</code></a></li>
<li><a href="#event-online">Event: <code>'online'</code></a></li>
<li><a href="#workergetheapsnapshot"><code>worker.getHeapSnapshot()</code></a></li>
<li><a href="#workerperformance"><code>worker.performance</code></a>
<ul>
<li><a href="#performanceeventlooputilizationutilization1-utilization2"><code>performance.eventLoopUtilization([utilization1[, utilization2]])</code></a></li>
</ul>
</li>
<li><a href="#workerpostmessagevalue-transferlist"><code>worker.postMessage(value[, transferList])</code></a></li>
<li><a href="#workerref"><code>worker.ref()</code></a></li>
<li><a href="#workerresourcelimits_1"><code>worker.resourceLimits</code></a></li>
<li><a href="#workerstderr"><code>worker.stderr</code></a></li>
<li><a href="#workerstdin"><code>worker.stdin</code></a></li>
<li><a href="#workerstdout"><code>worker.stdout</code></a></li>
<li><a href="#workerterminate"><code>worker.terminate()</code></a></li>
<li><a href="#workerthreadid_1"><code>worker.threadId</code></a></li>
<li><a href="#workerunref"><code>worker.unref()</code></a></li>
</ul>
</li>
<li><a href="#notes">Notes</a>
<ul>
<li><a href="#synchronous-blocking-of-stdio">Synchronous blocking of stdio</a></li>
<li><a href="#launching-worker-threads-from-preload-scripts">Launching worker threads from preload scripts</a></li>
</ul>
</li>
</ul>
</li>
</ul></details>
<div id="apicontent">
<h2>Worker threads<span><a class="mark" href="#worker-threads" id="worker-threads">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_threads"></a></h2>
<p></p><div class="api_stability api_stability_2"><a href="documentation.html#stability-index">Stability: 2</a> - Stable</div><p></p>
<p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v16.16.0/lib/worker_threads.js">lib/worker_threads.js</a></p>
<p>The <code>worker_threads</code> module enables the use of threads that execute JavaScript
in parallel. To access it:</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> worker = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>);</code></pre>
<p>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.</p>
<p>Unlike <code>child_process</code> or <code>cluster</code>, <code>worker_threads</code> can share memory. They do
so by transferring <code>ArrayBuffer</code> instances or sharing <code>SharedArrayBuffer</code>
instances.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> {
<span class="hljs-title class_">Worker</span>, isMainThread, parentPort, workerData
} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>);
<span class="hljs-keyword">if</span> (isMainThread) {
<span class="hljs-variable language_">module</span>.<span class="hljs-property">exports</span> = <span class="hljs-keyword">function</span> <span class="hljs-title function_">parseJSAsync</span>(<span class="hljs-params">script</span>) {
<span class="hljs-keyword">return</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Promise</span>(<span class="hljs-function">(<span class="hljs-params">resolve, reject</span>) =></span> {
<span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename, {
<span class="hljs-attr">workerData</span>: script
});
worker.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, resolve);
worker.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, reject);
worker.<span class="hljs-title function_">on</span>(<span class="hljs-string">'exit'</span>, <span class="hljs-function">(<span class="hljs-params">code</span>) =></span> {
<span class="hljs-keyword">if</span> (code !== <span class="hljs-number">0</span>)
<span class="hljs-title function_">reject</span>(<span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>(<span class="hljs-string">`Worker stopped with exit code <span class="hljs-subst">${code}</span>`</span>));
});
});
};
} <span class="hljs-keyword">else</span> {
<span class="hljs-keyword">const</span> { parse } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'some-js-parsing-library'</span>);
<span class="hljs-keyword">const</span> script = workerData;
parentPort.<span class="hljs-title function_">postMessage</span>(<span class="hljs-title function_">parse</span>(script));
}</code></pre>
<p>The above example spawns a Worker thread for each <code>parseJSAsync()</code> call. In
practice, use a pool of Workers for these kinds of tasks. Otherwise, the
overhead of creating Workers would likely exceed their benefit.</p>
<p>When implementing a worker pool, use the <a href="async_hooks.html#class-asyncresource"><code>AsyncResource</code></a> API to inform
diagnostic tools (e.g. to provide asynchronous stack traces) about the
correlation between tasks and their outcomes. See
<a href="async_context.html#using-asyncresource-for-a-worker-thread-pool">"Using <code>AsyncResource</code> for a <code>Worker</code> thread pool"</a>
in the <code>async_hooks</code> documentation for an example implementation.</p>
<p>Worker threads inherit non-process-specific options by default. Refer to
<a href="#new-workerfilename-options"><code>Worker constructor options</code></a> to know how to customize worker thread options,
specifically <code>argv</code> and <code>execArgv</code> options.</p>
<section><h3><code>worker.getEnvironmentData(key)</code><span><a class="mark" href="#workergetenvironmentdatakey" id="workergetenvironmentdatakey">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_getenvironmentdata_key"></a></h3>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v16.15.0</td>
<td><p>No longer experimental.</p></td></tr>
<tr><td>v15.12.0, v14.18.0</td>
<td><p><span>Added in: v15.12.0, v14.18.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<ul>
<li><code>key</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> Any arbitrary, cloneable JavaScript value that can be used as a
<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map" class="type"><Map></a> key.</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
</ul>
<p>Within a worker thread, <code>worker.getEnvironmentData()</code> returns a clone
of data passed to the spawning thread's <code>worker.setEnvironmentData()</code>.
Every new <code>Worker</code> receives its own copy of the environment data
automatically.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> {
<span class="hljs-title class_">Worker</span>,
isMainThread,
setEnvironmentData,
getEnvironmentData,
} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>);
<span class="hljs-keyword">if</span> (isMainThread) {
<span class="hljs-title function_">setEnvironmentData</span>(<span class="hljs-string">'Hello'</span>, <span class="hljs-string">'World!'</span>);
<span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename);
} <span class="hljs-keyword">else</span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">getEnvironmentData</span>(<span class="hljs-string">'Hello'</span>)); <span class="hljs-comment">// Prints 'World!'.</span>
}</code></pre>
</section><section><h3><code>worker.isMainThread</code><span><a class="mark" href="#workerismainthread" id="workerismainthread">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_ismainthread"></a></h3>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a></li>
</ul>
<p>Is <code>true</code> if this code is not running inside of a <a href="#class-worker"><code>Worker</code></a> thread.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">Worker</span>, isMainThread } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>);
<span class="hljs-keyword">if</span> (isMainThread) {
<span class="hljs-comment">// This re-loads the current file inside a Worker instance.</span>
<span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename);
} <span class="hljs-keyword">else</span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'Inside Worker!'</span>);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(isMainThread); <span class="hljs-comment">// Prints 'false'.</span>
}</code></pre>
</section><section><h3><code>worker.markAsUntransferable(object)</code><span><a class="mark" href="#workermarkasuntransferableobject" id="workermarkasuntransferableobject">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_markasuntransferable_object"></a></h3>
<div class="api_metadata">
<span>Added in: v14.5.0, v12.19.0</span>
</div>
<p>Mark an object as not transferable. If <code>object</code> occurs in the transfer list of
a <a href="#portpostmessagevalue-transferlist"><code>port.postMessage()</code></a> call, it is ignored.</p>
<p>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 <code>ArrayBuffer</code>s it uses for its
<a href="buffer.html#static-method-bufferallocunsafesize"><code>Buffer</code> pool</a> with this.</p>
<p>This operation cannot be undone.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">MessageChannel</span>, markAsUntransferable } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>);
<span class="hljs-keyword">const</span> pooledBuffer = <span class="hljs-keyword">new</span> <span class="hljs-title class_">ArrayBuffer</span>(<span class="hljs-number">8</span>);
<span class="hljs-keyword">const</span> typedArray1 = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Uint8Array</span>(pooledBuffer);
<span class="hljs-keyword">const</span> typedArray2 = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Float64Array</span>(pooledBuffer);
<span class="hljs-title function_">markAsUntransferable</span>(pooledBuffer);
<span class="hljs-keyword">const</span> { port1 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>();
port1.<span class="hljs-title function_">postMessage</span>(typedArray1, [ typedArray1.<span class="hljs-property">buffer</span> ]);
<span class="hljs-comment">// The following line prints the contents of typedArray1 -- it still owns</span>
<span class="hljs-comment">// its memory and has been cloned, not transferred. Without</span>
<span class="hljs-comment">// `markAsUntransferable()`, this would print an empty Uint8Array.</span>
<span class="hljs-comment">// typedArray2 is intact as well.</span>
