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data-id="worker_threads" class="interior"> 90 <header> 91 <div class="header-container"> 92 <h1>Node.js v14.20.1 documentation</h1> 93 <button class="theme-toggle-btn" id="theme-toggle-btn" title="Toggle dark mode/light mode" aria-label="Toggle dark mode/light mode" hidden> 94 <svg xmlns="http://www.w3.org/2000/svg" class="icon dark-icon" height="24" width="24"> 95 <path fill="none" d="M0 0h24v24H0z" /> 96 <path d="M11.1 12.08c-2.33-4.51-.5-8.48.53-10.07C6.27 2.2 1.98 6.59 1.98 12c0 .14.02.28.02.42.62-.27 1.29-.42 2-.42 1.66 0 3.18.83 4.1 2.15A4.01 4.01 0 0111 18c0 1.52-.87 2.83-2.12 3.51.98.32 2.03.5 3.11.5 3.5 0 6.58-1.8 8.37-4.52-2.36.23-6.98-.97-9.26-5.41z"/> 97 <path d="M7 16h-.18C6.4 14.84 5.3 14 4 14c-1.66 0-3 1.34-3 3s1.34 3 3 3h3c1.1 0 2-.9 2-2s-.9-2-2-2z"/> 98 </svg> 99 <svg xmlns="http://www.w3.org/2000/svg" class="icon light-icon" height="24" width="24"> 100 <path d="M0 0h24v24H0z" fill="none" /> 101 <path d="M6.76 4.84l-1.8-1.79-1.41 1.41 1.79 1.79 1.42-1.41zM4 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href="https://nodejs.org/docs/latest-v15.x/api/worker_threads.html">15.x</a></li> 123<li><a href="https://nodejs.org/docs/latest-v14.x/api/worker_threads.html">14.x <b>LTS</b></a></li> 124<li><a href="https://nodejs.org/docs/latest-v13.x/api/worker_threads.html">13.x</a></li> 125<li><a href="https://nodejs.org/docs/latest-v12.x/api/worker_threads.html">12.x <b>LTS</b></a></li> 126<li><a href="https://nodejs.org/docs/latest-v11.x/api/worker_threads.html">11.x</a></li> 127<li><a href="https://nodejs.org/docs/latest-v10.x/api/worker_threads.html">10.x</a></li></ol> 128 </li> 129 130 <li class="edit_on_github"><a href="https://github.com/nodejs/node/edit/master/doc/api/worker_threads.md">Edit on GitHub</a></li> 131 </ul> 132 </div> 133 <hr> 134 </header> 135 136 <details id="toc" open><summary>Table of contents</summary><ul> 137<li><span class="stability_2"><a href="#worker_threads_worker_threads">Worker threads</a></span> 138<ul> 139<li><span class="stability_1"><a href="#worker_threads_worker_getenvironmentdata_key"><code>worker.getEnvironmentData(key)</code></a></span></li> 140<li><a href="#worker_threads_worker_ismainthread"><code>worker.isMainThread</code></a></li> 141<li><a href="#worker_threads_worker_markasuntransferable_object"><code>worker.markAsUntransferable(object)</code></a></li> 142<li><a href="#worker_threads_worker_movemessageporttocontext_port_contextifiedsandbox"><code>worker.moveMessagePortToContext(port, contextifiedSandbox)</code></a></li> 143<li><a href="#worker_threads_worker_parentport"><code>worker.parentPort</code></a></li> 144<li><a href="#worker_threads_worker_receivemessageonport_port"><code>worker.receiveMessageOnPort(port)</code></a></li> 145<li><a href="#worker_threads_worker_resourcelimits"><code>worker.resourceLimits</code></a></li> 146<li><a href="#worker_threads_worker_share_env"><code>worker.SHARE_ENV</code></a></li> 147<li><span class="stability_1"><a href="#worker_threads_worker_setenvironmentdata_key_value"><code>worker.setEnvironmentData(key[, value])</code></a></span></li> 148<li><a href="#worker_threads_worker_threadid"><code>worker.threadId</code></a></li> 149<li><a href="#worker_threads_worker_workerdata"><code>worker.workerData</code></a></li> 150<li><a href="#worker_threads_class_messagechannel">Class: <code>MessageChannel</code></a></li> 151<li><a href="#worker_threads_class_messageport">Class: <code>MessagePort</code></a> 152<ul> 153<li><a href="#worker_threads_event_close">Event: <code>'close'</code></a></li> 154<li><a href="#worker_threads_event_message">Event: <code>'message'</code></a></li> 155<li><a href="#worker_threads_event_messageerror">Event: <code>'messageerror'</code></a></li> 156<li><a href="#worker_threads_port_close"><code>port.close()</code></a></li> 157<li><a href="#worker_threads_port_postmessage_value_transferlist"><code>port.postMessage(value[, transferList])</code></a> 158<ul> 159<li><a href="#worker_threads_considerations_when_transferring_typedarrays_and_buffers">Considerations when transferring TypedArrays and Buffers</a></li> 160</ul> 161</li> 162<li><a href="#worker_threads_port_ref"><code>port.ref()</code></a></li> 163<li><a href="#worker_threads_port_start"><code>port.start()</code></a></li> 164<li><a href="#worker_threads_port_unref"><code>port.unref()</code></a></li> 165</ul> 166</li> 167<li><a href="#worker_threads_class_worker">Class: <code>Worker</code></a> 168<ul> 169<li><a href="#worker_threads_new_worker_filename_options"><code>new Worker(filename[, options])</code></a></li> 170<li><a href="#worker_threads_event_error">Event: <code>'error'</code></a></li> 171<li><a href="#worker_threads_event_exit">Event: <code>'exit'</code></a></li> 172<li><a href="#worker_threads_event_message_1">Event: <code>'message'</code></a></li> 173<li><a href="#worker_threads_event_messageerror_1">Event: <code>'messageerror'</code></a></li> 174<li><a href="#worker_threads_event_online">Event: <code>'online'</code></a></li> 175<li><a href="#worker_threads_worker_getheapsnapshot"><code>worker.getHeapSnapshot()</code></a></li> 176<li><a href="#worker_threads_worker_performance"><code>worker.performance</code></a> 177<ul> 178<li><a href="#worker_threads_performance_eventlooputilization_utilization1_utilization2"><code>performance.eventLoopUtilization([utilization1[, utilization2]])</code></a></li> 179</ul> 180</li> 181<li><a href="#worker_threads_worker_postmessage_value_transferlist"><code>worker.postMessage(value[, transferList])</code></a></li> 182<li><a href="#worker_threads_worker_ref"><code>worker.ref()</code></a></li> 183<li><a href="#worker_threads_worker_resourcelimits_1"><code>worker.resourceLimits</code></a></li> 184<li><a href="#worker_threads_worker_stderr"><code>worker.stderr</code></a></li> 185<li><a href="#worker_threads_worker_stdin"><code>worker.stdin</code></a></li> 186<li><a href="#worker_threads_worker_stdout"><code>worker.stdout</code></a></li> 187<li><a href="#worker_threads_worker_terminate"><code>worker.terminate()</code></a></li> 188<li><a href="#worker_threads_worker_threadid_1"><code>worker.threadId</code></a></li> 189<li><a href="#worker_threads_worker_unref"><code>worker.unref()</code></a></li> 190</ul> 191</li> 192<li><a href="#worker_threads_notes">Notes</a> 193<ul> 194<li><a href="#worker_threads_synchronous_blocking_of_stdio">Synchronous blocking of stdio</a></li> 195<li><a href="#worker_threads_launching_worker_threads_from_preload_scripts">Launching worker threads from preload scripts</a></li> 196</ul> 197</li> 198</ul> 199</li> 200</ul></details> 201 202 <div id="apicontent"> 203 <h2>Worker threads<span><a class="mark" href="#worker_threads_worker_threads" id="worker_threads_worker_threads">#</a></span></h2> 204 205<p></p><div class="api_stability api_stability_2"><a href="documentation.html#documentation_stability_index">Stability: 2</a> - Stable</div><p></p> 206<p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v14.20.1/lib/worker_threads.js">lib/worker_threads.js</a></p> 207<p>The <code>worker_threads</code> module enables the use of threads that execute JavaScript 208in parallel. To access it:</p> 209<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> 210<p>Workers (threads) are useful for performing CPU-intensive JavaScript operations. 211They will not help much with I/O-intensive work. Node.js’s built-in asynchronous 212I/O operations are more efficient than Workers can be.</p> 213<p>Unlike <code>child_process</code> or <code>cluster</code>, <code>worker_threads</code> can share memory. They do 214so by transferring <code>ArrayBuffer</code> instances or sharing <code>SharedArrayBuffer</code> 215instances.</p> 216<pre><code class="language-js"><span class="hljs-keyword">const</span> { 217 <span class="hljs-title class_">Worker</span>, isMainThread, parentPort, workerData 218} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>); 219 220<span class="hljs-keyword">if</span> (isMainThread) { 221 <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>) { 222 <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> { 223 <span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename, { 224 <span class="hljs-attr">workerData</span>: script 225 }); 226 worker.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, resolve); 227 worker.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, reject); 228 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> { 229 <span class="hljs-keyword">if</span> (code !== <span class="hljs-number">0</span>) 230 <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>)); 231 }); 232 }); 233 }; 234} <span class="hljs-keyword">else</span> { 235 <span class="hljs-keyword">const</span> { parse } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'some-js-parsing-library'</span>); 236 <span class="hljs-keyword">const</span> script = workerData; 237 parentPort.