1<!DOCTYPE html> 2<html lang="en"> 3<head> 4 <meta charset="utf-8"> 5 <meta name="viewport" content="width=device-width"> 6 <meta name="nodejs.org:node-version" content="v14.21.2"> 7 <title>Worker threads | Node.js v14.21.2 Documentation</title> 8 <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Lato:400,700,400italic&display=fallback"> 9 <link rel="stylesheet" href="assets/style.css"> 10 <link rel="stylesheet" href="assets/hljs.css"> 11 <link rel="canonical" href="https://nodejs.org/api/worker_threads.html"> 12</head> 13<body class="alt apidoc" id="api-section-worker_threads"> 14 <div id="content" class="clearfix"> 15 <div id="column2" class="interior"> 16 <div id="intro" class="interior"> 17 <a href="/" title="Go back to the home page"> 18 Node.js 19 </a> 20 </div> 21 <ul> 22<li><a href="documentation.html" class="nav-documentation">About this documentation</a></li> 23<li><a href="synopsis.html" class="nav-synopsis">Usage and example</a></li> 24</ul> 25<hr class="line"> 26<ul> 27<li><a href="assert.html" class="nav-assert">Assertion testing</a></li> 28<li><a href="async_hooks.html" class="nav-async_hooks">Async hooks</a></li> 29<li><a href="buffer.html" class="nav-buffer">Buffer</a></li> 30<li><a href="addons.html" class="nav-addons">C++ addons</a></li> 31<li><a href="n-api.html" class="nav-n-api">C/C++ addons with Node-API</a></li> 32<li><a href="embedding.html" class="nav-embedding">C++ embedder API</a></li> 33<li><a href="child_process.html" class="nav-child_process">Child processes</a></li> 34<li><a href="cluster.html" class="nav-cluster">Cluster</a></li> 35<li><a href="cli.html" class="nav-cli">Command-line options</a></li> 36<li><a href="console.html" class="nav-console">Console</a></li> 37<li><a href="corepack.html" class="nav-corepack">Corepack</a></li> 38<li><a href="crypto.html" class="nav-crypto">Crypto</a></li> 39<li><a href="debugger.html" class="nav-debugger">Debugger</a></li> 40<li><a href="deprecations.html" class="nav-deprecations">Deprecated APIs</a></li> 41<li><a href="diagnostics_channel.html" class="nav-diagnostics_channel">Diagnostics Channel</a></li> 42<li><a href="dns.html" class="nav-dns">DNS</a></li> 43<li><a href="domain.html" class="nav-domain">Domain</a></li> 44<li><a href="errors.html" class="nav-errors">Errors</a></li> 45<li><a href="events.html" class="nav-events">Events</a></li> 46<li><a href="fs.html" class="nav-fs">File system</a></li> 47<li><a href="globals.html" class="nav-globals">Globals</a></li> 48<li><a href="http.html" class="nav-http">HTTP</a></li> 49<li><a href="http2.html" class="nav-http2">HTTP/2</a></li> 50<li><a href="https.html" class="nav-https">HTTPS</a></li> 51<li><a href="inspector.html" class="nav-inspector">Inspector</a></li> 52<li><a href="intl.html" class="nav-intl">Internationalization</a></li> 53<li><a href="modules.html" class="nav-modules">Modules: CommonJS modules</a></li> 54<li><a href="esm.html" class="nav-esm">Modules: ECMAScript modules</a></li> 55<li><a href="module.html" class="nav-module">Modules: <code>module</code> API</a></li> 56<li><a href="packages.html" class="nav-packages">Modules: Packages</a></li> 57<li><a href="net.html" class="nav-net">Net</a></li> 58<li><a href="os.html" class="nav-os">OS</a></li> 59<li><a href="path.html" class="nav-path">Path</a></li> 60<li><a href="perf_hooks.html" class="nav-perf_hooks">Performance hooks</a></li> 61<li><a href="policy.html" class="nav-policy">Policies</a></li> 62<li><a href="process.html" class="nav-process">Process</a></li> 63<li><a href="punycode.html" class="nav-punycode">Punycode</a></li> 64<li><a href="querystring.html" class="nav-querystring">Query strings</a></li> 65<li><a href="readline.html" class="nav-readline">Readline</a></li> 66<li><a href="repl.html" class="nav-repl">REPL</a></li> 67<li><a href="report.html" class="nav-report">Report</a></li> 68<li><a href="stream.html" class="nav-stream">Stream</a></li> 69<li><a href="string_decoder.html" class="nav-string_decoder">String decoder</a></li> 70<li><a href="timers.html" class="nav-timers">Timers</a></li> 71<li><a href="tls.html" class="nav-tls">TLS/SSL</a></li> 72<li><a href="tracing.html" class="nav-tracing">Trace events</a></li> 73<li><a href="tty.html" class="nav-tty">TTY</a></li> 74<li><a href="dgram.html" class="nav-dgram">UDP/datagram</a></li> 75<li><a href="url.html" class="nav-url">URL</a></li> 76<li><a href="util.html" class="nav-util">Utilities</a></li> 77<li><a href="v8.html" class="nav-v8">V8</a></li> 78<li><a href="vm.html" class="nav-vm">VM</a></li> 79<li><a href="wasi.html" class="nav-wasi">WASI</a></li> 80<li><a href="worker_threads.html" class="nav-worker_threads active">Worker threads</a></li> 81<li><a href="zlib.html" class="nav-zlib">Zlib</a></li> 82</ul> 83<hr class="line"> 84<ul> 85<li><a href="https://github.com/nodejs/node" class="nav-https-github-com-nodejs-node">Code repository and issue tracker</a></li> 86</ul> 87 </div> 88 89 <div id="column1" data-id="worker_threads" class="interior"> 90 <header> 91 <div class="header-container"> 92 <h1>Node.js v14.21.2 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 10.5H1v2h3v-2zm9-9.95h-2V3.5h2V.55zm7.45 3.91l-1.41-1.41-1.79 1.79 1.41 1.41 1.79-1.79zm-3.21 13.7l1.79 1.8 1.41-1.41-1.8-1.79-1.4 1.4zM20 10.5v2h3v-2h-3zm-8-5c-3.31 0-6 2.69-6 6s2.69 6 6 6 6-2.69 6-6-2.69-6-6-6zm-1 16.95h2V19.5h-2v2.95zm-7.45-3.91l1.41 1.41 1.79-1.8-1.41-1.41-1.79 1.8z"/> 102 </svg> 103 </button> 104 </div> 105 <div id="gtoc"> 106 <ul> 107 <li> 108 <a href="index.html">Index</a> 109 </li> 110 <li> 111 <a href="all.html">View on single page</a> 112 </li> 113 <li> 114 <a href="worker_threads.json">View as JSON</a> 115 </li> 116 117 <li class="version-picker"> 118 <a href="#">View another version <span>▼</span></a> 119 <ol class="version-picker"><li><a href="https://nodejs.org/docs/latest-v19.x/api/worker_threads.html">19.x</a></li> 120<li><a href="https://nodejs.org/docs/latest-v18.x/api/worker_threads.html">18.x <b>LTS</b></a></li> 121<li><a href="https://nodejs.org/docs/latest-v17.x/api/worker_threads.html">17.x</a></li> 122<li><a href="https://nodejs.org/docs/latest-v16.x/api/worker_threads.html">16.x <b>LTS</b></a></li> 123<li><a href="https://nodejs.org/docs/latest-v15.x/api/worker_threads.html">15.x</a></li> 124<li><a href="https://nodejs.org/docs/latest-v14.x/api/worker_threads.html">14.x <b>LTS</b></a></li> 125<li><a href="https://nodejs.org/docs/latest-v13.x/api/worker_threads.html">13.x</a></li> 126<li><a href="https://nodejs.org/docs/latest-v12.x/api/worker_threads.html">12.x</a></li> 127<li><a href="https://nodejs.org/docs/latest-v11.x/api/worker_threads.html">11.x</a></li> 128<li><a href="https://nodejs.org/docs/latest-v10.x/api/worker_threads.html">10.x</a></li></ol> 129 </li> 130 131 <li class="edit_on_github"><a href="https://github.com/nodejs/node/edit/master/doc/api/worker_threads.md">Edit on GitHub</a></li> 132 </ul> 133 </div> 134 <hr> 135 </header> 136 137 <details id="toc" open><summary>Table of contents</summary><ul> 138<li><span class="stability_2"><a href="#worker_threads_worker_threads">Worker threads</a></span> 139<ul> 140<li><span class="stability_1"><a href="#worker_threads_worker_getenvironmentdata_key"><code>worker.getEnvironmentData(key)</code></a></span></li> 141<li><a href="#worker_threads_worker_ismainthread"><code>worker.isMainThread</code></a></li> 142<li><a href="#worker_threads_worker_markasuntransferable_object"><code>worker.markAsUntransferable(object)</code></a></li> 143<li><a href="#worker_threads_worker_movemessageporttocontext_port_contextifiedsandbox"><code>worker.moveMessagePortToContext(port, contextifiedSandbox)</code></a></li> 144<li><a href="#worker_threads_worker_parentport"><code>worker.parentPort</code></a></li> 145<li><a href="#worker_threads_worker_receivemessageonport_port"><code>worker.receiveMessageOnPort(port)</code></a></li> 146<li><a href="#worker_threads_worker_resourcelimits"><code>worker.resourceLimits</code></a></li> 147<li><a href="#worker_threads_worker_share_env"><code>worker.SHARE_ENV</code></a></li> 148<li><span class="stability_1"><a href="#worker_threads_worker_setenvironmentdata_key_value"><code>worker.setEnvironmentData(key[, value])</code></a></span></li> 149<li><a href="#worker_threads_worker_threadid"><code>worker.threadId</code></a></li> 150<li><a href="#worker_threads_worker_workerdata"><code>worker.workerData</code></a></li> 151<li><a href="#worker_threads_class_messagechannel">Class: <code>MessageChannel</code></a></li> 152<li><a href="#worker_threads_class_messageport">Class: <code>MessagePort</code></a> 153<ul> 154<li><a href="#worker_threads_event_close">Event: <code>'close'</code></a></li> 155<li><a href="#worker_threads_event_message">Event: <code>'message'</code></a></li> 156<li><a href="#worker_threads_event_messageerror">Event: <code>'messageerror'</code></a></li> 157<li><a href="#worker_threads_port_close"><code>port.close()</code></a></li> 158<li><a