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(typedArray1);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(typedArray2);</code></pre>
<p>There is no equivalent to this API in browsers.</p>
</section><section><h3><code>worker.moveMessagePortToContext(port, contextifiedSandbox)</code><span><a class="mark" href="#workermovemessageporttocontextport-contextifiedsandbox" id="workermovemessageporttocontextport-contextifiedsandbox">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_movemessageporttocontext_port_contextifiedsandbox"></a></h3>
<div class="api_metadata">
<span>Added in: v11.13.0</span>
</div>
<ul>
<li>
<p><code>port</code> <a href="worker_threads.html#class-messageport" class="type"><MessagePort></a> The message port to transfer.</p>
</li>
<li>
<p><code>contextifiedSandbox</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> A <a href="vm.html#what-does-it-mean-to-contextify-an-object">contextified</a> object as returned by the
<code>vm.createContext()</code> method.</p>
</li>
<li>
<p>Returns: <a href="worker_threads.html#class-messageport" class="type"><MessagePort></a></p>
</li>
</ul>
<p>Transfer a <code>MessagePort</code> to a different <a href="vm.html"><code>vm</code></a> Context. The original <code>port</code>
object is rendered unusable, and the returned <code>MessagePort</code> instance
takes its place.</p>
<p>The returned <code>MessagePort</code> is an object in the target context and
inherits from its global <code>Object</code> class. Objects passed to the
<a href="https://developer.mozilla.org/en-US/docs/Web/API/MessagePort/onmessage"><code>port.onmessage()</code></a> listener are also created in the target context
and inherit from its global <code>Object</code> class.</p>
<p>However, the created <code>MessagePort</code> no longer inherits from
<a href="https://developer.mozilla.org/en-US/docs/Web/API/EventTarget"><code>EventTarget</code></a>, and only <a href="https://developer.mozilla.org/en-US/docs/Web/API/MessagePort/onmessage"><code>port.onmessage()</code></a> can be used to receive
events using it.</p>
</section><section><h3><code>worker.parentPort</code><span><a class="mark" href="#workerparentport" id="workerparentport">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_parentport"></a></h3>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Null_type" class="type"><null></a> | <a href="worker_threads.html#class-messageport" class="type"><MessagePort></a></li>
</ul>
<p>If this thread is a <a href="#class-worker"><code>Worker</code></a>, this is a <a href="#class-messageport"><code>MessagePort</code></a>
allowing communication with the parent thread. Messages sent using
<code>parentPort.postMessage()</code> are available in the parent thread
using <code>worker.on('message')</code>, and messages sent from the parent thread
using <code>worker.postMessage()</code> are available in this thread using
<code>parentPort.on('message')</code>.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">Worker</span>, isMainThread, parentPort } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>);
<span class="hljs-keyword">if</span> (isMainThread) {
<span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename);
worker.<span class="hljs-title function_">once</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">message</span>) =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(message); <span class="hljs-comment">// Prints 'Hello, world!'.</span>
});
worker.<span class="hljs-title function_">postMessage</span>(<span class="hljs-string">'Hello, world!'</span>);
} <span class="hljs-keyword">else</span> {
<span class="hljs-comment">// When a message from the parent thread is received, send it back:</span>
parentPort.<span class="hljs-title function_">once</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">message</span>) =></span> {
parentPort.<span class="hljs-title function_">postMessage</span>(message);
});
}</code></pre>
</section><section><h3><code>worker.receiveMessageOnPort(port)</code><span><a class="mark" href="#workerreceivemessageonportport" id="workerreceivemessageonportport">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_receivemessageonport_port"></a></h3>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v15.12.0</td>
<td><p>The port argument can also refer to a <code>BroadcastChannel</code> now.</p></td></tr>
<tr><td>v12.3.0</td>
<td><p><span>Added in: v12.3.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<ul>
<li>
<p><code>port</code> <a href="worker_threads.html#class-messageport" class="type"><MessagePort></a> | <a href="worker_threads.html#class-broadcastchannel-extends-eventtarget" class="type"><BroadcastChannel></a></p>
</li>
<li>
<p>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Undefined_type" class="type"><undefined></a></p>
</li>
</ul>
<p>Receive a single message from a given <code>MessagePort</code>. If no message is available,
<code>undefined</code> is returned, otherwise an object with a single <code>message</code> property
that contains the message payload, corresponding to the oldest message in the
<code>MessagePort</code>’s queue.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">MessageChannel</span>, receiveMessageOnPort } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>);
<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>();
port1.<span class="hljs-title function_">postMessage</span>({ <span class="hljs-attr">hello</span>: <span class="hljs-string">'world'</span> });
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">receiveMessageOnPort</span>(port2));
<span class="hljs-comment">// Prints: { message: { hello: 'world' } }</span>
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">receiveMessageOnPort</span>(port2));
<span class="hljs-comment">// Prints: undefined</span></code></pre>
<p>When this function is used, no <code>'message'</code> event is emitted and the
<code>onmessage</code> listener is not invoked.</p>
</section><section><h3><code>worker.resourceLimits</code><span><a class="mark" href="#workerresourcelimits" id="workerresourcelimits">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_resourcelimits"></a></h3>
<div class="api_metadata">
<span>Added in: v13.2.0, v12.16.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a>
<ul>
<li><code>maxYoungGenerationSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li>
<li><code>maxOldGenerationSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li>
<li><code>codeRangeSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li>
<li><code>stackSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li>
</ul>
</li>
</ul>
<p>Provides the set of JS engine resource constraints inside this Worker thread.
If the <code>resourceLimits</code> option was passed to the <a href="#class-worker"><code>Worker</code></a> constructor,
this matches its values.</p>
<p>If this is used in the main thread, its value is an empty object.</p>
</section><section><h3><code>worker.SHARE_ENV</code><span><a class="mark" href="#workershare_env" id="workershare_env">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_share_env"></a></h3>
<div class="api_metadata">
<span>Added in: v11.14.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a></li>
</ul>
<p>A special value that can be passed as the <code>env</code> option of the <a href="#class-worker"><code>Worker</code></a>
constructor, to indicate that the current thread and the Worker thread should
share read and write access to the same set of environment variables.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">Worker</span>, <span class="hljs-variable constant_">SHARE_ENV</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>);
<span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(<span class="hljs-string">'process.env.SET_IN_WORKER = "foo"'</span>, { <span class="hljs-attr">eval</span>: <span class="hljs-literal">true</span>, <span class="hljs-attr">env</span>: <span class="hljs-variable constant_">SHARE_ENV</span> })
.<span class="hljs-title function_">on</span>(<span class="hljs-string">'exit'</span>, <span class="hljs-function">() =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(process.<span class="hljs-property">env</span>.<span class="hljs-property">SET_IN_WORKER</span>); <span class="hljs-comment">// Prints 'foo'.</span>
});</code></pre>
</section><section><h3><code>worker.setEnvironmentData(key[, value])</code><span><a class="mark" href="#workersetenvironmentdatakey-value" id="workersetenvironmentdatakey-value">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_setenvironmentdata_key_value"></a></h3>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v16.15.0</td>
<td><p>No longer experimental.</p></td></tr>
<tr><td>v15.12.0, v14.18.0</td>
<td><p><span>Added in: v15.12.0, v14.18.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<ul>
<li><code>key</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> Any arbitrary, cloneable JavaScript value that can be used as a
<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map" class="type"><Map></a> key.</li>
<li><code>value</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> Any arbitrary, cloneable JavaScript value that will be cloned
and passed automatically to all new <code>Worker</code> instances. If <code>value</code> is passed
as <code>undefined</code>, any previously set value for the <code>key</code> will be deleted.</li>
</ul>
<p>The <code>worker.setEnvironmentData()</code> API sets the content of
<code>worker.getEnvironmentData()</code> in the current thread and all new <code>Worker</code>
instances spawned from the current context.</p>
</section><section><h3><code>worker.threadId</code><span><a class="mark" href="#workerthreadid" id="workerthreadid">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_threadid"></a></h3>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><integer></a></li>
</ul>
<p>An integer identifier for the current thread. On the corresponding worker object
(if there is any), it is available as <a href="#workerthreadid_1"><code>worker.threadId</code></a>.