<span class="hljs-title function_">postMessage</span>(<span class="hljs-title function_">parse</span>(script)); 238}</code></pre> 239<p>The above example spawns a Worker thread for each <code>parse()</code> call. In actual 240practice, use a pool of Workers instead for these kinds of tasks. Otherwise, the 241overhead of creating Workers would likely exceed their benefit.</p> 242<p>When implementing a worker pool, use the <a href="async_hooks.html#async_hooks_class_asyncresource"><code>AsyncResource</code></a> API to inform 243diagnostic tools (e.g. in order to provide asynchronous stack traces) about the 244correlation between tasks and their outcomes. See 245<a href="async_hooks.html#async-resource-worker-pool">"Using <code>AsyncResource</code> for a <code>Worker</code> thread pool"</a> 246in the <code>async_hooks</code> documentation for an example implementation.</p> 247<p>Worker threads inherit non-process-specific options by default. Refer to 248<a href="#worker_threads_new_worker_filename_options"><code>Worker constructor options</code></a> to know how to customize worker thread options, 249specifically <code>argv</code> and <code>execArgv</code> options.</p> 250<section><h3><code>worker.getEnvironmentData(key)</code><span><a class="mark" href="#worker_threads_worker_getenvironmentdata_key" id="worker_threads_worker_getenvironmentdata_key">#</a></span></h3> 251<div class="api_metadata"> 252<span>Added in: v14.18.0</span> 253</div> 254<p></p><div class="api_stability api_stability_1"><a href="documentation.html#documentation_stability_index">Stability: 1</a> - Experimental</div><p></p> 255<ul> 256<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 257<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map" class="type"><Map></a> key.</li> 258<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 259</ul> 260<p>Within a worker thread, <code>worker.getEnvironmentData()</code> returns a clone 261of data passed to the spawning thread's <code>worker.setEnvironmentData()</code>. 262Every new <code>Worker</code> receives its own copy of the environment data 263automatically.</p> 264<pre><code class="language-js"><span class="hljs-keyword">const</span> { 265 <span class="hljs-title class_">Worker</span>, 266 isMainThread, 267 setEnvironmentData, 268 getEnvironmentData, 269} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>); 270 271<span class="hljs-keyword">if</span> (isMainThread) { 272 <span class="hljs-title function_">setEnvironmentData</span>(<span class="hljs-string">'Hello'</span>, <span class="hljs-string">'World!'</span>); 273 <span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename); 274} <span class="hljs-keyword">else</span> { 275 <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> 276}</code></pre> 277</section><section><h3><code>worker.isMainThread</code><span><a class="mark" href="#worker_threads_worker_ismainthread" id="worker_threads_worker_ismainthread">#</a></span></h3> 278<div class="api_metadata"> 279<span>Added in: v10.5.0</span> 280</div> 281<ul> 282<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a></li> 283</ul> 284<p>Is <code>true</code> if this code is not running inside of a <a href="#worker_threads_class_worker"><code>Worker</code></a> thread.</p> 285<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>); 286 287<span class="hljs-keyword">if</span> (isMainThread) { 288 <span class="hljs-comment">// This re-loads the current file inside a Worker instance.</span> 289 <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename); 290} <span class="hljs-keyword">else</span> { 291 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'Inside Worker!'</span>); 292 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(isMainThread); <span class="hljs-comment">// Prints 'false'.</span> 293}</code></pre> 294</section><section><h3><code>worker.markAsUntransferable(object)</code><span><a class="mark" href="#worker_threads_worker_markasuntransferable_object" id="worker_threads_worker_markasuntransferable_object">#</a></span></h3> 295<div class="api_metadata"> 296<span>Added in: v14.5.0</span> 297</div> 298<p>Mark an object as not transferable. If <code>object</code> occurs in the transfer list of 299a <a href="#worker_threads_port_postmessage_value_transferlist"><code>port.postMessage()</code></a> call, it will be ignored.</p> 300<p>In particular, this makes sense for objects that can be cloned, rather than 301transferred, and which are used by other objects on the sending side. 302For example, Node.js marks the <code>ArrayBuffer</code>s it uses for its 303<a href="buffer.html#buffer_static_method_buffer_allocunsafe_size"><code>Buffer</code> pool</a> with this.</p> 304<p>This operation cannot be undone.</p> 305<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>); 306 307<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>); 308<span class="hljs-keyword">const</span> typedArray1 = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Uint</span>8<span class="hljs-built_in">Array</span>(pooledBuffer); 309<span class="hljs-keyword">const</span> typedArray2 = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Float</span>64<span class="hljs-built_in">Array</span>(pooledBuffer); 310 311<span class="hljs-title function_">markAsUntransferable</span>(pooledBuffer); 312 313<span class="hljs-keyword">const</span> { port1 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>(); 314port1.<span class="hljs-title function_">postMessage</span>(typedArray1, [ typedArray1.<span class="hljs-property">buffer</span> ]); 315 316<span class="hljs-comment">// The following line prints the contents of typedArray1 -- it still owns</span> 317<span class="hljs-comment">// its memory and has been cloned, not transferred. Without</span> 318<span class="hljs-comment">// `markAsUntransferable()`, this would print an empty Uint8Array.</span> 319<span class="hljs-comment">// typedArray2 is intact as well.</span> 320<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(typedArray1); 321<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(typedArray2);</code></pre> 322<p>There is no equivalent to this API in browsers.</p> 323</section><section><h3><code>worker.moveMessagePortToContext(port, contextifiedSandbox)</code><span><a class="mark" href="#worker_threads_worker_movemessageporttocontext_port_contextifiedsandbox" id="worker_threads_worker_movemessageporttocontext_port_contextifiedsandbox">#</a></span></h3> 324<div class="api_metadata"> 325<span>Added in: v11.13.0</span> 326</div> 327<ul> 328<li> 329<p><code>port</code> <a href="worker_threads.html#worker_threads_class_messageport" class="type"><MessagePort></a> The message port which will be transferred.</p> 330</li> 331<li> 332<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#vm_what_does_it_mean_to_contextify_an_object">contextified</a> object as returned by the 333<code>vm.createContext()</code> method.</p> 334</li> 335<li> 336<p>Returns: <a href="worker_threads.html#worker_threads_class_messageport" class="type"><MessagePort></a></p> 337</li> 338</ul> 339<p>Transfer a <code>MessagePort</code> to a different <a href="vm.html"><code>vm</code></a> Context. The original <code>port</code> 340object will be rendered unusable, and the returned <code>MessagePort</code> instance will 341take its place.</p> 342<p>The returned <code>MessagePort</code> will be an object in the target context, and will 343inherit from its global <code>Object</code> class. Objects passed to the 344<a href="https://developer.mozilla.org/en-US/docs/Web/API/MessagePort/onmessage"><code>port.onmessage()</code></a> listener will also be created in the target context 345and inherit from its global <code>Object</code> class.</p> 346<p>However, the created <code>MessagePort</code> will no longer inherit from 347<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 348events using it.</p> 349</section><section><h3><code>worker.parentPort</code><span><a class="mark" href="#worker_threads_worker_parentport" id="worker_threads_worker_parentport">#</a></span></h3> 350<div class="api_metadata"> 351<span>Added in: v10.5.0</span> 352</div> 353<ul> 354<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#worker_threads_class_messageport" class="type"><MessagePort></a></li> 355</ul> 356<p>If this thread was spawned as a <a href="#worker_threads_class_worker"><code>Worker</code></a>, this will be a <a href="#worker_threads_class_messageport"><code>MessagePort</code></a> 357allowing communication with the parent thread. Messages sent using 358<code>parentPort.postMessage()</code> will be available in the parent thread 359using <code>worker.on('message')</code>, and messages sent from the parent thread 360using <code>worker.postMessage()</code> will be available in this thread using 361<code>parentPort.on('message')</code>.</p> 362<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>); 363 364<span class="hljs-keyword">if</span> (isMainThread) { 365 <span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename); 366 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> { 367 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(message); <span class="hljs-comment">// Prints 'Hello, world!'