href="#worker_threads_port_postmessage_value_transferlist"><code>port.postMessage(value[, transferList])</code></a> 159<ul> 160<li><a href="#worker_threads_considerations_when_transferring_typedarrays_and_buffers">Considerations when transferring TypedArrays and Buffers</a></li> 161</ul> 162</li> 163<li><a href="#worker_threads_port_ref"><code>port.ref()</code></a></li> 164<li><a href="#worker_threads_port_start"><code>port.start()</code></a></li> 165<li><a href="#worker_threads_port_unref"><code>port.unref()</code></a></li> 166</ul> 167</li> 168<li><a href="#worker_threads_class_worker">Class: <code>Worker</code></a> 169<ul> 170<li><a href="#worker_threads_new_worker_filename_options"><code>new Worker(filename[, options])</code></a></li> 171<li><a href="#worker_threads_event_error">Event: <code>'error'</code></a></li> 172<li><a href="#worker_threads_event_exit">Event: <code>'exit'</code></a></li> 173<li><a href="#worker_threads_event_message_1">Event: <code>'message'</code></a></li> 174<li><a href="#worker_threads_event_messageerror_1">Event: <code>'messageerror'</code></a></li> 175<li><a href="#worker_threads_event_online">Event: <code>'online'</code></a></li> 176<li><a href="#worker_threads_worker_getheapsnapshot"><code>worker.getHeapSnapshot()</code></a></li> 177<li><a href="#worker_threads_worker_performance"><code>worker.performance</code></a> 178<ul> 179<li><a href="#worker_threads_performance_eventlooputilization_utilization1_utilization2"><code>performance.eventLoopUtilization([utilization1[, utilization2]])</code></a></li> 180</ul> 181</li> 182<li><a href="#worker_threads_worker_postmessage_value_transferlist"><code>worker.postMessage(value[, transferList])</code></a></li> 183<li><a href="#worker_threads_worker_ref"><code>worker.ref()</code></a></li> 184<li><a href="#worker_threads_worker_resourcelimits_1"><code>worker.resourceLimits</code></a></li> 185<li><a href="#worker_threads_worker_stderr"><code>worker.stderr</code></a></li> 186<li><a href="#worker_threads_worker_stdin"><code>worker.stdin</code></a></li> 187<li><a href="#worker_threads_worker_stdout"><code>worker.stdout</code></a></li> 188<li><a href="#worker_threads_worker_terminate"><code>worker.terminate()</code></a></li> 189<li><a href="#worker_threads_worker_threadid_1"><code>worker.threadId</code></a></li> 190<li><a href="#worker_threads_worker_unref"><code>worker.unref()</code></a></li> 191</ul> 192</li> 193<li><a href="#worker_threads_notes">Notes</a> 194<ul> 195<li><a href="#worker_threads_synchronous_blocking_of_stdio">Synchronous blocking of stdio</a></li> 196<li><a href="#worker_threads_launching_worker_threads_from_preload_scripts">Launching worker threads from preload scripts</a></li> 197</ul> 198</li> 199</ul> 200</li> 201</ul></details> 202 203 <div id="apicontent"> 204 <h2>Worker threads<span><a class="mark" href="#worker_threads_worker_threads" id="worker_threads_worker_threads">#</a></span></h2> 205 206<p></p><div class="api_stability api_stability_2"><a href="documentation.html#documentation_stability_index">Stability: 2</a> - Stable</div><p></p> 207<p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v14.21.2/lib/worker_threads.js">lib/worker_threads.js</a></p> 208<p>The <code>worker_threads</code> module enables the use of threads that execute JavaScript 209in parallel. To access it:</p> 210<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> 211<p>Workers (threads) are useful for performing CPU-intensive JavaScript operations. 212They will not help much with I/O-intensive work. Node.js’s built-in asynchronous 213I/O operations are more efficient than Workers can be.</p> 214<p>Unlike <code>child_process</code> or <code>cluster</code>, <code>worker_threads</code> can share memory. They do 215so by transferring <code>ArrayBuffer</code> instances or sharing <code>SharedArrayBuffer</code> 216instances.</p> 217<pre><code class="language-js"><span class="hljs-keyword">const</span> { 218 <span class="hljs-title class_">Worker</span>, isMainThread, parentPort, workerData 219} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>); 220 221<span class="hljs-keyword">if</span> (isMainThread) { 222 <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>) { 223 <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> { 224 <span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename, { 225 <span class="hljs-attr">workerData</span>: script 226 }); 227 worker.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, resolve); 228 worker.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, reject); 229 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> { 230 <span class="hljs-keyword">if</span> (code !== <span class="hljs-number">0</span>) 231 <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>)); 232 }); 233 }); 234 }; 235} <span class="hljs-keyword">else</span> { 236 <span class="hljs-keyword">const</span> { parse } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'some-js-parsing-library'</span>); 237 <span class="hljs-keyword">const</span> script = workerData; 238 parentPort.<span class="hljs-title function_">postMessage</span>(<span class="hljs-title function_">parse</span>(script)); 239}</code></pre> 240<p>The above example spawns a Worker thread for each <code>parse()</code> call. In actual 241practice, use a pool of Workers instead for these kinds of tasks. Otherwise, the 242overhead of creating Workers would likely exceed their benefit.</p> 243<p>When implementing a worker pool, use the <a href="async_hooks.html#async_hooks_class_asyncresource"><code>AsyncResource</code></a> API to inform 244diagnostic tools (e.g. in order to provide asynchronous stack traces) about the 245correlation between tasks and their outcomes. See 246<a href="async_hooks.html#async-resource-worker-pool">"Using <code>AsyncResource</code> for a <code>Worker</code> thread pool"</a> 247in the <code>async_hooks</code> documentation for an example implementation.</p> 248<p>Worker threads inherit non-process-specific options by default. Refer to 249<a href="#worker_threads_new_worker_filename_options"><code>Worker constructor options</code></a> to know how to customize worker thread options, 250specifically <code>argv</code> and <code>execArgv</code> options.</p> 251<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> 252<div class="api_metadata"> 253<span>Added in: v14.18.0</span> 254</div> 255<p></p><div class="api_stability api_stability_1"><a href="documentation.html#documentation_stability_index">Stability: 1</a> - Experimental</div><p></p> 256<ul> 257<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 258<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map" class="type"><Map></a> key.</li> 259<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 260</ul> 261<p>Within a worker thread, <code>worker.getEnvironmentData()</code> returns a clone 262of data passed to the spawning thread's <code>worker.setEnvironmentData()</code>. 263Every new <code>Worker</code> receives its own copy of the environment data 264automatically.</p> 265<pre><code class="language-js"><span class="hljs-keyword">const</span> { 266 <span class="hljs-title class_">Worker</span>, 267 isMainThread, 268 setEnvironmentData, 269 getEnvironmentData, 270} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>); 271 272<span class="hljs-keyword">if</span> (isMainThread) { 273 <span class="hljs-title function_">setEnvironmentData</span>(<span class="hljs-string">'Hello'</span>, <span class="hljs-string">'World!'</span>); 274 <span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename); 275} <span class="hljs-keyword">else</span> { 276 <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> 277}</code></pre> 278</section><section><h3><code>worker.isMainThread</code><span><a class="mark" href="#worker_threads_worker_ismainthread" id="worker_threads_worker_ismainthread">#</a></span></h3> 279<div class="api_metadata"> 280<span>Added in: v10.5.0</span> 281</div> 282<ul> 283<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a></li> 284</ul> 285<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> 286<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>); 287 288<span class="hljs-keyword">if</span> (isMainThread) { 289 <span class="hljs-comment">// This re-loads the current file inside a Worker instance.</span> 290 <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename); 291} <span class="hljs-keyword">else</span> { 292 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'Inside Worker!'</span>); 293 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(isMainThread); <span class="hljs-comment">// Prints 'false'.</span> 294}</code></pre> 295</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> 296<div class="api_metadata"> 297<span>Added in: v14.5.0</span> 298</div> 299<p>Mark an object as not transferable. If <code>object</code> occurs in the transfer list of 300a <a href="#worker_threads_port_postmessage_value_transferlist"><code>port.postMessage()</code></a> call, it will be ignored.