This value is unique for each <a href="#class-worker"><code>Worker</code></a> instance inside a single process.</p>
</section><section><h3><code>worker.workerData</code><span><a class="mark" href="#workerworkerdata" id="workerworkerdata">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_workerdata"></a></h3>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<p>An arbitrary JavaScript value that contains a clone of the data passed
to this thread’s <code>Worker</code> constructor.</p>
<p>The data is cloned as if using <a href="#portpostmessagevalue-transferlist"><code>postMessage()</code></a>,
according to the <a href="https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm">HTML structured clone algorithm</a>.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">Worker</span>, isMainThread, workerData } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>);
<span class="hljs-keyword">if</span> (isMainThread) {
<span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename, { <span class="hljs-attr">workerData</span>: <span class="hljs-string">'Hello, world!'</span> });
} <span class="hljs-keyword">else</span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(workerData); <span class="hljs-comment">// Prints 'Hello, world!'.</span>
}</code></pre>
</section><section><h3>Class: <code>BroadcastChannel extends EventTarget</code><span><a class="mark" href="#class-broadcastchannel-extends-eventtarget" id="class-broadcastchannel-extends-eventtarget">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_class_broadcastchannel_extends_eventtarget"></a></h3>
<div class="api_metadata">
<span>Added in: v15.4.0</span>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<p>Instances of <code>BroadcastChannel</code> allow asynchronous one-to-many communication
with all other <code>BroadcastChannel</code> instances bound to the same channel name.</p>
<pre><code class="language-js"><span class="hljs-meta">'use strict'</span>;
<span class="hljs-keyword">const</span> {
isMainThread,
<span class="hljs-title class_">BroadcastChannel</span>,
<span class="hljs-title class_">Worker</span>
} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>);
<span class="hljs-keyword">const</span> bc = <span class="hljs-keyword">new</span> <span class="hljs-title class_">BroadcastChannel</span>(<span class="hljs-string">'hello'</span>);
<span class="hljs-keyword">if</span> (isMainThread) {
<span class="hljs-keyword">let</span> c = <span class="hljs-number">0</span>;
bc.<span class="hljs-property">onmessage</span> = <span class="hljs-function">(<span class="hljs-params">event</span>) =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(event.<span class="hljs-property">data</span>);
<span class="hljs-keyword">if</span> (++c === <span class="hljs-number">10</span>) bc.<span class="hljs-title function_">close</span>();
};
<span class="hljs-keyword">for</span> (<span class="hljs-keyword">let</span> n = <span class="hljs-number">0</span>; n < <span class="hljs-number">10</span>; n++)
<span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename);
} <span class="hljs-keyword">else</span> {
bc.<span class="hljs-title function_">postMessage</span>(<span class="hljs-string">'hello from every worker'</span>);
bc.<span class="hljs-title function_">close</span>();
}</code></pre>
<h4><code>new BroadcastChannel(name)</code><span><a class="mark" href="#new-broadcastchannelname" id="new-broadcastchannelname">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_new_broadcastchannel_name"></a></h4>
<div class="api_metadata">
<span>Added in: v15.4.0</span>
</div>
<ul>
<li><code>name</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> The name of the channel to connect to. Any JavaScript value
that can be converted to a string using <code>`${name}`</code> is permitted.</li>
</ul>
<h4><code>broadcastChannel.close()</code><span><a class="mark" href="#broadcastchannelclose" id="broadcastchannelclose">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_broadcastchannel_close"></a></h4>
<div class="api_metadata">
<span>Added in: v15.4.0</span>
</div>
<p>Closes the <code>BroadcastChannel</code> connection.</p>
<h4><code>broadcastChannel.onmessage</code><span><a class="mark" href="#broadcastchannelonmessage" id="broadcastchannelonmessage">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_broadcastchannel_onmessage"></a></h4>
<div class="api_metadata">
<span>Added in: v15.4.0</span>
</div>
<ul>
<li>Type: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> Invoked with a single <code>MessageEvent</code> argument
when a message is received.</li>
</ul>
<h4><code>broadcastChannel.onmessageerror</code><span><a class="mark" href="#broadcastchannelonmessageerror" id="broadcastchannelonmessageerror">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_broadcastchannel_onmessageerror"></a></h4>
<div class="api_metadata">
<span>Added in: v15.4.0</span>
</div>
<ul>
<li>Type: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> Invoked with a received message cannot be
deserialized.</li>
</ul>
<h4><code>broadcastChannel.postMessage(message)</code><span><a class="mark" href="#broadcastchannelpostmessagemessage" id="broadcastchannelpostmessagemessage">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_broadcastchannel_postmessage_message"></a></h4>
<div class="api_metadata">
<span>Added in: v15.4.0</span>
</div>
<ul>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> Any cloneable JavaScript value.</li>
</ul>
<h4><code>broadcastChannel.ref()</code><span><a class="mark" href="#broadcastchannelref" id="broadcastchannelref">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_broadcastchannel_ref"></a></h4>
<div class="api_metadata">
<span>Added in: v15.4.0</span>
</div>
<p>Opposite of <code>unref()</code>. Calling <code>ref()</code> on a previously <code>unref()</code>ed
BroadcastChannel does <em>not</em> let the program exit if it's the only active handle
left (the default behavior). If the port is <code>ref()</code>ed, calling <code>ref()</code> again
has no effect.</p>
<h4><code>broadcastChannel.unref()</code><span><a class="mark" href="#broadcastchannelunref" id="broadcastchannelunref">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_broadcastchannel_unref"></a></h4>
<div class="api_metadata">
<span>Added in: v15.4.0</span>
</div>
<p>Calling <code>unref()</code> on a BroadcastChannel allows the thread to exit if this
is the only active handle in the event system. If the BroadcastChannel is
already <code>unref()</code>ed calling <code>unref()</code> again has no effect.</p>
</section><section><h3>Class: <code>MessageChannel</code><span><a class="mark" href="#class-messagechannel" id="class-messagechannel">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_class_messagechannel"></a></h3>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<p>Instances of the <code>worker.MessageChannel</code> class represent an asynchronous,
two-way communications channel.