.</span> 368 }); 369 worker.<span class="hljs-title function_">postMessage</span>(<span class="hljs-string">'Hello, world!'</span>); 370} <span class="hljs-keyword">else</span> { 371 <span class="hljs-comment">// When a message from the parent thread is received, send it back:</span> 372 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> { 373 parentPort.<span class="hljs-title function_">postMessage</span>(message); 374 }); 375}</code></pre> 376</section><section><h3><code>worker.receiveMessageOnPort(port)</code><span><a class="mark" href="#worker_threads_worker_receivemessageonport_port" id="worker_threads_worker_receivemessageonport_port">#</a></span></h3> 377<div class="api_metadata"> 378<span>Added in: v12.3.0</span> 379</div> 380<ul> 381<li> 382<p><code>port</code> <a href="worker_threads.html#worker_threads_class_messageport" class="type"><MessagePort></a></p> 383</li> 384<li> 385<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> 386</li> 387</ul> 388<p>Receive a single message from a given <code>MessagePort</code>. If no message is available, 389<code>undefined</code> is returned, otherwise an object with a single <code>message</code> property 390that contains the message payload, corresponding to the oldest message in the 391<code>MessagePort</code>’s queue.</p> 392<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>); 393<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>(); 394port1.<span class="hljs-title function_">postMessage</span>({ <span class="hljs-attr">hello</span>: <span class="hljs-string">'world'</span> }); 395 396<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">receiveMessageOnPort</span>(port2)); 397<span class="hljs-comment">// Prints: { message: { hello: 'world' } }</span> 398<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">receiveMessageOnPort</span>(port2)); 399<span class="hljs-comment">// Prints: undefined</span></code></pre> 400<p>When this function is used, no <code>'message'</code> event will be emitted and the 401<code>onmessage</code> listener will not be invoked.</p> 402</section><section><h3><code>worker.resourceLimits</code><span><a class="mark" href="#worker_threads_worker_resourcelimits" id="worker_threads_worker_resourcelimits">#</a></span></h3> 403<div class="api_metadata"> 404<span>Added in: v13.2.0, v12.16.0</span> 405</div> 406<ul> 407<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 408<ul> 409<li><code>maxYoungGenerationSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 410<li><code>maxOldGenerationSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 411<li><code>codeRangeSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 412<li><code>stackSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 413</ul> 414</li> 415</ul> 416<p>Provides the set of JS engine resource constraints inside this Worker thread. 417If the <code>resourceLimits</code> option was passed to the <a href="#worker_threads_class_worker"><code>Worker</code></a> constructor, 418this matches its values.</p> 419<p>If this is used in the main thread, its value is an empty object.</p> 420</section><section><h3><code>worker.SHARE_ENV</code><span><a class="mark" href="#worker_threads_worker_share_env" id="worker_threads_worker_share_env">#</a></span></h3> 421<div class="api_metadata"> 422<span>Added in: v11.14.0</span> 423</div> 424<ul> 425<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a></li> 426</ul> 427<p>A special value that can be passed as the <code>env</code> option of the <a href="#worker_threads_class_worker"><code>Worker</code></a> 428constructor, to indicate that the current thread and the Worker thread should 429share read and write access to the same set of environment variables.</p> 430<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>); 431<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> }) 432 .<span class="hljs-title function_">on</span>(<span class="hljs-string">'exit'</span>, <span class="hljs-function">() =></span> { 433 <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> 434 });</code></pre> 435</section><section><h3><code>worker.setEnvironmentData(key[, value])</code><span><a class="mark" href="#worker_threads_worker_setenvironmentdata_key_value" id="worker_threads_worker_setenvironmentdata_key_value">#</a></span></h3> 436<div class="api_metadata"> 437<span>Added in: v14.18.0</span> 438</div> 439<p></p><div class="api_stability api_stability_1"><a href="documentation.html#documentation_stability_index">Stability: 1</a> - Experimental</div><p></p> 440<ul> 441<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 442<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map" class="type"><Map></a> key.</li> 443<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 444and passed automatically to all new <code>Worker</code> instances. If <code>value</code> is passed 445as <code>undefined</code>, any previously set value for the <code>key</code> will be deleted.</li> 446</ul> 447<p>The <code>worker.setEnvironmentData()</code> API sets the content of 448<code>worker.getEnvironmentData()</code> in the current thread and all new <code>Worker</code> 449instances spawned from the current context.</p> 450</section><section><h3><code>worker.threadId</code><span><a class="mark" href="#worker_threads_worker_threadid" id="worker_threads_worker_threadid">#</a></span></h3> 451<div class="api_metadata"> 452<span>Added in: v10.5.0</span> 453</div> 454<ul> 455<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><integer></a></li> 456</ul> 457<p>An integer identifier for the current thread. On the corresponding worker object 458(if there is any), it is available as <a href="#worker_threads_worker_threadid_1"><code>worker.threadId</code></a>. 459This value is unique for each <a href="#worker_threads_class_worker"><code>Worker</code></a> instance inside a single process.</p> 460</section><section><h3><code>worker.workerData</code><span><a class="mark" href="#worker_threads_worker_workerdata" id="worker_threads_worker_workerdata">#</a></span></h3> 461<div class="api_metadata"> 462<span>Added in: v10.5.0</span> 463</div> 464<p>An arbitrary JavaScript value that contains a clone of the data passed 465to this thread’s <code>Worker</code> constructor.</p> 466<p>The data is cloned as if using <a href="#worker_threads_port_postmessage_value_transferlist"><code>postMessage()</code></a>, 467according 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> 468<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>); 469 470<span class="hljs-keyword">if</span> (isMainThread) { 471 <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> }); 472} <span class="hljs-keyword">else</span> { 473 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(workerData); <span class="hljs-comment">// Prints 'Hello, world!'.</span> 474}</code></pre> 475</section><section><h3>Class: <code>MessageChannel</code><span><a class="mark" href="#worker_threads_class_messagechannel" id="worker_threads_class_messagechannel">#</a></span></h3> 476<div class="api_metadata"> 477<span>Added in: v10.5.0</span> 478</div> 479<p>Instances of the <code>worker.MessageChannel</code> class represent an asynchronous, 480two-way communications channel. 481The <code>MessageChannel</code> has no methods of its own. <code>new MessageChannel()</code> 482yields an object with <code>port1</code> and <code>port2</code> properties, which refer to linked 483<a href="#worker_threads_class_messageport"><code>MessagePort</code></a> instances.</p> 484<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>); 485 486<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>(); 487port1.<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)); 488port2.<span class="hljs-title function_">postMessage</span>({ <span class="hljs-attr">foo</span>: <span class="hljs-string">'bar'</span> }); 489<span class="hljs-comment">// Prints: received { foo: 'bar' } from the `port1.on('message')` listener</span></code></pre> 490</section><section><h3>Class: <code>MessagePort</code><span><a class="mark" href="#worker_threads_class_messageport" id="worker_threads_class_messageport">#</a></span></h3> 491<div class="api_metadata"> 492<details class="changelog"><summary>History</summary> 493<table> 494<tbody><tr><th>Version</th><th>Changes</th></tr> 495<tr><td>v14.7.0</td> 496<td><p>This class now inherits from <code>EventTarget</code> rather than from <code>EventEmitter</code>.</p></td></tr> 497<tr><td>v10.5.0</td> 498<td><p><span>Added in: v10.5.0</span></p></td></tr> 499</tbody></table> 500</details> 501</div> 502<ul> 503<li>Extends: <a href="events.html#events_class_eventtarget" class="type"><EventTarget></a></li> 504</ul> 505<p>Instances of the <code>worker.MessagePort</code> class represent one end of an 506asynchronous, two-way communications channel. It can be used to transfer 507structured data, memory regions and other <code>MessagePort</code>s between different 508<a href="#worker_threads_class_worker"><code>Worker</code></a>s.