</p> 301<p>In particular, this makes sense for objects that can be cloned, rather than 302transferred, and which are used by other objects on the sending side. 303For example, Node.js marks the <code>ArrayBuffer</code>s it uses for its 304<a href="buffer.html#buffer_static_method_buffer_allocunsafe_size"><code>Buffer</code> pool</a> with this.</p> 305<p>This operation cannot be undone.</p> 306<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>); 307 308<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>); 309<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); 310<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); 311 312<span class="hljs-title function_">markAsUntransferable</span>(pooledBuffer); 313 314<span class="hljs-keyword">const</span> { port1 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>(); 315port1.<span class="hljs-title function_">postMessage</span>(typedArray1, [ typedArray1.<span class="hljs-property">buffer</span> ]); 316 317<span class="hljs-comment">// The following line prints the contents of typedArray1 -- it still owns</span> 318<span class="hljs-comment">// its memory and has been cloned, not transferred. Without</span> 319<span class="hljs-comment">// `markAsUntransferable()`, this would print an empty Uint8Array.</span> 320<span class="hljs-comment">// typedArray2 is intact as well.</span> 321<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(typedArray1); 322<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(typedArray2);</code></pre> 323<p>There is no equivalent to this API in browsers.</p> 324</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> 325<div class="api_metadata"> 326<span>Added in: v11.13.0</span> 327</div> 328<ul> 329<li> 330<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> 331</li> 332<li> 333<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 334<code>vm.createContext()</code> method.</p> 335</li> 336<li> 337<p>Returns: <a href="worker_threads.html#worker_threads_class_messageport" class="type"><MessagePort></a></p> 338</li> 339</ul> 340<p>Transfer a <code>MessagePort</code> to a different <a href="vm.html"><code>vm</code></a> Context. The original <code>port</code> 341object will be rendered unusable, and the returned <code>MessagePort</code> instance will 342take its place.</p> 343<p>The returned <code>MessagePort</code> will be an object in the target context, and will 344inherit from its global <code>Object</code> class. Objects passed to the 345<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 346and inherit from its global <code>Object</code> class.</p> 347<p>However, the created <code>MessagePort</code> will no longer inherit from 348<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 349events using it.</p> 350</section><section><h3><code>worker.parentPort</code><span><a class="mark" href="#worker_threads_worker_parentport" id="worker_threads_worker_parentport">#</a></span></h3> 351<div class="api_metadata"> 352<span>Added in: v10.5.0</span> 353</div> 354<ul> 355<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> 356</ul> 357<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> 358allowing communication with the parent thread. Messages sent using 359<code>parentPort.postMessage()</code> will be available in the parent thread 360using <code>worker.on('message')</code>, and messages sent from the parent thread 361using <code>worker.postMessage()</code> will be available in this thread using 362<code>parentPort.on('message')</code>.</p> 363<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>); 364 365<span class="hljs-keyword">if</span> (isMainThread) { 366 <span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename); 367 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> { 368 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(message); <span class="hljs-comment">// Prints 'Hello, world!'.</span> 369 }); 370 worker.<span class="hljs-title function_">postMessage</span>(<span class="hljs-string">'Hello, world!'</span>); 371} <span class="hljs-keyword">else</span> { 372 <span class="hljs-comment">// When a message from the parent thread is received, send it back:</span> 373 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> { 374 parentPort.<span class="hljs-title function_">postMessage</span>(message); 375 }); 376}</code></pre> 377</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> 378<div class="api_metadata"> 379<span>Added in: v12.3.0</span> 380</div> 381<ul> 382<li> 383<p><code>port</code> <a href="worker_threads.html#worker_threads_class_messageport" class="type"><MessagePort></a></p> 384</li> 385<li> 386<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> 387</li> 388</ul> 389<p>Receive a single message from a given <code>MessagePort</code>. If no message is available, 390<code>undefined</code> is returned, otherwise an object with a single <code>message</code> property 391that contains the message payload, corresponding to the oldest message in the 392<code>MessagePort</code>’s queue.</p> 393<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>); 394<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>(); 395port1.<span class="hljs-title function_">postMessage</span>({ <span class="hljs-attr">hello</span>: <span class="hljs-string">'world'</span> }); 396 397<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">receiveMessageOnPort</span>(port2)); 398<span class="hljs-comment">// Prints: { message: { hello: 'world' } }</span> 399<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">receiveMessageOnPort</span>(port2)); 400<span class="hljs-comment">// Prints: undefined</span></code></pre> 401<p>When this function is used, no <code>'message'</code> event will be emitted and the 402<code>onmessage</code> listener will not be invoked.</p> 403</section><section><h3><code>worker.resourceLimits</code><span><a class="mark" href="#worker_threads_worker_resourcelimits" id="worker_threads_worker_resourcelimits">#</a></span></h3> 404<div class="api_metadata"> 405<span>Added in: v13.2.0, v12.16.0</span> 406</div> 407<ul> 408<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 409<ul> 410<li><code>maxYoungGenerationSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 411<li><code>maxOldGenerationSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 412<li><code>codeRangeSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 413<li><code>stackSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 414</ul> 415</li> 416</ul> 417<p>Provides the set of JS engine resource constraints inside this Worker thread. 418If the <code>resourceLimits</code> option was passed to the <a href="#worker_threads_class_worker"><code>Worker</code></a> constructor, 419this matches its values.</p> 420<p>If this is used in the main thread, its value is an empty object.</p> 421</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> 422<div class="api_metadata"> 423<span>Added in: v11.14.0</span> 424</div> 425<ul> 426<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a></li> 427</ul> 428<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> 429constructor, to indicate that the current thread and the Worker thread should 430share read and write access to the same set of environment variables.</p> 431<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>); 432<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> }) 433 .<span class="hljs-title function_">on</span>(<span class="hljs-string">'exit'</span>, <span class="hljs-function">() =></span> { 434 <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> 435 });</code></pre> 436</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> 437<div class="api_metadata"> 438<span>Added in: v14.18.0</span> 439</div> 440<p></p><div class="api_stability api_stability_1"><a href="documentation.html#documentation_stability_index">Stability: 1</a> - Experimental</div><p></p> 441<ul> 442<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 443<a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map" class="type"><Map></a> key.</li> 444<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 445and passed automatically to all new <code>Worker</code> instances. If <code>value</code> is passed 446as <code>undefined</code>, any previously set value for the <code>key</code> will be deleted.</li> 447</ul> 448<p>The <code>worker.setEnvironmentData()</code> API sets the content of 449<code>worker.getEnvironmentData()</code> in the current thread and all new <code>Worker</code> 450instances spawned from the current context.</p> 451</section><section><h3><code>worker.threadId</code><span><a class="mark" href="#worker_threads_worker_threadid" id="worker_threads_worker_threadid">#</a></span></h3> 452<div class="api_metadata"> 453<span>Added in: v10.5.0</span> 454</div> 455<ul> 456<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><integer></a></li> 457</ul> 458<p>An integer identifier for the current thread. On the corresponding worker object 459(if there is any), it is available as <a href="#worker_threads_worker_threadid_1"><code>worker.threadId</code></a>. 