The <code>MessageChannel</code> has no methods of its own. <code>new MessageChannel()</code>
yields an object with <code>port1</code> and <code>port2</code> properties, which refer to linked
<a href="#class-messageport"><code>MessagePort</code></a> instances.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">MessageChannel</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>);
<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>();
port1.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">message</span>) =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'received'</span>, message));
port2.<span class="hljs-title function_">postMessage</span>({ <span class="hljs-attr">foo</span>: <span class="hljs-string">'bar'</span> });
<span class="hljs-comment">// Prints: received { foo: 'bar' } from the `port1.on('message')` listener</span></code></pre>
</section><section><h3>Class: <code>MessagePort</code><span><a class="mark" href="#class-messageport" id="class-messageport">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_class_messageport"></a></h3>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v14.7.0</td>
<td><p>This class now inherits from <code>EventTarget</code> rather than from <code>EventEmitter</code>.</p></td></tr>
<tr><td>v10.5.0</td>
<td><p><span>Added in: v10.5.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<ul>
<li>Extends: <a href="events.html#class-eventtarget" class="type"><EventTarget></a></li>
</ul>
<p>Instances of the <code>worker.MessagePort</code> class represent one end of an
asynchronous, two-way communications channel. It can be used to transfer
structured data, memory regions and other <code>MessagePort</code>s between different
<a href="#class-worker"><code>Worker</code></a>s.</p>
<p>This implementation matches <a href="https://developer.mozilla.org/en-US/docs/Web/API/MessagePort">browser <code>MessagePort</code></a>s.</p>
<h4>Event: <code>'close'</code><span><a class="mark" href="#event-close" id="event-close">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_event_close"></a></h4>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<p>The <code>'close'</code> event is emitted once either side of the channel has been
disconnected.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">MessageChannel</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>);
<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>();
<span class="hljs-comment">// Prints:</span>
<span class="hljs-comment">// foobar</span>
<span class="hljs-comment">// closed!</span>
port2.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">message</span>) =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(message));
port2.<span class="hljs-title function_">on</span>(<span class="hljs-string">'close'</span>, <span class="hljs-function">() =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'closed!'</span>));
port1.<span class="hljs-title function_">postMessage</span>(<span class="hljs-string">'foobar'</span>);
port1.<span class="hljs-title function_">close</span>();</code></pre>
<h4>Event: <code>'message'</code><span><a class="mark" href="#event-message" id="event-message">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_event_message"></a></h4>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<ul>
<li><code>value</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> The transmitted value</li>
</ul>
<p>The <code>'message'</code> event is emitted for any incoming message, containing the cloned
input of <a href="#portpostmessagevalue-transferlist"><code>port.postMessage()</code></a>.</p>
<p>Listeners on this event receive a clone of the <code>value</code> parameter as passed
to <code>postMessage()</code> and no further arguments.</p>
<h4>Event: <code>'messageerror'</code><span><a class="mark" href="#event-messageerror" id="event-messageerror">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_event_messageerror"></a></h4>
<div class="api_metadata">
<span>Added in: v14.5.0, v12.19.0</span>
</div>
<ul>
<li><code>error</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a> An Error object</li>
</ul>
<p>The <code>'messageerror'</code> event is emitted when deserializing a message failed.</p>
<p>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 <code>vm.Context</code> (where Node.js APIs are currently
unavailable).</p>
<h4><code>port.close()</code><span><a class="mark" href="#portclose" id="portclose">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_port_close"></a></h4>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<p>Disables further sending of messages on either side of the connection.
This method can be called when no further communication will happen over this
<code>MessagePort</code>.</p>
<p>The <a href="#event-close"><code>'close'</code> event</a> is emitted on both <code>MessagePort</code> instances that
are part of the channel.</p>
<h4><code>port.postMessage(value[, transferList])</code><span><a class="mark" href="#portpostmessagevalue-transferlist" id="portpostmessagevalue-transferlist">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_port_postmessage_value_transferlist"></a></h4>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v15.14.0</td>
<td><p>Add 'BlockList' to the list of cloneable types.</p></td></tr>
<tr><td>v15.9.0</td>
<td><p>Add 'Histogram' types to the list of cloneable types.</p></td></tr>
<tr><td>v15.6.0</td>
<td><p>Added <code>X509Certificate</code> to the list of cloneable types.</p></td></tr>
<tr><td>v15.0.0</td>
<td><p>Added <code>CryptoKey</code> to the list of cloneable types.</p></td></tr>
<tr><td>v14.5.0, v12.19.0</td>
<td><p>Added <code>KeyObject</code> to the list of cloneable types.</p></td></tr>
<tr><td>v14.5.0, v12.19.0</td>
<td><p>Added <code>FileHandle</code> to the list of transferable types.</p></td></tr>
<tr><td>v10.5.0</td>
<td><p><span>Added in: v10.5.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<ul>
<li><code>value</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>transferList</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object[]></a></li>
</ul>
<p>Sends a JavaScript value to the receiving side of this channel.
<code>value</code> is transferred in a way which is compatible with
the <a href="https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm">HTML structured clone algorithm</a>.</p>
<p>In particular, the significant differences to <code>JSON</code> are:</p>
<ul>
<li><code>value</code> may contain circular references.</li>
<li><code>value</code> may contain instances of builtin JS types such as <code>RegExp</code>s,
<code>BigInt</code>s, <code>Map</code>s, <code>Set</code>s, etc.</li>
<li><code>value</code> may contain typed arrays, both using <code>ArrayBuffer</code>s
and <code>SharedArrayBuffer</code>s.</li>
<li><code>value</code> may contain <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/Module"><code>WebAssembly.Module</code></a> instances.</li>
<li><code>value</code> may not contain native (C++-backed) objects other than:
<ul>
<li><a href="webcrypto.html#class-cryptokey" class="type"><CryptoKey></a>s,</li>
<li><a href="fs.html#class-filehandle" class="type"><FileHandle></a>s,</li>
<li><a href="perf_hooks.html#class-histogram" class="type"><Histogram></a>s,</li>
<li><a href="crypto.html#class-keyobject" class="type"><KeyObject></a>s,</li>
<li><a href="worker_threads.html#class-messageport" class="type"><MessagePort></a>s,</li>
<li><a href="net.html#class-netblocklist" class="type"><net.BlockList></a>s,</li>
<li><a href="net.html#class-netsocketaddress" class="type"><net.SocketAddress></a>es,</li>
<li><a href="crypto.html#class-x509certificate" class="type"><X509Certificate></a>s.</li>
</ul>
</li>
</ul>
<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">MessageChannel</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>);
<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>();
port1.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">message</span>) =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(message));
<span class="hljs-keyword">const</span> circularData = {};
circularData.<span class="hljs-property">foo</span> = circularData;
<span class="hljs-comment">// Prints: { foo: [Circular] }</span>
port2.<span class="hljs-title function_">postMessage</span>(circularData);</code></pre>
<p><code>transferList</code> may be a list of <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer"><code>ArrayBuffer</code></a>, <a href="#class-messageport"><code>MessagePort</code></a> and
<a href="fs.html#class-filehandle"><code>FileHandle</code></a> objects.
After transferring, they are not usable on the sending side of the channel
anymore (even if they are not contained in <code>value</code>). Unlike with
<a href="child_process.html">child processes</a>, transferring handles such as network sockets is currently
not supported.</p>
<p>If <code>value</code> contains <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer"><code>SharedArrayBuffer</code></a> instances, those are accessible
from either thread. They cannot be listed in <code>transferList</code>.</p>
<p><code>value</code> may still contain <code>ArrayBuffer</code> instances that are not in
<code>transferList</code>; in that case, the underlying memory is copied rather than moved.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">MessageChannel</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>);
<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>();
port1.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">message</span>) =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(message));
<span class="hljs-keyword">const</span> uint8Array = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Uint8Array</span>([ <span class="hljs-number">1</span>, <span class="hljs-number">2</span>, <span class="hljs-number">3</span>, <span class="hljs-number">4</span> ]);
<span class="hljs-comment">// This posts a copy of `uint8Array`:</span>
port2.<span class="hljs-title function_">postMessage</span>(uint8Array);
<span class="hljs-comment">// This does not copy data, but renders `uint8Array` unusable:</span>
port2.<span class="hljs-title function_">postMessage</span>(uint8Array, [ uint8Array.<span class="hljs-property">buffer</span> ]);
<span class="hljs-comment">// The memory for the `sharedUint8Array` is accessible from both the</span>
<span class="hljs-comment">// original and the copy received by `.on('message')`:</span>
<span class="hljs-keyword">const</span> sharedUint8Array = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Uint8Array</span>(<span class="hljs-keyword">new</span> <span class="hljs-title class_">SharedArrayBuffer</span>(<span class="hljs-number">4</span>));
port2.<span class="hljs-title function_">postMessage</span>(sharedUint8Array);
<span class="hljs-comment">// This transfers a freshly created message port to the receiver.</span>
<span class="hljs-comment">// This can be used, for example, to create communication channels between</span>
<span class="hljs-comment">// multiple `Worker` threads that are children of the same parent thread.</span>
<span class="hljs-keyword">const</span> otherChannel = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>();
port2.<span class="hljs-title function_">postMessage</span>({ <span class="hljs-attr">port</span>: otherChannel.<span class="hljs-property">port1</span> }, [ otherChannel.<span class="hljs-property">port1</span> ]);</code></pre>
<p>The message object is cloned immediately, and can be modified after
posting without having side effects.</p>
<p>For more information on the serialization and deserialization mechanisms
behind this API, see the <a href="v8.html#serialization-api">serialization API of the <code>v8</code> module</a>.</p>
<h5>Considerations when transferring TypedArrays and Buffers<span><a class="mark" href="#considerations-when-transferring-typedarrays-and-buffers" id="considerations-when-transferring-typedarrays-and-buffers">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_considerations_when_transferring_typedarrays_and_buffers"></a></h5>
<p>All <code>TypedArray</code> and <code>Buffer</code> instances are views over an underlying
<code>ArrayBuffer</code>. That is, it is the <code>ArrayBuffer</code> that actually stores
the raw data while the <code>TypedArray</code> and <code>Buffer</code> objects provide a
way of viewing and manipulating the data. It is possible and common
for multiple views to be created over the same <code>ArrayBuffer</code> instance.