</p> 509<p>This implementation matches <a href="https://developer.mozilla.org/en-US/docs/Web/API/MessagePort">browser <code>MessagePort</code></a>s.</p> 510<h4>Event: <code>'close'</code><span><a class="mark" href="#worker_threads_event_close" id="worker_threads_event_close">#</a></span></h4> 511<div class="api_metadata"> 512<span>Added in: v10.5.0</span> 513</div> 514<p>The <code>'close'</code> event is emitted once either side of the channel has been 515disconnected.</p> 516<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>); 517<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>(); 518 519<span class="hljs-comment">// Prints:</span> 520<span class="hljs-comment">// foobar</span> 521<span class="hljs-comment">// closed!</span> 522port2.<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)); 523port2.<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>)); 524 525port1.<span class="hljs-title function_">postMessage</span>(<span class="hljs-string">'foobar'</span>); 526port1.<span class="hljs-title function_">close</span>();</code></pre> 527<h4>Event: <code>'message'</code><span><a class="mark" href="#worker_threads_event_message" id="worker_threads_event_message">#</a></span></h4> 528<div class="api_metadata"> 529<span>Added in: v10.5.0</span> 530</div> 531<ul> 532<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> 533</ul> 534<p>The <code>'message'</code> event is emitted for any incoming message, containing the cloned 535input of <a href="#worker_threads_port_postmessage_value_transferlist"><code>port.postMessage()</code></a>.</p> 536<p>Listeners on this event will receive a clone of the <code>value</code> parameter as passed 537to <code>postMessage()</code> and no further arguments.</p> 538<h4>Event: <code>'messageerror'</code><span><a class="mark" href="#worker_threads_event_messageerror" id="worker_threads_event_messageerror">#</a></span></h4> 539<div class="api_metadata"> 540<span>Added in: v14.5.0</span> 541</div> 542<ul> 543<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> 544</ul> 545<p>The <code>'messageerror'</code> event is emitted when deserializing a message failed.</p> 546<p>Currently, this event is emitted when there is an error occurring while 547instantiating the posted JS object on the receiving end. Such situations 548are rare, but can happen, for instance, when certain Node.js API objects 549are received in a <code>vm.Context</code> (where Node.js APIs are currently 550unavailable).</p> 551<h4><code>port.close()</code><span><a class="mark" href="#worker_threads_port_close" id="worker_threads_port_close">#</a></span></h4> 552<div class="api_metadata"> 553<span>Added in: v10.5.0</span> 554</div> 555<p>Disables further sending of messages on either side of the connection. 556This method can be called when no further communication will happen over this 557<code>MessagePort</code>.</p> 558<p>The <a href="#worker_threads_event_close"><code>'close'</code> event</a> will be emitted on both <code>MessagePort</code> instances that 559are part of the channel.</p> 560<h4><code>port.postMessage(value[, transferList])</code><span><a class="mark" href="#worker_threads_port_postmessage_value_transferlist" id="worker_threads_port_postmessage_value_transferlist">#</a></span></h4> 561<div class="api_metadata"> 562<details class="changelog"><summary>History</summary> 563<table> 564<tbody><tr><th>Version</th><th>Changes</th></tr> 565<tr><td>v14.18.0</td> 566<td><p>Add 'BlockList' to the list of cloneable types.</p></td></tr> 567<tr><td>v14.18.0</td> 568<td><p>Add 'Histogram' types to the list of cloneable types.</p></td></tr> 569<tr><td>v14.5.0</td> 570<td><p>Added <code>KeyObject</code> to the list of cloneable types.</p></td></tr> 571<tr><td>v14.5.0</td> 572<td><p>Added <code>FileHandle</code> to the list of transferable types.</p></td></tr> 573<tr><td>v10.5.0</td> 574<td><p><span>Added in: v10.5.0</span></p></td></tr> 575</tbody></table> 576</details> 577</div> 578<ul> 579<li><code>value</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 580<li><code>transferList</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object[]></a></li> 581</ul> 582<p>Sends a JavaScript value to the receiving side of this channel. 583<code>value</code> will be transferred in a way which is compatible with 584the <a href="https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm">HTML structured clone algorithm</a>.</p> 585<p>In particular, the significant differences to <code>JSON</code> are:</p> 586<ul> 587<li><code>value</code> may contain circular references.</li> 588<li><code>value</code> may contain instances of builtin JS types such as <code>RegExp</code>s, 589<code>BigInt</code>s, <code>Map</code>s, <code>Set</code>s, etc.</li> 590<li><code>value</code> may contain typed arrays, both using <code>ArrayBuffer</code>s 591and <code>SharedArrayBuffer</code>s.</li> 592<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> 593<li><code>value</code> may not contain native (C++-backed) objects other than: 594<ul> 595<li><a href="fs.html#fs_class_filehandle" class="type"><FileHandle></a>s,</li> 596<li><a href="perf_hooks.html#perf_hooks_class_histogram" class="type"><Histogram></a>s,</li> 597<li><a href="crypto.html#crypto_class_keyobject" class="type"><KeyObject></a>s,</li> 598<li><a href="worker_threads.html#worker_threads_class_messageport" class="type"><MessagePort></a>s,</li> 599<li><a href="net.html#net_class_net_blocklist" class="type"><net.BlockList></a>s,</li> 600<li><a href="net.html#net_class_net_socketaddress" class="type"><net.SocketAddress></a>es,</li> 601</ul> 602</li> 603</ul> 604<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>); 605<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>(); 606 607port1.<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)); 608 609<span class="hljs-keyword">const</span> circularData = {}; 610circularData.<span class="hljs-property">foo</span> = circularData; 611<span class="hljs-comment">// Prints: { foo: [Circular] }</span> 612port2.<span class="hljs-title function_">postMessage</span>(circularData);</code></pre> 613<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="#worker_threads_class_messageport"><code>MessagePort</code></a> and 614<a href="fs.html#fs_class_filehandle"><code>FileHandle</code></a> objects. 615After transferring, they will not be usable on the sending side of the channel 616anymore (even if they are not contained in <code>value</code>). Unlike with 617<a href="child_process.html">child processes</a>, transferring handles such as network sockets is currently 618not supported.</p> 619<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 will be accessible 620from either thread. They cannot be listed in <code>transferList</code>.</p> 621<p><code>value</code> may still contain <code>ArrayBuffer</code> instances that are not in 622<code>transferList</code>; in that case, the underlying memory is copied rather than moved.</p> 623<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>); 624<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>(); 625 626port1.<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)); 627 628<span class="hljs-keyword">const</span> uint8Array = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Uint</span>8<span class="hljs-built_in">Array</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> ]); 629<span class="hljs-comment">// This posts a copy of `uint8Array`:</span> 630port2.<span class="hljs-title function_">postMessage</span>(uint8Array); 631<span class="hljs-comment">// This does not copy data, but renders `uint8Array` unusable:</span> 632port2.<span class="hljs-title function_">postMessage</span>(uint8Array, [ uint8Array.<span class="hljs-property">buffer</span> ]); 633 634<span class="hljs-comment">// The memory for the `sharedUint8Array` will be accessible from both the</span> 635<span class="hljs-comment">// original and the copy received by `.on('message')`:</span> 636<span class="hljs-keyword">const</span> sharedUint8Array = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Uint</span>8<span class="hljs-built_in">Array</span>(<span class="hljs-keyword">new</span> <span class="hljs-title class_">SharedArrayBuffer</span>(<span class="hljs-number">4</span>)); 637port2.<span class="hljs-title function_">postMessage</span>(sharedUint8Array); 638 639<span class="hljs-comment">// This transfers a freshly created message port to the receiver.</span> 640<span class="hljs-comment">// This can be used, for example, to create communication channels between</span> 641<span class="hljs-comment">// multiple `Worker` threads that are children of the same parent thread.</span> 642<span class="hljs-keyword">const</span> otherChannel = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>(); 643port2.