460This value is unique for each <a href="#worker_threads_class_worker"><code>Worker</code></a> instance inside a single process.</p> 461</section><section><h3><code>worker.workerData</code><span><a class="mark" href="#worker_threads_worker_workerdata" id="worker_threads_worker_workerdata">#</a></span></h3> 462<div class="api_metadata"> 463<span>Added in: v10.5.0</span> 464</div> 465<p>An arbitrary JavaScript value that contains a clone of the data passed 466to this thread’s <code>Worker</code> constructor.</p> 467<p>The data is cloned as if using <a href="#worker_threads_port_postmessage_value_transferlist"><code>postMessage()</code></a>, 468according 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> 469<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>); 470 471<span class="hljs-keyword">if</span> (isMainThread) { 472 <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> }); 473} <span class="hljs-keyword">else</span> { 474 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(workerData); <span class="hljs-comment">// Prints 'Hello, world!'.</span> 475}</code></pre> 476</section><section><h3>Class: <code>MessageChannel</code><span><a class="mark" href="#worker_threads_class_messagechannel" id="worker_threads_class_messagechannel">#</a></span></h3> 477<div class="api_metadata"> 478<span>Added in: v10.5.0</span> 479</div> 480<p>Instances of the <code>worker.MessageChannel</code> class represent an asynchronous, 481two-way communications channel. 482The <code>MessageChannel</code> has no methods of its own. <code>new MessageChannel()</code> 483yields an object with <code>port1</code> and <code>port2</code> properties, which refer to linked 484<a href="#worker_threads_class_messageport"><code>MessagePort</code></a> instances.</p> 485<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>); 486 487<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>(); 488port1.<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)); 489port2.<span class="hljs-title function_">postMessage</span>({ <span class="hljs-attr">foo</span>: <span class="hljs-string">'bar'</span> }); 490<span class="hljs-comment">// Prints: received { foo: 'bar' } from the `port1.on('message')` listener</span></code></pre> 491</section><section><h3>Class: <code>MessagePort</code><span><a class="mark" href="#worker_threads_class_messageport" id="worker_threads_class_messageport">#</a></span></h3> 492<div class="api_metadata"> 493<details class="changelog"><summary>History</summary> 494<table> 495<tbody><tr><th>Version</th><th>Changes</th></tr> 496<tr><td>v14.7.0</td> 497<td><p>This class now inherits from <code>EventTarget</code> rather than from <code>EventEmitter</code>.</p></td></tr> 498<tr><td>v10.5.0</td> 499<td><p><span>Added in: v10.5.0</span></p></td></tr> 500</tbody></table> 501</details> 502</div> 503<ul> 504<li>Extends: <a href="events.html#events_class_eventtarget" class="type"><EventTarget></a></li> 505</ul> 506<p>Instances of the <code>worker.MessagePort</code> class represent one end of an 507asynchronous, two-way communications channel. It can be used to transfer 508structured data, memory regions and other <code>MessagePort</code>s between different 509<a href="#worker_threads_class_worker"><code>Worker</code></a>s.</p> 510<p>This implementation matches <a href="https://developer.mozilla.org/en-US/docs/Web/API/MessagePort">browser <code>MessagePort</code></a>s.</p> 511<h4>Event: <code>'close'</code><span><a class="mark" href="#worker_threads_event_close" id="worker_threads_event_close">#</a></span></h4> 512<div class="api_metadata"> 513<span>Added in: v10.5.0</span> 514</div> 515<p>The <code>'close'</code> event is emitted once either side of the channel has been 516disconnected.</p> 517<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>); 518<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>(); 519 520<span class="hljs-comment">// Prints:</span> 521<span class="hljs-comment">// foobar</span> 522<span class="hljs-comment">// closed!</span> 523port2.<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)); 524port2.<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>)); 525 526port1.<span class="hljs-title function_">postMessage</span>(<span class="hljs-string">'foobar'</span>); 527port1.<span class="hljs-title function_">close</span>();</code></pre> 528<h4>Event: <code>'message'</code><span><a class="mark" href="#worker_threads_event_message" id="worker_threads_event_message">#</a></span></h4> 529<div class="api_metadata"> 530<span>Added in: v10.5.0</span> 531</div> 532<ul> 533<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> 534</ul> 535<p>The <code>'message'</code> event is emitted for any incoming message, containing the cloned 536input of <a href="#worker_threads_port_postmessage_value_transferlist"><code>port.postMessage()</code></a>.</p> 537<p>Listeners on this event will receive a clone of the <code>value</code> parameter as passed 538to <code>postMessage()</code> and no further arguments.</p> 539<h4>Event: <code>'messageerror'</code><span><a class="mark" href="#worker_threads_event_messageerror" id="worker_threads_event_messageerror">#</a></span></h4> 540<div class="api_metadata"> 541<span>Added in: v14.5.0</span> 542</div> 543<ul> 544<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> 545</ul> 546<p>The <code>'messageerror'</code> event is emitted when deserializing a message failed.</p> 547<p>Currently, this event is emitted when there is an error occurring while 548instantiating the posted JS object on the receiving end. Such situations 549are rare, but can happen, for instance, when certain Node.js API objects 550are received in a <code>vm.Context</code> (where Node.js APIs are currently 551unavailable).</p> 552<h4><code>port.close()</code><span><a class="mark" href="#worker_threads_port_close" id="worker_threads_port_close">#</a></span></h4> 553<div class="api_metadata"> 554<span>Added in: v10.5.0</span> 555</div> 556<p>Disables further sending of messages on either side of the connection. 557This method can be called when no further communication will happen over this 558<code>MessagePort</code>.</p> 559<p>The <a href="#worker_threads_event_close"><code>'close'</code> event</a> will be emitted on both <code>MessagePort</code> instances that 560are part of the channel.</p> 561<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> 562<div class="api_metadata"> 563<details class="changelog"><summary>History</summary> 564<table> 565<tbody><tr><th>Version</th><th>Changes</th></tr> 566<tr><td>v14.18.0</td> 567<td><p>Add 'BlockList' to the list of cloneable types.</p></td></tr> 568<tr><td>v14.18.0</td> 569<td><p>Add 'Histogram' types to the list of cloneable types.</p></td></tr> 570<tr><td>v14.5.0</td> 571<td><p>Added <code>KeyObject</code> to the list of cloneable types.</p></td></tr> 572<tr><td>v14.5.0</td> 573<td><p>Added <code>FileHandle</code> to the list of transferable types.</p></td></tr> 574<tr><td>v10.5.0</td> 575<td><p><span>Added in: v10.5.0</span></p></td></tr> 576</tbody></table> 577</details> 578</div> 579<ul> 580<li><code>value</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 581<li><code>transferList</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object[]></a></li> 582</ul> 583<p>Sends a JavaScript value to the receiving side of this channel. 584<code>value</code> will be transferred in a way which is compatible with 585the <a href="https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm">HTML structured clone algorithm</a>.</p> 586<p>In particular, the significant differences to <code>JSON</code> are:</p> 587<ul> 588<li><code>value</code> may contain circular references.</li> 589<li><code>value</code> may contain instances of builtin JS types such as <code>RegExp</code>s, 590<code>BigInt</code>s, <code>Map</code>s, <code>Set</code>s, etc.</li> 591<li><code>value</code> may contain typed arrays, both using <code>ArrayBuffer</code>s 592and <code>SharedArrayBuffer</code>s.</li> 593<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> 594<li><code>value</code> may not contain native (C++-backed) objects other than: 595<ul> 596<li><a href="fs.html#fs_class_filehandle" class="type"><FileHandle></a>s,</li> 597<li><a href="perf_hooks.html#perf_hooks_class_histogram" class="type"><Histogram></a>s,</li> 598<li><a href="crypto.html#crypto_class_keyobject" class="type"><KeyObject></a>s,</li> 599<li><a href="worker_threads.html#worker_threads_class_messageport" class="type"><MessagePort></a>s,</li> 600<li><a href="net.html#net_class_net_blocklist" class="type"><net.BlockList></a>s,</li> 601<li><a href="net.html#net_class_net_socketaddress" class="type"><net.SocketAddress></a>es,</li> 602</ul> 603</li> 604</ul> 605<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>); 606<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>(); 607 608port1.<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)); 609 610<span class="hljs-keyword">const</span> circularData = {}; 611circularData.