Great care must be taken when using a transfer list to transfer an
<code>ArrayBuffer</code> as doing so causes all <code>TypedArray</code> and <code>Buffer</code>
instances that share that same <code>ArrayBuffer</code> to become unusable.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> ab = <span class="hljs-keyword">new</span> <span class="hljs-title class_">ArrayBuffer</span>(<span class="hljs-number">10</span>);
<span class="hljs-keyword">const</span> u1 = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Uint8Array</span>(ab);
<span class="hljs-keyword">const</span> u2 = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Uint16Array</span>(ab);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(u2.<span class="hljs-property">length</span>); <span class="hljs-comment">// prints 5</span>
port.<span class="hljs-title function_">postMessage</span>(u1, [u1.<span class="hljs-property">buffer</span>]);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(u2.<span class="hljs-property">length</span>); <span class="hljs-comment">// prints 0</span></code></pre>
<p>For <code>Buffer</code> instances, specifically, whether the underlying
<code>ArrayBuffer</code> can be transferred or cloned depends entirely on how
instances were created, which often cannot be reliably determined.</p>
<p>An <code>ArrayBuffer</code> can be marked with <a href="#workermarkasuntransferableobject"><code>markAsUntransferable()</code></a> to indicate
that it should always be cloned and never transferred.</p>
<p>Depending on how a <code>Buffer</code> instance was created, it may or may
not own its underlying <code>ArrayBuffer</code>. An <code>ArrayBuffer</code> must not
be transferred unless it is known that the <code>Buffer</code> instance
owns it. In particular, for <code>Buffer</code>s created from the internal
<code>Buffer</code> pool (using, for instance <code>Buffer.from()</code> or <code>Buffer.allocUnsafe()</code>),
transferring them is not possible and they are always cloned,
which sends a copy of the entire <code>Buffer</code> pool.
This behavior may come with unintended higher memory
usage and possible security concerns.</p>
<p>See <a href="buffer.html#static-method-bufferallocunsafesize"><code>Buffer.allocUnsafe()</code></a> for more details on <code>Buffer</code> pooling.</p>
<p>The <code>ArrayBuffer</code>s for <code>Buffer</code> instances created using
<code>Buffer.alloc()</code> or <code>Buffer.allocUnsafeSlow()</code> can always be
transferred but doing so renders all other existing views of
those <code>ArrayBuffer</code>s unusable.</p>
<h5>Considerations when cloning objects with prototypes, classes, and accessors<span><a class="mark" href="#considerations-when-cloning-objects-with-prototypes-classes-and-accessors" id="considerations-when-cloning-objects-with-prototypes-classes-and-accessors">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_considerations_when_cloning_objects_with_prototypes_classes_and_accessors"></a></h5>
<p>Because object cloning uses the <a href="https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm">HTML structured clone algorithm</a>,
non-enumerable properties, property accessors, and object prototypes are
not preserved. In particular, <a href="buffer.html"><code>Buffer</code></a> objects will be read as
plain <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array"><code>Uint8Array</code></a>s on the receiving side, and instances of JavaScript
classes will be cloned as plain JavaScript objects.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> b = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'b'</span>);
<span class="hljs-keyword">class</span> <span class="hljs-title class_">Foo</span> {
#a = <span class="hljs-number">1</span>;
<span class="hljs-title function_">constructor</span>(<span class="hljs-params"></span>) {
<span class="hljs-variable language_">this</span>[b] = <span class="hljs-number">2</span>;
<span class="hljs-variable language_">this</span>.<span class="hljs-property">c</span> = <span class="hljs-number">3</span>;
}
<span class="hljs-keyword">get</span> <span class="hljs-title function_">d</span>() { <span class="hljs-keyword">return</span> <span class="hljs-number">4</span>; }
}
<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>();
port1.<span class="hljs-property">onmessage</span> = <span class="hljs-function">(<span class="hljs-params">{ data }</span>) =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(data);
port2.<span class="hljs-title function_">postMessage</span>(<span class="hljs-keyword">new</span> <span class="hljs-title class_">Foo</span>());
<span class="hljs-comment">// Prints: { c: 3 }</span></code></pre>
<p>This limitation extends to many built-in objects, such as the global <code>URL</code>
object:</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>();
port1.<span class="hljs-property">onmessage</span> = <span class="hljs-function">(<span class="hljs-params">{ data }</span>) =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(data);
port2.<span class="hljs-title function_">postMessage</span>(<span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(<span class="hljs-string">'https://example.org'</span>));
<span class="hljs-comment">// Prints: { }</span></code></pre>
<h4><code>port.ref()</code><span><a class="mark" href="#portref" id="portref">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_port_ref"></a></h4>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<p>Opposite of <code>unref()</code>. Calling <code>ref()</code> on a previously <code>unref()</code>ed port does
<em>not</em> let the program exit if it's the only active handle left (the default
behavior). If the port is <code>ref()</code>ed, calling <code>ref()</code> again has no effect.</p>
<p>If listeners are attached or removed using <code>.on('message')</code>, the port
is <code>ref()</code>ed and <code>unref()</code>ed automatically depending on whether
listeners for the event exist.</p>
<h4><code>port.start()</code><span><a class="mark" href="#portstart" id="portstart">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_port_start"></a></h4>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<p>Starts receiving messages on this <code>MessagePort</code>. When using this port
as an event emitter, this is called automatically once <code>'message'</code>
listeners are attached.</p>
<p>This method exists for parity with the Web <code>MessagePort</code> 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 <code>.onmessage</code>. Setting it
automatically calls <code>.start()</code>, but unsetting it lets messages queue up
until a new handler is set or the port is discarded.</p>
<h4><code>port.unref()</code><span><a class="mark" href="#portunref" id="portunref">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_port_unref"></a></h4>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<p>Calling <code>unref()</code> on a port allows the thread to exit if this is the only
active handle in the event system. If the port is already <code>unref()</code>ed calling
<code>unref()</code> again has no effect.</p>
<p>If listeners are attached or removed using <code>.on('message')</code>, the port is
<code>ref()</code>ed and <code>unref()</code>ed automatically depending on whether
listeners for the event exist.</p>
</section><section><h3>Class: <code>Worker</code><span><a class="mark" href="#class-worker" id="class-worker">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_class_worker"></a></h3>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<ul>
<li>Extends: <a href="events.html#class-eventemitter" class="type"><EventEmitter></a></li>
</ul>
<p>The <code>Worker</code> class represents an independent JavaScript execution thread.