<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> 644<p>Because the object cloning uses the structured clone algorithm, 645non-enumerable properties, property accessors, and object prototypes are 646not preserved. In particular, <a href="buffer.html"><code>Buffer</code></a> objects will be read as 647plain <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array"><code>Uint8Array</code></a>s on the receiving side.</p> 648<p>The message object will be cloned immediately, and can be modified after 649posting without having side effects.</p> 650<p>For more information on the serialization and deserialization mechanisms 651behind this API, see the <a href="v8.html#v8_serialization_api">serialization API of the <code>v8</code> module</a>.</p> 652<h5>Considerations when transferring TypedArrays and Buffers<span><a class="mark" href="#worker_threads_considerations_when_transferring_typedarrays_and_buffers" id="worker_threads_considerations_when_transferring_typedarrays_and_buffers">#</a></span></h5> 653<p>All <code>TypedArray</code> and <code>Buffer</code> instances are views over an underlying 654<code>ArrayBuffer</code>. That is, it is the <code>ArrayBuffer</code> that actually stores 655the raw data while the <code>TypedArray</code> and <code>Buffer</code> objects provide a 656way of viewing and manipulating the data. It is possible and common 657for multiple views to be created over the same <code>ArrayBuffer</code> instance. 658Great care must be taken when using a transfer list to transfer an 659<code>ArrayBuffer</code> as doing so will cause all <code>TypedArray</code> and <code>Buffer</code> 660instances that share that same <code>ArrayBuffer</code> to become unusable.</p> 661<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>); 662 663<span class="hljs-keyword">const</span> u1 = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Uint</span>8<span class="hljs-built_in">Array</span>(ab); 664<span class="hljs-keyword">const</span> u2 = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Uint</span>16<span class="hljs-built_in">Array</span>(ab); 665 666<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> 667 668port.<span class="hljs-title function_">postMessage</span>(u1, [u1.<span class="hljs-property">buffer</span>]); 669 670<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> 671<p>For <code>Buffer</code> instances, specifically, whether the underlying 672<code>ArrayBuffer</code> can be transferred or cloned depends entirely on how 673instances were created, which often cannot be reliably determined.</p> 674<p>An <code>ArrayBuffer</code> can be marked with <a href="#worker_threads_worker_markasuntransferable_object"><code>markAsUntransferable()</code></a> to indicate 675that it should always be cloned and never transferred.</p> 676<p>Depending on how a <code>Buffer</code> instance was created, it may or may 677not own its underlying <code>ArrayBuffer</code>. An <code>ArrayBuffer</code> must not 678be transferred unless it is known that the <code>Buffer</code> instance 679owns it. In particular, for <code>Buffer</code>s created from the internal 680<code>Buffer</code> pool (using, for instance <code>Buffer.from()</code> or <code>Buffer.alloc()</code>), 681transferring them is not possible and they will always be cloned, 682which sends a copy of the entire <code>Buffer</code> pool. 683This behavior may come with unintended higher memory 684usage and possible security concerns.</p> 685<p>See <a href="buffer.html#buffer_static_method_buffer_allocunsafe_size"><code>Buffer.allocUnsafe()</code></a> for more details on <code>Buffer</code> pooling.</p> 686<p>The <code>ArrayBuffer</code>s for <code>Buffer</code> instances created using 687<code>Buffer.alloc()</code> or <code>Buffer.allocUnsafeSlow()</code> can always be 688transferred but doing so will render all other existing views of 689those <code>ArrayBuffer</code>s unusable.</p> 690<h4><code>port.ref()</code><span><a class="mark" href="#worker_threads_port_ref" id="worker_threads_port_ref">#</a></span></h4> 691<div class="api_metadata"> 692<span>Added in: v10.5.0</span> 693</div> 694<p>Opposite of <code>unref()</code>. Calling <code>ref()</code> on a previously <code>unref()</code>ed port will 695<em>not</em> let the program exit if it's the only active handle left (the default 696behavior). If the port is <code>ref()</code>ed, calling <code>ref()</code> again will have no effect.</p> 697<p>If listeners are attached or removed using <code>.on('message')</code>, the port will 698be <code>ref()</code>ed and <code>unref()</code>ed automatically depending on whether 699listeners for the event exist.</p> 700<h4><code>port.start()</code><span><a class="mark" href="#worker_threads_port_start" id="worker_threads_port_start">#</a></span></h4> 701<div class="api_metadata"> 702<span>Added in: v10.5.0</span> 703</div> 704<p>Starts receiving messages on this <code>MessagePort</code>. When using this port 705as an event emitter, this will be called automatically once <code>'message'</code> 706listeners are attached.</p> 707<p>This method exists for parity with the Web <code>MessagePort</code> API. In Node.js, 708it is only useful for ignoring messages when no event listener is present. 709Node.js also diverges in its handling of <code>.onmessage</code>. Setting it will 710automatically call <code>.start()</code>, but unsetting it will let messages queue up 711until a new handler is set or the port is discarded.</p> 712<h4><code>port.unref()</code><span><a class="mark" href="#worker_threads_port_unref" id="worker_threads_port_unref">#</a></span></h4> 713<div class="api_metadata"> 714<span>Added in: v10.5.0</span> 715</div> 716<p>Calling <code>unref()</code> on a port will allow the thread to exit if this is the only 717active handle in the event system. If the port is already <code>unref()</code>ed calling 718<code>unref()</code> again will have no effect.</p> 719<p>If listeners are attached or removed using <code>.on('message')</code>, the port will 720be <code>ref()</code>ed and <code>unref()</code>ed automatically depending on whether 721listeners for the event exist.</p> 722</section><section><h3>Class: <code>Worker</code><span><a class="mark" href="#worker_threads_class_worker" id="worker_threads_class_worker">#</a></span></h3> 723<div class="api_metadata"> 724<span>Added in: v10.5.0</span> 725</div> 726<ul> 727<li>Extends: <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a></li> 728</ul> 729<p>The <code>Worker</code> class represents an independent JavaScript execution thread. 730Most Node.js APIs are available inside of it.</p> 731<p>Notable differences inside a Worker environment are:</p> 732<ul> 733<li>The <a href="process.html#process_process_stdin"><code>process.stdin</code></a>, <a href="process.html#process_process_stdout"><code>process.stdout</code></a> and <a href="process.html#process_process_stderr"><code>process.stderr</code></a> 734may be redirected by the parent thread.</li> 735<li>The <a href="#worker_threads_worker_ismainthread"><code>require('worker_threads').isMainThread</code></a> property is set to <code>false</code>.</li> 736<li>The <a href="#worker_threads_worker_parentport"><code>require('worker_threads').parentPort</code></a> message port is available.</li> 737<li><a href="process.html#process_process_exit_code"><code>process.exit()</code></a> does not stop the whole program, just the single thread, 738and <a href="process.html#process_process_abort"><code>process.abort()</code></a> is not available.</li> 739<li><a href="process.html#process_process_chdir_directory"><code>process.chdir()</code></a> and <code>process</code> methods that set group or user ids 740are not available.</li> 741<li><a href="process.html#process_process_env"><code>process.env</code></a> is a copy of the parent thread's environment variables, 742unless otherwise specified. Changes to one copy will not be visible in other 743threads, and will not be visible to native add-ons (unless 744<a href="#worker_threads_worker_share_env"><code>worker.SHARE_ENV</code></a> has been passed as the <code>env</code> option to the 745<a href="#worker_threads_class_worker"><code>Worker</code></a> constructor).</li> 746<li><a href="process.html#process_process_title"><code>process.title</code></a> cannot be modified.</li> 747<li>Signals will not be delivered through <a href="process.html#process_signal_events"><code>process.on('...')</code></a>.</li> 748<li>Execution may stop at any point as a result of <a href="#worker_threads_worker_terminate"><code>worker.terminate()</code></a> 749being invoked.</li> 750<li>IPC channels from parent processes are not accessible.</li> 751<li>The <a href="tracing.html"><code>trace_events</code></a> module is not supported.</li> 752<li>Native add-ons can only be loaded from multiple threads if they fulfill 753<a href="addons.html#addons_worker_support">certain conditions</a>.</li> 754</ul> 755<p>Creating <code>Worker</code> instances inside of other <code>Worker</code>s is possible.