<span class="hljs-property">foo</span> = circularData; 612<span class="hljs-comment">// Prints: { foo: [Circular] }</span> 613port2.<span class="hljs-title function_">postMessage</span>(circularData);</code></pre> 614<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 615<a href="fs.html#fs_class_filehandle"><code>FileHandle</code></a> objects. 616After transferring, they will not be usable on the sending side of the channel 617anymore (even if they are not contained in <code>value</code>). Unlike with 618<a href="child_process.html">child processes</a>, transferring handles such as network sockets is currently 619not supported.</p> 620<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 621from either thread. They cannot be listed in <code>transferList</code>.</p> 622<p><code>value</code> may still contain <code>ArrayBuffer</code> instances that are not in 623<code>transferList</code>; in that case, the underlying memory is copied rather than moved.</p> 624<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>); 625<span class="hljs-keyword">const</span> { port1, port2 } = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>(); 626 627port1.<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)); 628 629<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> ]); 630<span class="hljs-comment">// This posts a copy of `uint8Array`:</span> 631port2.<span class="hljs-title function_">postMessage</span>(uint8Array); 632<span class="hljs-comment">// This does not copy data, but renders `uint8Array` unusable:</span> 633port2.<span class="hljs-title function_">postMessage</span>(uint8Array, [ uint8Array.<span class="hljs-property">buffer</span> ]); 634 635<span class="hljs-comment">// The memory for the `sharedUint8Array` will be accessible from both the</span> 636<span class="hljs-comment">// original and the copy received by `.on('message')`:</span> 637<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>)); 638port2.<span class="hljs-title function_">postMessage</span>(sharedUint8Array); 639 640<span class="hljs-comment">// This transfers a freshly created message port to the receiver.</span> 641<span class="hljs-comment">// This can be used, for example, to create communication channels between</span> 642<span class="hljs-comment">// multiple `Worker` threads that are children of the same parent thread.</span> 643<span class="hljs-keyword">const</span> otherChannel = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>(); 644port2.<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> 645<p>Because the object cloning uses the structured clone algorithm, 646non-enumerable properties, property accessors, and object prototypes are 647not preserved. In particular, <a href="buffer.html"><code>Buffer</code></a> objects will be read as 648plain <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> 649<p>The message object will be cloned immediately, and can be modified after 650posting without having side effects.</p> 651<p>For more information on the serialization and deserialization mechanisms 652behind this API, see the <a href="v8.html#v8_serialization_api">serialization API of the <code>v8</code> module</a>.</p> 653<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> 654<p>All <code>TypedArray</code> and <code>Buffer</code> instances are views over an underlying 655<code>ArrayBuffer</code>. That is, it is the <code>ArrayBuffer</code> that actually stores 656the raw data while the <code>TypedArray</code> and <code>Buffer</code> objects provide a 657way of viewing and manipulating the data. It is possible and common 658for multiple views to be created over the same <code>ArrayBuffer</code> instance. 659Great care must be taken when using a transfer list to transfer an 660<code>ArrayBuffer</code> as doing so will cause all <code>TypedArray</code> and <code>Buffer</code> 661instances that share that same <code>ArrayBuffer</code> to become unusable.</p> 662<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>); 663 664<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); 665<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); 666 667<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> 668 669port.<span class="hljs-title function_">postMessage</span>(u1, [u1.<span class="hljs-property">buffer</span>]); 670 671<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> 672<p>For <code>Buffer</code> instances, specifically, whether the underlying 673<code>ArrayBuffer</code> can be transferred or cloned depends entirely on how 674instances were created, which often cannot be reliably determined.</p> 675<p>An <code>ArrayBuffer</code> can be marked with <a href="#worker_threads_worker_markasuntransferable_object"><code>markAsUntransferable()</code></a> to indicate 676that it should always be cloned and never transferred.</p> 677<p>Depending on how a <code>Buffer</code> instance was created, it may or may 678not own its underlying <code>ArrayBuffer</code>. An <code>ArrayBuffer</code> must not 679be transferred unless it is known that the <code>Buffer</code> instance 680owns it. In particular, for <code>Buffer</code>s created from the internal 681<code>Buffer</code> pool (using, for instance <code>Buffer.from()</code> or <code>Buffer.alloc()</code>), 682transferring them is not possible and they will always be cloned, 683which sends a copy of the entire <code>Buffer</code> pool. 684This behavior may come with unintended higher memory 685usage and possible security concerns.</p> 686<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> 687<p>The <code>ArrayBuffer</code>s for <code>Buffer</code> instances created using 688<code>Buffer.alloc()</code> or <code>Buffer.allocUnsafeSlow()</code> can always be 689transferred but doing so will render all other existing views of 690those <code>ArrayBuffer</code>s unusable.</p> 691<h4><code>port.ref()</code><span><a class="mark" href="#worker_threads_port_ref" id="worker_threads_port_ref">#</a></span></h4> 692<div class="api_metadata"> 693<span>Added in: v10.5.0</span> 694</div> 695<p>Opposite of <code>unref()</code>. Calling <code>ref()</code> on a previously <code>unref()</code>ed port will 696<em>not</em> let the program exit if it's the only active handle left (the default 697behavior). If the port is <code>ref()</code>ed, calling <code>ref()</code> again will have no effect.</p> 698<p>If listeners are attached or removed using <code>.on('message')</code>, the port will 699be <code>ref()</code>ed and <code>unref()</code>ed automatically depending on whether 700listeners for the event exist.</p> 701<h4><code>port.start()</code><span><a class="mark" href="#worker_threads_port_start" id="worker_threads_port_start">#</a></span></h4> 702<div class="api_metadata"> 703<span>Added in: v10.5.0</span> 704</div> 705<p>Starts receiving messages on this <code>MessagePort</code>. When using this port 706as an event emitter, this will be called automatically once <code>'message'</code> 707listeners are attached.</p> 708<p>This method exists for parity with the Web <code>MessagePort</code> API. In Node.js, 709it is only useful for ignoring messages when no event listener is present. 710Node.js also diverges in its handling of <code>.onmessage</code>. Setting it will 711automatically call <code>.start()</code>, but unsetting it will let messages queue up 712until a new handler is set or the port is discarded.</p> 713<h4><code>port.unref()</code><span><a class="mark" href="#worker_threads_port_unref" id="worker_threads_port_unref">#</a></span></h4> 714<div class="api_metadata"> 715<span>Added in: v10.5.0</span> 716</div> 717<p>Calling <code>unref()</code> on a port will allow the thread to exit if this is the only 718active handle in the event system. If the port is already <code>unref()</code>ed calling 719<code>unref()</code> again will have no effect.</p> 720<p>If listeners are attached or removed using <code>.on('message')</code>, the port will 721be <code>ref()</code>ed and <code>unref()</code>ed automatically depending on whether 722listeners for the event exist.</p> 723</section><section><h3>Class: <code>Worker</code><span><a class="mark" href="#worker_threads_class_worker" id="worker_threads_class_worker">#</a></span></h3> 724<div class="api_metadata"> 725<span>Added in: v10.5.0</span> 726</div> 727<ul> 728<li>Extends: <a href="events.html#events_class_eventemitter" class="type"><EventEmitter></a></li> 729</ul> 730<p>The <code>Worker</code> class represents an independent JavaScript execution thread. 731Most Node.js APIs are available inside of it.</p> 732<p>Notable differences inside a Worker environment are:</p> 733<ul> 734<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> 735may be redirected by the parent thread.</li> 736<li>The <a href="#worker_threads_worker_ismainthread"><code>require('worker_threads').isMainThread</code></a> property is set to <code>false</code>.