Most Node.js APIs are available inside of it.</p>
<p>Notable differences inside a Worker environment are:</p>
<ul>
<li>The <a href="process.html#processstdin"><code>process.stdin</code></a>, <a href="process.html#processstdout"><code>process.stdout</code></a> and <a href="process.html#processstderr"><code>process.stderr</code></a>
may be redirected by the parent thread.</li>
<li>The <a href="#workerismainthread"><code>require('worker_threads').isMainThread</code></a> property is set to <code>false</code>.</li>
<li>The <a href="#workerparentport"><code>require('worker_threads').parentPort</code></a> message port is available.</li>
<li><a href="process.html#processexitcode"><code>process.exit()</code></a> does not stop the whole program, just the single thread,
and <a href="process.html#processabort"><code>process.abort()</code></a> is not available.</li>
<li><a href="process.html#processchdirdirectory"><code>process.chdir()</code></a> and <code>process</code> methods that set group or user ids
are not available.</li>
<li><a href="process.html#processenv"><code>process.env</code></a> 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
<a href="#workershare_env"><code>worker.SHARE_ENV</code></a> is passed as the <code>env</code> option to the
<a href="#class-worker"><code>Worker</code></a> constructor).</li>
<li><a href="process.html#processtitle"><code>process.title</code></a> cannot be modified.</li>
<li>Signals are not delivered through <a href="process.html#signal-events"><code>process.on('...')</code></a>.</li>
<li>Execution may stop at any point as a result of <a href="#workerterminate"><code>worker.terminate()</code></a>
being invoked.</li>
<li>IPC channels from parent processes are not accessible.</li>
<li>The <a href="tracing.html"><code>trace_events</code></a> module is not supported.</li>
<li>Native add-ons can only be loaded from multiple threads if they fulfill
<a href="addons.html#worker-support">certain conditions</a>.</li>
</ul>
<p>Creating <code>Worker</code> instances inside of other <code>Worker</code>s is possible.</p>
<p>Like <a href="https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API">Web Workers</a> and the <a href="cluster.html"><code>cluster</code> module</a>, two-way communication can be
achieved through inter-thread message passing. Internally, a <code>Worker</code> has a
built-in pair of <a href="#class-messageport"><code>MessagePort</code></a>s that are already associated with each other
when the <code>Worker</code> is created. While the <code>MessagePort</code> object on the parent side
is not directly exposed, its functionalities are exposed through
<a href="#workerpostmessagevalue-transferlist"><code>worker.postMessage()</code></a> and the <a href="#event-message_1"><code>worker.on('message')</code></a> event
on the <code>Worker</code> object for the parent thread.</p>
<p>To create custom messaging channels (which is encouraged over using the default
global channel because it facilitates separation of concerns), users can create
a <code>MessageChannel</code> object on either thread and pass one of the
<code>MessagePort</code>s on that <code>MessageChannel</code> to the other thread through a
pre-existing channel, such as the global one.</p>
<p>See <a href="#portpostmessagevalue-transferlist"><code>port.postMessage()</code></a> for more information on how messages are passed,
and what kind of JavaScript values can be successfully transported through
the thread barrier.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> assert = <span class="hljs-built_in">require</span>(<span class="hljs-string">'assert'</span>);
<span class="hljs-keyword">const</span> {
<span class="hljs-title class_">Worker</span>, <span class="hljs-title class_">MessageChannel</span>, <span class="hljs-title class_">MessagePort</span>, isMainThread, parentPort
} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>);
<span class="hljs-keyword">if</span> (isMainThread) {
<span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename);
<span class="hljs-keyword">const</span> subChannel = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>();
worker.<span class="hljs-title function_">postMessage</span>({ <span class="hljs-attr">hereIsYourPort</span>: subChannel.<span class="hljs-property">port1</span> }, [subChannel.<span class="hljs-property">port1</span>]);
subChannel.<span class="hljs-property">port2</span>.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">value</span>) =></span> {
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'received:'</span>, value);
});
} <span class="hljs-keyword">else</span> {
parentPort.<span class="hljs-title function_">once</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">value</span>) =></span> {
<span class="hljs-title function_">assert</span>(value.<span class="hljs-property">hereIsYourPort</span> <span class="hljs-keyword">instanceof</span> <span class="hljs-title class_">MessagePort</span>);
value.<span class="hljs-property">hereIsYourPort</span>.<span class="hljs-title function_">postMessage</span>(<span class="hljs-string">'the worker is sending this'</span>);
value.<span class="hljs-property">hereIsYourPort</span>.<span class="hljs-title function_">close</span>();
});
}</code></pre>
<h4><code>new Worker(filename[, options])</code><span><a class="mark" href="#new-workerfilename-options" id="new-workerfilename-options">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_new_worker_filename_options"></a></h4>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v14.9.0</td>
<td><p>The <code>filename</code> parameter can be a WHATWG <code>URL</code> object using <code>data:</code> protocol.</p></td></tr>
<tr><td>v14.9.0</td>
<td><p>The <code>trackUnmanagedFds</code> option was set to <code>true</code> by default.</p></td></tr>
<tr><td>v14.6.0, v12.19.0</td>
<td><p>The <code>trackUnmanagedFds</code> option was introduced.</p></td></tr>
<tr><td>v13.13.0, v12.17.0</td>
<td><p>The <code>transferList</code> option was introduced.</p></td></tr>
<tr><td>v13.12.0, v12.17.0</td>
<td><p>The <code>filename</code> parameter can be a WHATWG <code>URL</code> object using <code>file:</code> protocol.</p></td></tr>
<tr><td>v13.4.0, v12.16.0</td>
<td><p>The <code>argv</code> option was introduced.</p></td></tr>
<tr><td>v13.2.0, v12.16.0</td>
<td><p>The <code>resourceLimits</code> option was introduced.</p></td></tr>
<tr><td>v10.5.0</td>
<td><p><span>Added in: v10.5.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<ul>
<li><code>filename</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="url.html#the-whatwg-url-api" class="type"><URL></a> 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 <code>./</code> or <code>../</code>, or a WHATWG <code>URL</code>
object using <code>file:</code> or <code>data:</code> protocol.
When using a <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URIs"><code>data:</code> URL</a>, the data is interpreted based on MIME type using
the <a href="esm.html#data-imports">ECMAScript module loader</a>.
If <code>options.eval</code> is <code>true</code>, this is a string containing JavaScript code
rather than a path.</li>
<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a>
<ul>
<li><code>argv</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any[]></a> List of arguments which would be stringified and appended to
<code>process.argv</code> in the worker. This is mostly similar to the <code>workerData</code>
but the values are available on the global <code>process.argv</code> as if they
were passed as CLI options to the script.</li>
<li><code>env</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> If set, specifies the initial value of <code>process.env</code> inside
the Worker thread. As a special value, <a href="#workershare_env"><code>worker.SHARE_ENV</code></a> 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 <code>process.env</code>
object affect the other thread as well. <strong>Default:</strong> <code>process.env</code>.</li>
<li><code>eval</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> If <code>true</code> and the first argument is a <code>string</code>, interpret
the first argument to the constructor as a script that is executed once the
worker is online.</li>
<li><code>execArgv</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string[]></a> List of node CLI options passed to the worker.