</p> 756<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 757achieved through inter-thread message passing. Internally, a <code>Worker</code> has a 758built-in pair of <a href="#worker_threads_class_messageport"><code>MessagePort</code></a>s that are already associated with each other 759when the <code>Worker</code> is created. While the <code>MessagePort</code> object on the parent side 760is not directly exposed, its functionalities are exposed through 761<a href="#worker_threads_worker_postmessage_value_transferlist"><code>worker.postMessage()</code></a> and the <a href="#worker_threads_event_message_1"><code>worker.on('message')</code></a> event 762on the <code>Worker</code> object for the parent thread.</p> 763<p>To create custom messaging channels (which is encouraged over using the default 764global channel because it facilitates separation of concerns), users can create 765a <code>MessageChannel</code> object on either thread and pass one of the 766<code>MessagePort</code>s on that <code>MessageChannel</code> to the other thread through a 767pre-existing channel, such as the global one.</p> 768<p>See <a href="#worker_threads_port_postmessage_value_transferlist"><code>port.postMessage()</code></a> for more information on how messages are passed, 769and what kind of JavaScript values can be successfully transported through 770the thread barrier.</p> 771<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>); 772<span class="hljs-keyword">const</span> { 773 <span class="hljs-title class_">Worker</span>, <span class="hljs-title class_">MessageChannel</span>, <span class="hljs-title class_">MessagePort</span>, isMainThread, parentPort 774} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>); 775<span class="hljs-keyword">if</span> (isMainThread) { 776 <span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename); 777 <span class="hljs-keyword">const</span> subChannel = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>(); 778 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>]); 779 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> { 780 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'received:'</span>, value); 781 }); 782} <span class="hljs-keyword">else</span> { 783 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> { 784 <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>); 785 value.<span class="hljs-property">hereIsYourPort</span>.<span class="hljs-title function_">postMessage</span>(<span class="hljs-string">'the worker is sending this'</span>); 786 value.<span class="hljs-property">hereIsYourPort</span>.<span class="hljs-title function_">close</span>(); 787 }); 788}</code></pre> 789<h4><code>new Worker(filename[, options])</code><span><a class="mark" href="#worker_threads_new_worker_filename_options" id="worker_threads_new_worker_filename_options">#</a></span></h4> 790<div class="api_metadata"> 791<details class="changelog"><summary>History</summary> 792<table> 793<tbody><tr><th>Version</th><th>Changes</th></tr> 794<tr><td>v14.9.0</td> 795<td><p>The <code>filename</code> parameter can be a WHATWG <code>URL</code> object using <code>data:</code> protocol.</p></td></tr> 796<tr><td>v14.9.0</td> 797<td><p>The <code>trackUnmanagedFds</code> option was set to <code>true</code> by default.</p></td></tr> 798<tr><td>v14.6.0</td> 799<td><p>The <code>trackUnmanagedFds</code> option was introduced.</p></td></tr> 800<tr><td>v14.0.0</td> 801<td><p>The <code>transferList</code> option was introduced.</p></td></tr> 802<tr><td>v13.12.0</td> 803<td><p>The <code>filename</code> parameter can be a WHATWG <code>URL</code> object using <code>file:</code> protocol.</p></td></tr> 804<tr><td>v13.4.0, v12.16.0</td> 805<td><p>The <code>argv</code> option was introduced.</p></td></tr> 806<tr><td>v13.2.0, v12.16.0</td> 807<td><p>The <code>resourceLimits</code> option was introduced.</p></td></tr> 808<tr><td>v10.5.0</td> 809<td><p><span>Added in: v10.5.0</span></p></td></tr> 810</tbody></table> 811</details> 812</div> 813<ul> 814<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#url_the_whatwg_url_api" class="type"><URL></a> The path to the Worker’s main script or module. Must 815be either an absolute path or a relative path (i.e. relative to the 816current working directory) starting with <code>./</code> or <code>../</code>, or a WHATWG <code>URL</code> 817object using <code>file:</code> or <code>data:</code> protocol. 818When 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 819the <a href="esm.html#esm_data_imports">ECMAScript module loader</a>. 820If <code>options.eval</code> is <code>true</code>, this is a string containing JavaScript code 821rather than a path.</li> 822<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 823<ul> 824<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 825<code>process.argv</code> in the worker. This is mostly similar to the <code>workerData</code> 826but the values will be available on the global <code>process.argv</code> as if they 827were passed as CLI options to the script.</li> 828<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 829the Worker thread. As a special value, <a href="#worker_threads_worker_share_env"><code>worker.SHARE_ENV</code></a> may be used 830to specify that the parent thread and the child thread should share their 831environment variables; in that case, changes to one thread’s <code>process.env</code> 832object will affect the other thread as well. <strong>Default:</strong> <code>process.env</code>.</li> 833<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 834the first argument to the constructor as a script that is executed once the 835worker is online.</li> 836<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. 837V8 options (such as <code>--max-old-space-size</code>) and options that affect the 838process (such as <code>--title</code>) are not supported. If set, this will be provided 839as <a href="process.html#process_process_execargv"><code>process.execArgv</code></a> inside the worker. By default, options will be 840inherited from the parent thread.</li> 841<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> will 842provide a writable stream whose contents will appear as <code>process.stdin</code> 843inside the Worker. By default, no data is provided.</li> 844<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> will 845not automatically be piped through to <code>process.stdout</code> in the parent.</li> 846<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> will 847not automatically be piped through to <code>process.stderr</code> in the parent.</li> 848<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 will be cloned and made 849available as <a href="#worker_threads_worker_workerdata"><code>require('worker_threads').workerData</code></a>. The cloning will 850occur 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 851will be thrown if the object cannot be cloned (e.g. because it contains 852<code>function</code>s).</li> 853<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 will 854track raw file descriptors managed through <a href="fs.html#fs_fs_open_path_flags_mode_callback"><code>fs.open()</code></a> and 855<a href="fs.html#fs_fs_close_fd_callback"><code>fs.close()</code></a>, and close them when the Worker exits, similar to other 856resources like network sockets or file descriptors managed through 857the <a href="fs.html#fs_class_filehandle"><code>FileHandle</code></a> API. This option is automatically inherited by all 858nested <code>Worker</code>s. <strong>Default</strong>: <code>false</code>.</li> 859<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 860are passed in <code>workerData</code>, a <code>transferList</code> is required for those 861items or <a href="errors.html#errors_err_missing_message_port_in_transfer_list"><code>ERR_MISSING_MESSAGE_PORT_IN_TRANSFER_LIST</code></a> will be thrown. 862See <a href="#worker_threads_port_postmessage_value_transferlist"><code>port.postMessage()</code></a> for more information.</li> 863<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 864JS engine instance. Reaching these limits will lead to termination of the 865<code>Worker</code> instance. These limits only affect the JS engine, and no external 866data, including no <code>ArrayBuffer</code>s. Even if these limits are set, the process 867may still abort if it encounters a global out-of-memory situation. 868<ul> 869<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> 870<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 871recently created objects.</li> 872<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 873used for generated code.</li> 874<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. 875Small values may lead to unusable Worker instances. <strong>Default:</strong> <code>4</code>.