</li> 737<li>The <a href="#worker_threads_worker_parentport"><code>require('worker_threads').parentPort</code></a> message port is available.</li> 738<li><a href="process.html#process_process_exit_code"><code>process.exit()</code></a> does not stop the whole program, just the single thread, 739and <a href="process.html#process_process_abort"><code>process.abort()</code></a> is not available.</li> 740<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 741are not available.</li> 742<li><a href="process.html#process_process_env"><code>process.env</code></a> is a copy of the parent thread's environment variables, 743unless otherwise specified. Changes to one copy will not be visible in other 744threads, and will not be visible to native add-ons (unless 745<a href="#worker_threads_worker_share_env"><code>worker.SHARE_ENV</code></a> has been passed as the <code>env</code> option to the 746<a href="#worker_threads_class_worker"><code>Worker</code></a> constructor).</li> 747<li><a href="process.html#process_process_title"><code>process.title</code></a> cannot be modified.</li> 748<li>Signals will not be delivered through <a href="process.html#process_signal_events"><code>process.on('...')</code></a>.</li> 749<li>Execution may stop at any point as a result of <a href="#worker_threads_worker_terminate"><code>worker.terminate()</code></a> 750being invoked.</li> 751<li>IPC channels from parent processes are not accessible.</li> 752<li>The <a href="tracing.html"><code>trace_events</code></a> module is not supported.</li> 753<li>Native add-ons can only be loaded from multiple threads if they fulfill 754<a href="addons.html#addons_worker_support">certain conditions</a>.</li> 755</ul> 756<p>Creating <code>Worker</code> instances inside of other <code>Worker</code>s is possible.</p> 757<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 758achieved through inter-thread message passing. Internally, a <code>Worker</code> has a 759built-in pair of <a href="#worker_threads_class_messageport"><code>MessagePort</code></a>s that are already associated with each other 760when the <code>Worker</code> is created. While the <code>MessagePort</code> object on the parent side 761is not directly exposed, its functionalities are exposed through 762<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 763on the <code>Worker</code> object for the parent thread.</p> 764<p>To create custom messaging channels (which is encouraged over using the default 765global channel because it facilitates separation of concerns), users can create 766a <code>MessageChannel</code> object on either thread and pass one of the 767<code>MessagePort</code>s on that <code>MessageChannel</code> to the other thread through a 768pre-existing channel, such as the global one.</p> 769<p>See <a href="#worker_threads_port_postmessage_value_transferlist"><code>port.postMessage()</code></a> for more information on how messages are passed, 770and what kind of JavaScript values can be successfully transported through 771the thread barrier.</p> 772<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>); 773<span class="hljs-keyword">const</span> { 774 <span class="hljs-title class_">Worker</span>, <span class="hljs-title class_">MessageChannel</span>, <span class="hljs-title class_">MessagePort</span>, isMainThread, parentPort 775} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>); 776<span class="hljs-keyword">if</span> (isMainThread) { 777 <span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename); 778 <span class="hljs-keyword">const</span> subChannel = <span class="hljs-keyword">new</span> <span class="hljs-title class_">MessageChannel</span>(); 779 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>]); 780 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> { 781 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'received:'</span>, value); 782 }); 783} <span class="hljs-keyword">else</span> { 784 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> { 785 <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>); 786 value.<span class="hljs-property">hereIsYourPort</span>.<span class="hljs-title function_">postMessage</span>(<span class="hljs-string">'the worker is sending this'</span>); 787 value.<span class="hljs-property">hereIsYourPort</span>.<span class="hljs-title function_">close</span>(); 788 }); 789}</code></pre> 790<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> 791<div class="api_metadata"> 792<details class="changelog"><summary>History</summary> 793<table> 794<tbody><tr><th>Version</th><th>Changes</th></tr> 795<tr><td>v14.9.0</td> 796<td><p>The <code>filename</code> parameter can be a WHATWG <code>URL</code> object using <code>data:</code> protocol.</p></td></tr> 797<tr><td>v14.9.0</td> 798<td><p>The <code>trackUnmanagedFds</code> option was set to <code>true</code> by default.</p></td></tr> 799<tr><td>v14.6.0</td> 800<td><p>The <code>trackUnmanagedFds</code> option was introduced.</p></td></tr> 801<tr><td>v14.0.0</td> 802<td><p>The <code>transferList</code> option was introduced.</p></td></tr> 803<tr><td>v13.12.0</td> 804<td><p>The <code>filename</code> parameter can be a WHATWG <code>URL</code> object using <code>file:</code> protocol.</p></td></tr> 805<tr><td>v13.4.0, v12.16.0</td> 806<td><p>The <code>argv</code> option was introduced.</p></td></tr> 807<tr><td>v13.2.0, v12.16.0</td> 808<td><p>The <code>resourceLimits</code> option was introduced.</p></td></tr> 809<tr><td>v10.5.0</td> 810<td><p><span>Added in: v10.5.0</span></p></td></tr> 811</tbody></table> 812</details> 813</div> 814<ul> 815<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 816be either an absolute path or a relative path (i.e. relative to the 817current working directory) starting with <code>./</code> or <code>../</code>, or a WHATWG <code>URL</code> 818object using <code>file:</code> or <code>data:</code> protocol. 819When 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 820the <a href="esm.html#esm_data_imports">ECMAScript module loader</a>. 821If <code>options.eval</code> is <code>true</code>, this is a string containing JavaScript code 822rather than a path.</li> 823<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 824<ul> 825<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 826<code>process.argv</code> in the worker. This is mostly similar to the <code>workerData</code> 827but the values will be available on the global <code>process.argv</code> as if they 828were passed as CLI options to the script.</li> 829<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 830the Worker thread. As a special value, <a href="#worker_threads_worker_share_env"><code>worker.SHARE_ENV</code></a> may be used 831to specify that the parent thread and the child thread should share their 832environment variables; in that case, changes to one thread’s <code>process.env</code> 833object will affect the other thread as well. <strong>Default:</strong> <code>process.env</code>.</li> 834<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 835the first argument to the constructor as a script that is executed once the 836worker is online.</li> 837<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. 838V8 options (such as <code>--max-old-space-size</code>) and options that affect the 839process (such as <code>--title</code>) are not supported. If set, this will be provided 840as <a href="process.html#process_process_execargv"><code>process.execArgv</code></a> inside the worker. By default, options will be 841inherited from the parent thread.</li> 842<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 843provide a writable stream whose contents will appear as <code>process.stdin</code> 844inside the Worker. By default, no data is provided.</li> 845<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 846not automatically be piped through to <code>process.stdout</code> in the parent.</li> 847<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 848not automatically be piped through to <code>process.stderr</code> in the parent.</li> 849<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 850available as <a href="#worker_threads_worker_workerdata"><code>require('worker_threads').workerData</code></a>. The cloning will 851occur 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 852will be thrown if the object cannot be cloned (e.g. because it contains 853<code>function</code>s).</li> 854<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 855track raw file descriptors managed through <a href="fs.html#fs_fs_open_path_flags_mode_callback"><code>fs.open()</code></a> and 856<a href="fs.html#fs_fs_close_fd_callback"><code>fs.close()</code></a>, and close them when the Worker exits, similar to other 857resources like network sockets or file descriptors managed through 858the <a href="fs.html#fs_class_filehandle"><code>FileHandle</code></a> API. This option is automatically inherited by all 859nested <code>Worker</code>s. <strong>Default</strong>: <code>false</code>.