V8 options (such as <code>--max-old-space-size</code>) and options that affect the
process (such as <code>--title</code>) are not supported. If set, this is provided
as <a href="process.html#processexecargv"><code>process.execArgv</code></a> inside the worker. By default, options are
inherited from the parent thread.</li>
<li><code>stdin</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> If this is set to <code>true</code>, then <code>worker.stdin</code>
provides a writable stream whose contents appear as <code>process.stdin</code>
inside the Worker. By default, no data is provided.</li>
<li><code>stdout</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> If this is set to <code>true</code>, then <code>worker.stdout</code> is
not automatically piped through to <code>process.stdout</code> in the parent.</li>
<li><code>stderr</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> If this is set to <code>true</code>, then <code>worker.stderr</code> is
not automatically piped through to <code>process.stderr</code> in the parent.</li>
<li><code>workerData</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> Any JavaScript value that is cloned and made
available as <a href="#workerworkerdata"><code>require('worker_threads').workerData</code></a>. The cloning
occurs as described in the <a href="https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm">HTML structured clone algorithm</a>, and an error
is thrown if the object cannot be cloned (e.g. because it contains
<code>function</code>s).</li>
<li><code>trackUnmanagedFds</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> If this is set to <code>true</code>, then the Worker
tracks raw file descriptors managed through <a href="fs.html#fsopenpath-flags-mode-callback"><code>fs.open()</code></a> and
<a href="fs.html#fsclosefd-callback"><code>fs.close()</code></a>, and closes them when the Worker exits, similar to other
resources like network sockets or file descriptors managed through
the <a href="fs.html#class-filehandle"><code>FileHandle</code></a> API. This option is automatically inherited by all
nested <code>Worker</code>s. <strong>Default:</strong> <code>true</code>.</li>
<li><code>transferList</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object[]></a> If one or more <code>MessagePort</code>-like objects
are passed in <code>workerData</code>, a <code>transferList</code> is required for those
items or <a href="errors.html#err_missing_message_port_in_transfer_list"><code>ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST</code></a> is thrown.
See <a href="#portpostmessagevalue-transferlist"><code>port.postMessage()</code></a> for more information.</li>
<li><code>resourceLimits</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> An optional set of resource limits for the new
JS engine instance. Reaching these limits leads to termination of the
<code>Worker</code> instance. These limits only affect the JS engine, and no external
data, including no <code>ArrayBuffer</code>s. Even if these limits are set, the process
may still abort if it encounters a global out-of-memory situation.
<ul>
<li><code>maxOldGenerationSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The maximum size of the main heap in MB.</li>
<li><code>maxYoungGenerationSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The maximum size of a heap space for
recently created objects.</li>
<li><code>codeRangeSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The size of a pre-allocated memory range
used for generated code.</li>
<li><code>stackSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The default maximum stack size for the thread.
Small values may lead to unusable Worker instances. <strong>Default:</strong> <code>4</code>.</li>
</ul>
</li>
</ul>
</li>
</ul>
<h4>Event: <code>'error'</code><span><a class="mark" href="#event-error" id="event-error">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_event_error"></a></h4>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<ul>
<li><code>err</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a></li>
</ul>
<p>The <code>'error'</code> event is emitted if the worker thread throws an uncaught
exception. In that case, the worker is terminated.</p>
<h4>Event: <code>'exit'</code><span><a class="mark" href="#event-exit" id="event-exit">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_event_exit"></a></h4>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<ul>
<li><code>exitCode</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><integer></a></li>
</ul>
<p>The <code>'exit'</code> event is emitted once the worker has stopped. If the worker
exited by calling <a href="process.html#processexitcode"><code>process.exit()</code></a>, the <code>exitCode</code> parameter is the
passed exit code. If the worker was terminated, the <code>exitCode</code> parameter is
<code>1</code>.</p>
<p>This is the final event emitted by any <code>Worker</code> instance.</p>
<h4>Event: <code>'message'</code><span><a class="mark" href="#event-message_1" id="event-message_1">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_event_message_1"></a></h4>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<ul>
<li><code>value</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> The transmitted value</li>
</ul>
<p>The <code>'message'</code> event is emitted when the worker thread has invoked
<a href="#workerpostmessagevalue-transferlist"><code>require('worker_threads').parentPort.postMessage()</code></a>.
See the <a href="#event-message"><code>port.on('message')</code></a> event for more details.</p>
<p>All messages sent from the worker thread are emitted before the
<a href="#event-exit"><code>'exit'</code> event</a> is emitted on the <code>Worker</code> object.</p>
<h4>Event: <code>'messageerror'</code><span><a class="mark" href="#event-messageerror_1" id="event-messageerror_1">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_event_messageerror_1"></a></h4>
<div class="api_metadata">
<span>Added in: v14.5.0, v12.19.0</span>
</div>
<ul>
<li><code>error</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a> An Error object</li>
</ul>
<p>The <code>'messageerror'</code> event is emitted when deserializing a message failed.</p>
<h4>Event: <code>'online'</code><span><a class="mark" href="#event-online" id="event-online">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_event_online"></a></h4>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<p>The <code>'online'</code> event is emitted when the worker thread has started executing
JavaScript code.</p>
<h4><code>worker.getHeapSnapshot()</code><span><a class="mark" href="#workergetheapsnapshot" id="workergetheapsnapshot">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_getheapsnapshot"></a></h4>
<div class="api_metadata">
<span>Added in: v13.9.0, v12.17.0</span>
</div>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise" class="type"><Promise></a> A promise for a Readable Stream containing
a V8 heap snapshot</li>
</ul>
<p>Returns a readable stream for a V8 snapshot of the current state of the Worker.
See <a href="v8.html#v8getheapsnapshot"><code>v8.getHeapSnapshot()</code></a> for more details.</p>
<p>If the Worker thread is no longer running, which may occur before the
<a href="#event-exit"><code>'exit'</code> event</a> is emitted, the returned <code>Promise</code> is rejected
immediately with an <a href="errors.html#err_worker_not_running"><code>ERR_WORKER_NOT_RUNNING</code></a> error.</p>
<h4><code>worker.performance</code><span><a class="mark" href="#workerperformance" id="workerperformance">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_performance"></a></h4>
<div class="api_metadata">
<span>Added in: v15.1.0, v12.22.0</span>
</div>
<p>An object that can be used to query performance information from a worker
instance. Similar to <a href="perf_hooks.html#perf_hooksperformance"><code>perf_hooks.performance</code></a>.</p>
<h5><code>performance.eventLoopUtilization([utilization1[, utilization2]])</code><span><a class="mark" href="#performanceeventlooputilizationutilization1-utilization2" id="performanceeventlooputilizationutilization1-utilization2">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_performance_eventlooputilization_utilization1_utilization2"></a></h5>
<div class="api_metadata">
<span>Added in: v15.1.0, v12.22.0</span>
</div>
<ul>
<li><code>utilization1</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> The result of a previous call to
<code>eventLoopUtilization()</code>.</li>
<li><code>utilization2</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> The result of a previous call to
<code>eventLoopUtilization()</code> prior to <code>utilization1</code>.</li>
<li>Returns <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a>
<ul>
<li><code>idle</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li>
<li><code>active</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li>
<li><code>utilization</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li>
</ul>
</li>
</ul>
<p>The same call as <a href="perf_hooks.html#performanceeventlooputilizationutilization1-utilization2"><code>perf_hooks</code> <code>eventLoopUtilization()</code></a>, except the values
of the worker instance are returned.</p>
<p>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.</p>
<p>An <code>idle</code> 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 <code>idle</code> time, but is still be able to process
messages.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">Worker</span>, isMainThread, parentPort } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>);
<span class="hljs-keyword">if</span> (isMainThread) {
<span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename);
<span class="hljs-built_in">setInterval</span>(<span class="hljs-function">() =></span> {
worker.<span class="hljs-title function_">postMessage</span>(<span class="hljs-string">'hi'</span>);
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(worker.<span class="hljs-property">performance</span>.<span class="hljs-title function_">eventLoopUtilization</span>());
}, <span class="hljs-number">100</span>).<span class="hljs-title function_">unref</span>();
<span class="hljs-keyword">return</span>;
}
parentPort.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">() =></span> <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'msg'</span>)).<span class="hljs-title function_">unref</span>();
(<span class="hljs-keyword">function</span> <span class="hljs-title function_">r</span>(<span class="hljs-params">n</span>) {
<span class="hljs-keyword">if</span> (--n < <span class="hljs-number">0</span>) <span class="hljs-keyword">return</span>;
<span class="hljs-keyword">const</span> t = <span class="hljs-title class_">Date</span>.<span class="hljs-title function_">now</span>();
<span class="hljs-keyword">while</span> (<span class="hljs-title class_">Date</span>.<span class="hljs-title function_">now</span>() - t < <span class="hljs-number">300</span>);
<span class="hljs-title function_">setImmediate</span>(r, n);
})(<span class="hljs-number">10</span>);</code></pre>
<p>The event loop utilization of a worker is available only after the <a href="#event-online"><code>'online'</code>
event</a> emitted, and if called before this, or after the <a href="#event-exit"><code>'exit'</code>
event</a>, then all properties have the value of <code>0</code>.</p>
<h4><code>worker.postMessage(value[, transferList])</code><span><a class="mark" href="#workerpostmessagevalue-transferlist" id="workerpostmessagevalue-transferlist">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_postmessage_value_transferlist"></a></h4>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<ul>
<li><code>value</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li>
<li><code>transferList</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object[]></a></li>
</ul>
<p>Send a message to the worker that is received via
<a href="#event-message"><code>require('worker_threads').parentPort.on('message')</code></a>.