</li> 876</ul> 877</li> 878</ul> 879</li> 880</ul> 881<h4>Event: <code>'error'</code><span><a class="mark" href="#worker_threads_event_error" id="worker_threads_event_error">#</a></span></h4> 882<div class="api_metadata"> 883<span>Added in: v10.5.0</span> 884</div> 885<ul> 886<li><code>err</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a></li> 887</ul> 888<p>The <code>'error'</code> event is emitted if the worker thread throws an uncaught 889exception. In that case, the worker will be terminated.</p> 890<h4>Event: <code>'exit'</code><span><a class="mark" href="#worker_threads_event_exit" id="worker_threads_event_exit">#</a></span></h4> 891<div class="api_metadata"> 892<span>Added in: v10.5.0</span> 893</div> 894<ul> 895<li><code>exitCode</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><integer></a></li> 896</ul> 897<p>The <code>'exit'</code> event is emitted once the worker has stopped. If the worker 898exited by calling <a href="process.html#process_process_exit_code"><code>process.exit()</code></a>, the <code>exitCode</code> parameter will be the 899passed exit code. If the worker was terminated, the <code>exitCode</code> parameter will 900be <code>1</code>.</p> 901<p>This is the final event emitted by any <code>Worker</code> instance.</p> 902<h4>Event: <code>'message'</code><span><a class="mark" href="#worker_threads_event_message_1" id="worker_threads_event_message_1">#</a></span></h4> 903<div class="api_metadata"> 904<span>Added in: v10.5.0</span> 905</div> 906<ul> 907<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> 908</ul> 909<p>The <code>'message'</code> event is emitted when the worker thread has invoked 910<a href="#worker_threads_worker_postmessage_value_transferlist"><code>require('worker_threads').parentPort.postMessage()</code></a>. 911See the <a href="#worker_threads_event_message"><code>port.on('message')</code></a> event for more details.</p> 912<p>All messages sent from the worker thread will be emitted before the 913<a href="#worker_threads_event_exit"><code>'exit'</code> event</a> is emitted on the <code>Worker</code> object.</p> 914<h4>Event: <code>'messageerror'</code><span><a class="mark" href="#worker_threads_event_messageerror_1" id="worker_threads_event_messageerror_1">#</a></span></h4> 915<div class="api_metadata"> 916<span>Added in: v14.5.0</span> 917</div> 918<ul> 919<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> 920</ul> 921<p>The <code>'messageerror'</code> event is emitted when deserializing a message failed.</p> 922<h4>Event: <code>'online'</code><span><a class="mark" href="#worker_threads_event_online" id="worker_threads_event_online">#</a></span></h4> 923<div class="api_metadata"> 924<span>Added in: v10.5.0</span> 925</div> 926<p>The <code>'online'</code> event is emitted when the worker thread has started executing 927JavaScript code.</p> 928<h4><code>worker.getHeapSnapshot()</code><span><a class="mark" href="#worker_threads_worker_getheapsnapshot" id="worker_threads_worker_getheapsnapshot">#</a></span></h4> 929<div class="api_metadata"> 930<span>Added in: v13.9.0</span> 931</div> 932<ul> 933<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 934a V8 heap snapshot</li> 935</ul> 936<p>Returns a readable stream for a V8 snapshot of the current state of the Worker. 937See <a href="v8.html#v8_v8_getheapsnapshot"><code>v8.getHeapSnapshot()</code></a> for more details.</p> 938<p>If the Worker thread is no longer running, which may occur before the 939<a href="#worker_threads_event_exit"><code>'exit'</code> event</a> is emitted, the returned <code>Promise</code> will be rejected 940immediately with an <a href="errors.html#ERR_WORKER_NOT_RUNNING"><code>ERR_WORKER_NOT_RUNNING</code></a> error.</p> 941<h4><code>worker.performance</code><span><a class="mark" href="#worker_threads_worker_performance" id="worker_threads_worker_performance">#</a></span></h4> 942<div class="api_metadata"> 943<span>Added in: v14.17.0</span> 944</div> 945<p>An object that can be used to query performance information from a worker 946instance. Similar to <a href="perf_hooks.html#perf_hooks_perf_hooks_performance"><code>perf_hooks.performance</code></a>.</p> 947<h5><code>performance.eventLoopUtilization([utilization1[, utilization2]])</code><span><a class="mark" href="#worker_threads_performance_eventlooputilization_utilization1_utilization2" id="worker_threads_performance_eventlooputilization_utilization1_utilization2">#</a></span></h5> 948<div class="api_metadata"> 949<span>Added in: v14.17.0</span> 950</div> 951<ul> 952<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 953<code>eventLoopUtilization()</code>.</li> 954<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 955<code>eventLoopUtilization()</code> prior to <code>utilization1</code>.</li> 956<li>Returns <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 957<ul> 958<li><code>idle</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 959<li><code>active</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 960<li><code>utilization</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 961</ul> 962</li> 963</ul> 964<p>The same call as <a href="perf_hooks.html#perf_hooks_performance_eventlooputilization_utilization1_utilization2"><code>perf_hooks</code> <code>eventLoopUtilization()</code></a>, except the values 965of the worker instance are returned.</p> 966<p>One difference is that, unlike the main thread, bootstrapping within a worker 967is done within the event loop. So the event loop utilization will be 968immediately available once the worker's script begins execution.</p> 969<p>An <code>idle</code> time that does not increase does not indicate that the worker is 970stuck in bootstrap. The following examples shows how the worker's entire 971lifetime will never accumulate any <code>idle</code> time, but is still be able to process 972messages.</p> 973<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>); 974 975<span class="hljs-keyword">if</span> (isMainThread) { 976 <span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename); 977 <span class="hljs-built_in">setInterval</span>(<span class="hljs-function">() =></span> { 978 worker.<span class="hljs-title function_">postMessage</span>(<span class="hljs-string">'hi'</span>); 979 <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>()); 980 }, <span class="hljs-number">100</span>).<span class="hljs-title function_">unref</span>(); 981 <span class="hljs-keyword">return</span>; 982} 983 984parentPort.<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>(); 985(<span class="hljs-keyword">function</span> <span class="hljs-title function_">r</span>(<span class="hljs-params">n</span>) { 986 <span class="hljs-keyword">if</span> (--n < <span class="hljs-number">0</span>) <span class="hljs-keyword">return</span>; 987 <span class="hljs-keyword">const</span> t = <span class="hljs-title class_">Date</span>.<span class="hljs-title function_">now</span>(); 988 <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>); 989 <span class="hljs-title function_">setImmediate</span>(r, n); 990})(<span class="hljs-number">10</span>);</code></pre> 991<p>The event loop utilization of a worker is available only after the <a href="#worker_threads_event_online"><code>'online'</code> 992event</a> emitted, and if called before this, or after the <a href="#worker_threads_event_exit"><code>'exit'</code> 993event</a>, then all properties have the value of <code>0</code>.</p> 994<h4><code>worker.postMessage(value[, transferList])</code><span><a class="mark" href="#worker_threads_worker_postmessage_value_transferlist" id="worker_threads_worker_postmessage_value_transferlist">#</a></span></h4> 995<div class="api_metadata"> 996<span>Added in: v10.5.0</span> 997</div> 998<ul> 999<li><code>value</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 1000<li><code>transferList</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object[]></a></li> 1001</ul> 1002<p>Send a message to the worker that will be received via 1003<a href="#worker_threads_event_message"><code>require('worker_threads').parentPort.on('message')</code></a>. 1004See <a href="#worker_threads_port_postmessage_value_transferlist"><code>port.postMessage()</code></a> for more details.</p> 1005<h4><code>worker.ref()</code><span><a class="mark" href="#worker_threads_worker_ref" id="worker_threads_worker_ref">#</a></span></h4> 1006<div class="api_metadata"> 1007<span>Added in: v10.5.0</span> 1008</div> 1009<p>Opposite of <code>unref()</code>, calling <code>ref()</code> on a previously <code>unref()</code>ed worker will 1010<em>not</em> let the program exit if it's the only active handle left (the default 1011behavior). If the worker is <code>ref()</code>ed, calling <code>ref()</code> again will have 1012no effect.</p> 1013<h4><code>worker.resourceLimits</code><span><a class="mark" href="#worker_threads_worker_resourcelimits_1" id="worker_threads_worker_resourcelimits_1">#</a></span></h4> 1014<div class="api_metadata"> 1015<span>Added in: v13.2.0, v12.16.0</span> 1016</div> 1017<ul> 1018<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 1019<ul> 1020<li><code>maxYoungGenerationSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 1021<li><code>maxOldGenerationSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 1022<li><code>codeRangeSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 1023<li><code>stackSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 1024</ul> 1025</li> 1026</ul> 1027<p>Provides the set of JS engine resource constraints for this Worker thread. 