</li> 860<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 861are passed in <code>workerData</code>, a <code>transferList</code> is required for those 862items 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. 863See <a href="#worker_threads_port_postmessage_value_transferlist"><code>port.postMessage()</code></a> for more information.</li> 864<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 865JS engine instance. Reaching these limits will lead to termination of the 866<code>Worker</code> instance. These limits only affect the JS engine, and no external 867data, including no <code>ArrayBuffer</code>s. Even if these limits are set, the process 868may still abort if it encounters a global out-of-memory situation. 869<ul> 870<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> 871<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 872recently created objects.</li> 873<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 874used for generated code.</li> 875<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. 876Small values may lead to unusable Worker instances. <strong>Default:</strong> <code>4</code>.</li> 877</ul> 878</li> 879</ul> 880</li> 881</ul> 882<h4>Event: <code>'error'</code><span><a class="mark" href="#worker_threads_event_error" id="worker_threads_event_error">#</a></span></h4> 883<div class="api_metadata"> 884<span>Added in: v10.5.0</span> 885</div> 886<ul> 887<li><code>err</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error" class="type"><Error></a></li> 888</ul> 889<p>The <code>'error'</code> event is emitted if the worker thread throws an uncaught 890exception. In that case, the worker will be terminated.</p> 891<h4>Event: <code>'exit'</code><span><a class="mark" href="#worker_threads_event_exit" id="worker_threads_event_exit">#</a></span></h4> 892<div class="api_metadata"> 893<span>Added in: v10.5.0</span> 894</div> 895<ul> 896<li><code>exitCode</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><integer></a></li> 897</ul> 898<p>The <code>'exit'</code> event is emitted once the worker has stopped. If the worker 899exited by calling <a href="process.html#process_process_exit_code"><code>process.exit()</code></a>, the <code>exitCode</code> parameter will be the 900passed exit code. If the worker was terminated, the <code>exitCode</code> parameter will 901be <code>1</code>.</p> 902<p>This is the final event emitted by any <code>Worker</code> instance.</p> 903<h4>Event: <code>'message'</code><span><a class="mark" href="#worker_threads_event_message_1" id="worker_threads_event_message_1">#</a></span></h4> 904<div class="api_metadata"> 905<span>Added in: v10.5.0</span> 906</div> 907<ul> 908<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> 909</ul> 910<p>The <code>'message'</code> event is emitted when the worker thread has invoked 911<a href="#worker_threads_worker_postmessage_value_transferlist"><code>require('worker_threads').parentPort.postMessage()</code></a>. 912See the <a href="#worker_threads_event_message"><code>port.on('message')</code></a> event for more details.</p> 913<p>All messages sent from the worker thread will be emitted before the 914<a href="#worker_threads_event_exit"><code>'exit'</code> event</a> is emitted on the <code>Worker</code> object.</p> 915<h4>Event: <code>'messageerror'</code><span><a class="mark" href="#worker_threads_event_messageerror_1" id="worker_threads_event_messageerror_1">#</a></span></h4> 916<div class="api_metadata"> 917<span>Added in: v14.5.0</span> 918</div> 919<ul> 920<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> 921</ul> 922<p>The <code>'messageerror'</code> event is emitted when deserializing a message failed.</p> 923<h4>Event: <code>'online'</code><span><a class="mark" href="#worker_threads_event_online" id="worker_threads_event_online">#</a></span></h4> 924<div class="api_metadata"> 925<span>Added in: v10.5.0</span> 926</div> 927<p>The <code>'online'</code> event is emitted when the worker thread has started executing 928JavaScript code.</p> 929<h4><code>worker.getHeapSnapshot()</code><span><a class="mark" href="#worker_threads_worker_getheapsnapshot" id="worker_threads_worker_getheapsnapshot">#</a></span></h4> 930<div class="api_metadata"> 931<span>Added in: v13.9.0</span> 932</div> 933<ul> 934<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 935a V8 heap snapshot</li> 936</ul> 937<p>Returns a readable stream for a V8 snapshot of the current state of the Worker. 938See <a href="v8.html#v8_v8_getheapsnapshot"><code>v8.getHeapSnapshot()</code></a> for more details.</p> 939<p>If the Worker thread is no longer running, which may occur before the 940<a href="#worker_threads_event_exit"><code>'exit'</code> event</a> is emitted, the returned <code>Promise</code> will be rejected 941immediately with an <a href="errors.html#ERR_WORKER_NOT_RUNNING"><code>ERR_WORKER_NOT_RUNNING</code></a> error.</p> 942<h4><code>worker.performance</code><span><a class="mark" href="#worker_threads_worker_performance" id="worker_threads_worker_performance">#</a></span></h4> 943<div class="api_metadata"> 944<span>Added in: v14.17.0</span> 945</div> 946<p>An object that can be used to query performance information from a worker 947instance. Similar to <a href="perf_hooks.html#perf_hooks_perf_hooks_performance"><code>perf_hooks.performance</code></a>.</p> 948<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> 949<div class="api_metadata"> 950<span>Added in: v14.17.0</span> 951</div> 952<ul> 953<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 954<code>eventLoopUtilization()</code>.</li> 955<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 956<code>eventLoopUtilization()</code> prior to <code>utilization1</code>.</li> 957<li>Returns <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 958<ul> 959<li><code>idle</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 960<li><code>active</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 961<li><code>utilization</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 962</ul> 963</li> 964</ul> 965<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 966of the worker instance are returned.</p> 967<p>One difference is that, unlike the main thread, bootstrapping within a worker 968is done within the event loop. So the event loop utilization will be 969immediately available once the worker's script begins execution.</p> 970<p>An <code>idle</code> time that does not increase does not indicate that the worker is 971stuck in bootstrap. The following examples shows how the worker's entire 972lifetime will never accumulate any <code>idle</code> time, but is still be able to process 973messages.</p> 974<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>); 975 976<span class="hljs-keyword">if</span> (isMainThread) { 977 <span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename); 978 <span class="hljs-built_in">setInterval</span>(<span class="hljs-function">() =></span> { 979 worker.<span class="hljs-title function_">postMessage</span>(<span class="hljs-string">'hi'</span>); 980 <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>()); 981 }, <span class="hljs-number">100</span>).<span class="hljs-title function_">unref</span>(); 982 <span class="hljs-keyword">return</span>; 983} 984 985parentPort.<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>(); 986(<span class="hljs-keyword">function</span> <span class="hljs-title function_">r</span>(<span class="hljs-params">n</span>) { 987 <span class="hljs-keyword">if</span> (--n < <span class="hljs-number">0</span>) <span class="hljs-keyword">return</span>; 988 <span class="hljs-keyword">const</span> t = <span class="hljs-title class_">Date</span>.<span class="hljs-title function_">now</span>(); 989 <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>); 990 <span class="hljs-title function_">setImmediate</span>(r, n); 991})(<span class="hljs-number">10</span>);</code></pre> 992<p>The event loop utilization of a worker is available only after the <a href="#worker_threads_event_online"><code>'online'</code> 993event</a> emitted, and if called before this, or after the <a href="#worker_threads_event_exit"><code>'exit'</code> 994event</a>, then all properties have the value of <code>0</code>.</p> 995<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> 996<div class="api_metadata"> 997<span>Added in: v10.5.0</span> 998</div> 999<ul> 1000<li><code>value</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 1001<li><code>transferList</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object[]></a></li> 1002</ul> 1003<p>Send a message to the worker that will be received via 1004<a href="#worker_threads_event_message"><code>require('worker_threads').parentPort.on('message')</code></a>. 1005See <a href="#worker_threads_port_postmessage_value_transferlist"><code>port.postMessage()</code></a> for more details.