See <a href="#portpostmessagevalue-transferlist"><code>port.postMessage()</code></a> for more details.</p>
<h4><code>worker.ref()</code><span><a class="mark" href="#workerref" id="workerref">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_ref"></a></h4>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<p>Opposite of <code>unref()</code>, calling <code>ref()</code> on a previously <code>unref()</code>ed worker does
<em>not</em> let the program exit if it's the only active handle left (the default
behavior). If the worker is <code>ref()</code>ed, calling <code>ref()</code> again has
no effect.</p>
<h4><code>worker.resourceLimits</code><span><a class="mark" href="#workerresourcelimits_1" id="workerresourcelimits_1">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_resourcelimits_1"></a></h4>
<div class="api_metadata">
<span>Added in: v13.2.0, v12.16.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a>
<ul>
<li><code>maxYoungGenerationSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li>
<li><code>maxOldGenerationSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li>
<li><code>codeRangeSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li>
<li><code>stackSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li>
</ul>
</li>
</ul>
<p>Provides the set of JS engine resource constraints for this Worker thread.
If the <code>resourceLimits</code> option was passed to the <a href="#class-worker"><code>Worker</code></a> constructor,
this matches its values.</p>
<p>If the worker has stopped, the return value is an empty object.</p>
<h4><code>worker.stderr</code><span><a class="mark" href="#workerstderr" id="workerstderr">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_stderr"></a></h4>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<ul>
<li><a href="stream.html#class-streamreadable" class="type"><stream.Readable></a></li>
</ul>
<p>This is a readable stream which contains data written to <a href="process.html#processstderr"><code>process.stderr</code></a>
inside the worker thread. If <code>stderr: true</code> was not passed to the
<a href="#class-worker"><code>Worker</code></a> constructor, then data is piped to the parent thread's
<a href="process.html#processstderr"><code>process.stderr</code></a> stream.</p>
<h4><code>worker.stdin</code><span><a class="mark" href="#workerstdin" id="workerstdin">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_stdin"></a></h4>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Null_type" class="type"><null></a> | <a href="stream.html#class-streamwritable" class="type"><stream.Writable></a></li>
</ul>
<p>If <code>stdin: true</code> was passed to the <a href="#class-worker"><code>Worker</code></a> constructor, this is a
writable stream. The data written to this stream will be made available in
the worker thread as <a href="process.html#processstdin"><code>process.stdin</code></a>.</p>
<h4><code>worker.stdout</code><span><a class="mark" href="#workerstdout" id="workerstdout">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_stdout"></a></h4>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<ul>
<li><a href="stream.html#class-streamreadable" class="type"><stream.Readable></a></li>
</ul>
<p>This is a readable stream which contains data written to <a href="process.html#processstdout"><code>process.stdout</code></a>
inside the worker thread. If <code>stdout: true</code> was not passed to the
<a href="#class-worker"><code>Worker</code></a> constructor, then data is piped to the parent thread's
<a href="process.html#processstdout"><code>process.stdout</code></a> stream.</p>
<h4><code>worker.terminate()</code><span><a class="mark" href="#workerterminate" id="workerterminate">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_terminate"></a></h4>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v12.5.0</td>
<td><p>This function now returns a Promise. Passing a callback is deprecated, and was useless up to this version, as the Worker was actually terminated synchronously. Terminating is now a fully asynchronous operation.</p></td></tr>
<tr><td>v10.5.0</td>
<td><p><span>Added in: v10.5.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise" class="type"><Promise></a></li>
</ul>
<p>Stop all JavaScript execution in the worker thread as soon as possible.
Returns a Promise for the exit code that is fulfilled when the
<a href="#event-exit"><code>'exit'</code> event</a> is emitted.</p>
<h4><code>worker.threadId</code><span><a class="mark" href="#workerthreadid_1" id="workerthreadid_1">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_threadid_1"></a></h4>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<ul>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><integer></a></li>
</ul>
<p>An integer identifier for the referenced thread. Inside the worker thread,
it is available as <a href="#workerthreadid"><code>require('worker_threads').threadId</code></a>.
This value is unique for each <code>Worker</code> instance inside a single process.</p>
<h4><code>worker.unref()</code><span><a class="mark" href="#workerunref" id="workerunref">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_worker_unref"></a></h4>
<div class="api_metadata">
<span>Added in: v10.5.0</span>
</div>
<p>Calling <code>unref()</code> on a worker allows the thread to exit if this is the only
active handle in the event system. If the worker is already <code>unref()</code>ed calling
<code>unref()</code> again has no effect.</p>
</section><section><h3>Notes<span><a class="mark" href="#notes" id="notes">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_notes"></a></h3>
<h4>Synchronous blocking of stdio<span><a class="mark" href="#synchronous-blocking-of-stdio" id="synchronous-blocking-of-stdio">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_synchronous_blocking_of_stdio"></a></h4>
<p><code>Worker</code>s utilize message passing via <a href="worker_threads.html#class-messageport" class="type"><MessagePort></a> to implement interactions
with <code>stdio</code>. This means that <code>stdio</code> output originating from a <code>Worker</code> can
get blocked by synchronous code on the receiving end that is blocking the
Node.js event loop.</p>
<pre><input class="js-flavor-selector" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> {
<span class="hljs-title class_">Worker</span>,
isMainThread,
} <span class="hljs-keyword">from</span> <span class="hljs-string">'worker_threads'</span>;
<span class="hljs-keyword">if</span> (isMainThread) {
<span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(<span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(<span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>));
<span class="hljs-keyword">for</span> (<span class="hljs-keyword">let</span> n = <span class="hljs-number">0</span>; n < <span class="hljs-number">1e10</span>; n++) {
<span class="hljs-comment">// Looping to simulate work.</span>
}
} <span class="hljs-keyword">else</span> {
<span class="hljs-comment">// This output will be blocked by the for loop in the main thread.</span>
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'foo'</span>);
}</code><code class="language-js cjs"><span class="hljs-meta">'use strict'</span>;
<span class="hljs-keyword">const</span> {
<span class="hljs-title class_">Worker</span>,
isMainThread,
} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>);
<span class="hljs-keyword">if</span> (isMainThread) {
<span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename);
<span class="hljs-keyword">for</span> (<span class="hljs-keyword">let</span> n = <span class="hljs-number">0</span>; n < <span class="hljs-number">1e10</span>; n++) {
<span class="hljs-comment">// Looping to simulate work.</span>
}
} <span class="hljs-keyword">else</span> {
<span class="hljs-comment">// This output will be blocked by the for loop in the main thread.</span>
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'foo'</span>);
}</code></pre>
<h4>Launching worker threads from preload scripts<span><a class="mark" href="#launching-worker-threads-from-preload-scripts" id="launching-worker-threads-from-preload-scripts">#</a></span><a aria-hidden="true" class="legacy" id="worker_threads_launching_worker_threads_from_preload_scripts"></a></h4>
<p>Take care when launching worker threads from preload scripts (scripts loaded
and run using the <code>-r</code> command line flag). Unless the <code>execArgv</code> 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.</p></section>
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