1028If the <code>resourceLimits</code> option was passed to the <a href="#worker_threads_class_worker"><code>Worker</code></a> constructor, 1029this matches its values.</p> 1030<p>If the worker has stopped, the return value is an empty object.</p> 1031<h4><code>worker.stderr</code><span><a class="mark" href="#worker_threads_worker_stderr" id="worker_threads_worker_stderr">#</a></span></h4> 1032<div class="api_metadata"> 1033<span>Added in: v10.5.0</span> 1034</div> 1035<ul> 1036<li><a href="stream.html#stream_class_stream_readable" class="type"><stream.Readable></a></li> 1037</ul> 1038<p>This is a readable stream which contains data written to <a href="process.html#process_process_stderr"><code>process.stderr</code></a> 1039inside the worker thread. If <code>stderr: true</code> was not passed to the 1040<a href="#worker_threads_class_worker"><code>Worker</code></a> constructor, then data will be piped to the parent thread's 1041<a href="process.html#process_process_stderr"><code>process.stderr</code></a> stream.</p> 1042<h4><code>worker.stdin</code><span><a class="mark" href="#worker_threads_worker_stdin" id="worker_threads_worker_stdin">#</a></span></h4> 1043<div class="api_metadata"> 1044<span>Added in: v10.5.0</span> 1045</div> 1046<ul> 1047<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Null_type" class="type"><null></a> | <a href="stream.html#stream_class_stream_writable" class="type"><stream.Writable></a></li> 1048</ul> 1049<p>If <code>stdin: true</code> was passed to the <a href="#worker_threads_class_worker"><code>Worker</code></a> constructor, this is a 1050writable stream. The data written to this stream will be made available in 1051the worker thread as <a href="process.html#process_process_stdin"><code>process.stdin</code></a>.</p> 1052<h4><code>worker.stdout</code><span><a class="mark" href="#worker_threads_worker_stdout" id="worker_threads_worker_stdout">#</a></span></h4> 1053<div class="api_metadata"> 1054<span>Added in: v10.5.0</span> 1055</div> 1056<ul> 1057<li><a href="stream.html#stream_class_stream_readable" class="type"><stream.Readable></a></li> 1058</ul> 1059<p>This is a readable stream which contains data written to <a href="process.html#process_process_stdout"><code>process.stdout</code></a> 1060inside the worker thread. If <code>stdout: true</code> was not passed to the 1061<a href="#worker_threads_class_worker"><code>Worker</code></a> constructor, then data will be piped to the parent thread's 1062<a href="process.html#process_process_stdout"><code>process.stdout</code></a> stream.</p> 1063<h4><code>worker.terminate()</code><span><a class="mark" href="#worker_threads_worker_terminate" id="worker_threads_worker_terminate">#</a></span></h4> 1064<div class="api_metadata"> 1065<details class="changelog"><summary>History</summary> 1066<table> 1067<tbody><tr><th>Version</th><th>Changes</th></tr> 1068<tr><td>v12.5.0</td> 1069<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> 1070<tr><td>v10.5.0</td> 1071<td><p><span>Added in: v10.5.0</span></p></td></tr> 1072</tbody></table> 1073</details> 1074</div> 1075<ul> 1076<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise" class="type"><Promise></a></li> 1077</ul> 1078<p>Stop all JavaScript execution in the worker thread as soon as possible. 1079Returns a Promise for the exit code that is fulfilled when the 1080<a href="#worker_threads_event_exit"><code>'exit'</code> event</a> is emitted.</p> 1081<h4><code>worker.threadId</code><span><a class="mark" href="#worker_threads_worker_threadid_1" id="worker_threads_worker_threadid_1">#</a></span></h4> 1082<div class="api_metadata"> 1083<span>Added in: v10.5.0</span> 1084</div> 1085<ul> 1086<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><integer></a></li> 1087</ul> 1088<p>An integer identifier for the referenced thread. Inside the worker thread, 1089it is available as <a href="#worker_threads_worker_threadid"><code>require('worker_threads').threadId</code></a>. 1090This value is unique for each <code>Worker</code> instance inside a single process.</p> 1091<h4><code>worker.unref()</code><span><a class="mark" href="#worker_threads_worker_unref" id="worker_threads_worker_unref">#</a></span></h4> 1092<div class="api_metadata"> 1093<span>Added in: v10.5.0</span> 1094</div> 1095<p>Calling <code>unref()</code> on a worker will allow the thread to exit if this is the only 1096active handle in the event system. If the worker is already <code>unref()</code>ed calling 1097<code>unref()</code> again will have no effect.</p> 1098</section><section><h3>Notes<span><a class="mark" href="#worker_threads_notes" id="worker_threads_notes">#</a></span></h3> 1099<h4>Synchronous blocking of stdio<span><a class="mark" href="#worker_threads_synchronous_blocking_of_stdio" id="worker_threads_synchronous_blocking_of_stdio">#</a></span></h4> 1100<p><code>Worker</code>s utilize message passing via <a href="worker_threads.html#worker_threads_class_messageport" class="type"><MessagePort></a> to implement interactions 1101with <code>stdio</code>. This means that <code>stdio</code> output originating from a <code>Worker</code> can 1102get blocked by synchronous code on the receiving end that is blocking the 1103Node.js event loop.</p> 1104 1105<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> { 1106 <span class="hljs-title class_">Worker</span>, 1107 isMainThread, 1108} <span class="hljs-keyword">from</span> <span class="hljs-string">'worker_threads'</span>; 1109 1110<span class="hljs-keyword">if</span> (isMainThread) { 1111 <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>)); 1112 <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++) {} 1113} <span class="hljs-keyword">else</span> { 1114 <span class="hljs-comment">// This output will be blocked by the for loop in the main thread.</span> 1115 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'foo'</span>); 1116}</code><code class="language-js cjs"><span class="hljs-meta">'use strict'</span>; 1117 1118<span class="hljs-keyword">const</span> { 1119 <span class="hljs-title class_">Worker</span>, 1120 isMainThread, 1121} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>); 1122 1123<span class="hljs-keyword">if</span> (isMainThread) { 1124 <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename); 1125 <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++) {} 1126} <span class="hljs-keyword">else</span> { 1127 <span class="hljs-comment">// This output will be blocked by the for loop in the main thread.</span> 1128 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'foo'</span>); 1129}</code></pre> 1130<h4>Launching worker threads from preload scripts<span><a class="mark" href="#worker_threads_launching_worker_threads_from_preload_scripts" id="worker_threads_launching_worker_threads_from_preload_scripts">#</a></span></h4> 1131<p>Take care when launching worker threads from preload scripts (scripts loaded 1132and run using the <code>-r</code> command line flag). Unless the <code>execArgv</code> option is 1133explicitly set, new Worker threads automatically inherit the command line flags 1134from the running process and will preload the same preload scripts as the main 1135thread. If the preload script unconditionally launches a worker thread, every 1136thread spawned will spawn another until the application crashes.</p></section> 1137 <!-- API END --> 1138 </div> 1139 </div> 1140 </div> 1141 <script> 1142 'use strict'; 1143 { 1144 const kCustomPreference = 'customDarkTheme'; 1145 const userSettings = sessionStorage.getItem(kCustomPreference); 1146 const themeToggleButton = document.getElementById('theme-toggle-btn'); 1147 if (userSettings === null && window.matchMedia) { 1148 const mq = window.matchMedia('(prefers-color-scheme: dark)'); 1149 if ('onchange' in mq) { 1150 function mqChangeListener(e) { 1151 document.body.classList.toggle('dark-mode', e.matches); 1152 } 1153 mq.addEventListener('change', mqChangeListener); 1154 if (themeToggleButton) { 1155 themeToggleButton.addEventListener('click', function() { 1156 mq.removeEventListener('change', mqChangeListener); 1157 }, { once: true }); 1158 } 1159 } 1160 if (mq.matches) { 1161 document.body.classList.add('dark-mode'); 1162 } 1163 } else if (userSettings === 'true') { 1164 document.body.classList.add('dark-mode'); 1165 } 1166 if (themeToggleButton) { 1167 themeToggleButton.hidden = false; 1168 themeToggleButton.addEventListener('click', function() { 1169 sessionStorage.setItem( 1170 kCustomPreference, 1171 document.body.classList.toggle('dark-mode') 1172 ); 1173 }); 1174 } 1175 } 1176 </script> 1177</body> 1178</html> 1179