</p> 1006<h4><code>worker.ref()</code><span><a class="mark" href="#worker_threads_worker_ref" id="worker_threads_worker_ref">#</a></span></h4> 1007<div class="api_metadata"> 1008<span>Added in: v10.5.0</span> 1009</div> 1010<p>Opposite of <code>unref()</code>, calling <code>ref()</code> on a previously <code>unref()</code>ed worker will 1011<em>not</em> let the program exit if it's the only active handle left (the default 1012behavior). If the worker is <code>ref()</code>ed, calling <code>ref()</code> again will have 1013no effect.</p> 1014<h4><code>worker.resourceLimits</code><span><a class="mark" href="#worker_threads_worker_resourcelimits_1" id="worker_threads_worker_resourcelimits_1">#</a></span></h4> 1015<div class="api_metadata"> 1016<span>Added in: v13.2.0, v12.16.0</span> 1017</div> 1018<ul> 1019<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 1020<ul> 1021<li><code>maxYoungGenerationSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 1022<li><code>maxOldGenerationSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 1023<li><code>codeRangeSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 1024<li><code>stackSizeMb</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 1025</ul> 1026</li> 1027</ul> 1028<p>Provides the set of JS engine resource constraints for this Worker thread. 1029If the <code>resourceLimits</code> option was passed to the <a href="#worker_threads_class_worker"><code>Worker</code></a> constructor, 1030this matches its values.</p> 1031<p>If the worker has stopped, the return value is an empty object.</p> 1032<h4><code>worker.stderr</code><span><a class="mark" href="#worker_threads_worker_stderr" id="worker_threads_worker_stderr">#</a></span></h4> 1033<div class="api_metadata"> 1034<span>Added in: v10.5.0</span> 1035</div> 1036<ul> 1037<li><a href="stream.html#stream_class_stream_readable" class="type"><stream.Readable></a></li> 1038</ul> 1039<p>This is a readable stream which contains data written to <a href="process.html#process_process_stderr"><code>process.stderr</code></a> 1040inside the worker thread. If <code>stderr: true</code> was not passed to the 1041<a href="#worker_threads_class_worker"><code>Worker</code></a> constructor, then data will be piped to the parent thread's 1042<a href="process.html#process_process_stderr"><code>process.stderr</code></a> stream.</p> 1043<h4><code>worker.stdin</code><span><a class="mark" href="#worker_threads_worker_stdin" id="worker_threads_worker_stdin">#</a></span></h4> 1044<div class="api_metadata"> 1045<span>Added in: v10.5.0</span> 1046</div> 1047<ul> 1048<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> 1049</ul> 1050<p>If <code>stdin: true</code> was passed to the <a href="#worker_threads_class_worker"><code>Worker</code></a> constructor, this is a 1051writable stream. The data written to this stream will be made available in 1052the worker thread as <a href="process.html#process_process_stdin"><code>process.stdin</code></a>.</p> 1053<h4><code>worker.stdout</code><span><a class="mark" href="#worker_threads_worker_stdout" id="worker_threads_worker_stdout">#</a></span></h4> 1054<div class="api_metadata"> 1055<span>Added in: v10.5.0</span> 1056</div> 1057<ul> 1058<li><a href="stream.html#stream_class_stream_readable" class="type"><stream.Readable></a></li> 1059</ul> 1060<p>This is a readable stream which contains data written to <a href="process.html#process_process_stdout"><code>process.stdout</code></a> 1061inside the worker thread. If <code>stdout: true</code> was not passed to the 1062<a href="#worker_threads_class_worker"><code>Worker</code></a> constructor, then data will be piped to the parent thread's 1063<a href="process.html#process_process_stdout"><code>process.stdout</code></a> stream.</p> 1064<h4><code>worker.terminate()</code><span><a class="mark" href="#worker_threads_worker_terminate" id="worker_threads_worker_terminate">#</a></span></h4> 1065<div class="api_metadata"> 1066<details class="changelog"><summary>History</summary> 1067<table> 1068<tbody><tr><th>Version</th><th>Changes</th></tr> 1069<tr><td>v12.5.0</td> 1070<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> 1071<tr><td>v10.5.0</td> 1072<td><p><span>Added in: v10.5.0</span></p></td></tr> 1073</tbody></table> 1074</details> 1075</div> 1076<ul> 1077<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise" class="type"><Promise></a></li> 1078</ul> 1079<p>Stop all JavaScript execution in the worker thread as soon as possible. 1080Returns a Promise for the exit code that is fulfilled when the 1081<a href="#worker_threads_event_exit"><code>'exit'</code> event</a> is emitted.</p> 1082<h4><code>worker.threadId</code><span><a class="mark" href="#worker_threads_worker_threadid_1" id="worker_threads_worker_threadid_1">#</a></span></h4> 1083<div class="api_metadata"> 1084<span>Added in: v10.5.0</span> 1085</div> 1086<ul> 1087<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><integer></a></li> 1088</ul> 1089<p>An integer identifier for the referenced thread. Inside the worker thread, 1090it is available as <a href="#worker_threads_worker_threadid"><code>require('worker_threads').threadId</code></a>. 1091This value is unique for each <code>Worker</code> instance inside a single process.</p> 1092<h4><code>worker.unref()</code><span><a class="mark" href="#worker_threads_worker_unref" id="worker_threads_worker_unref">#</a></span></h4> 1093<div class="api_metadata"> 1094<span>Added in: v10.5.0</span> 1095</div> 1096<p>Calling <code>unref()</code> on a worker will allow the thread to exit if this is the only 1097active handle in the event system. If the worker is already <code>unref()</code>ed calling 1098<code>unref()</code> again will have no effect.</p> 1099</section><section><h3>Notes<span><a class="mark" href="#worker_threads_notes" id="worker_threads_notes">#</a></span></h3> 1100<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> 1101<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 1102with <code>stdio</code>. This means that <code>stdio</code> output originating from a <code>Worker</code> can 1103get blocked by synchronous code on the receiving end that is blocking the 1104Node.js event loop.</p> 1105 1106<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> { 1107 <span class="hljs-title class_">Worker</span>, 1108 isMainThread, 1109} <span class="hljs-keyword">from</span> <span class="hljs-string">'worker_threads'</span>; 1110 1111<span class="hljs-keyword">if</span> (isMainThread) { 1112 <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>)); 1113 <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++) {} 1114} <span class="hljs-keyword">else</span> { 1115 <span class="hljs-comment">// This output will be blocked by the for loop in the main thread.</span> 1116 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'foo'</span>); 1117}</code><code class="language-js cjs"><span class="hljs-meta">'use strict'</span>; 1118 1119<span class="hljs-keyword">const</span> { 1120 <span class="hljs-title class_">Worker</span>, 1121 isMainThread, 1122} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>); 1123 1124<span class="hljs-keyword">if</span> (isMainThread) { 1125 <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(__filename); 1126 <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++) {} 1127} <span class="hljs-keyword">else</span> { 1128 <span class="hljs-comment">// This output will be blocked by the for loop in the main thread.</span> 1129 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'foo'</span>); 1130}</code></pre> 1131<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> 1132<p>Take care when launching worker threads from preload scripts (scripts loaded 1133and run using the <code>-r</code> command line flag). Unless the <code>execArgv</code> option is 1134explicitly set, new Worker threads automatically inherit the command line flags 1135from the running process and will preload the same preload scripts as the main 1136thread. If the preload script unconditionally launches a worker thread, every 1137thread spawned will spawn another until the application crashes.</p></section> 1138 <!-- API END --> 1139 </div> 1140 </div> 1141 </div> 1142 <script> 1143 'use strict'; 1144 { 1145 const kCustomPreference = 'customDarkTheme'; 1146 const userSettings = sessionStorage.getItem(kCustomPreference); 1147 const themeToggleButton = document.getElementById('theme-toggle-btn'); 1148 if (userSettings === null && window.matchMedia) { 1149 const mq = window.matchMedia('(prefers-color-scheme: dark)'); 1150 if ('onchange' in mq) { 1151 function mqChangeListener(e) { 1152 document.body.classList.toggle('dark-mode', e.matches); 1153 } 1154 mq.addEventListener('change', mqChangeListener); 1155 if (themeToggleButton) { 1156 themeToggleButton.addEventListener('click', function() { 1157 mq.removeEventListener('change', mqChangeListener); 1158 }, { once: true }); 1159 } 1160 } 1161 if (mq.matches) { 1162 document.body.classList.add('dark-mode'); 1163 } 1164 } else if (userSettings === 'true') { 1165 document.body.classList.add('dark-mode'); 1166 } 1167 if (themeToggleButton) { 1168 themeToggleButton.hidden = false; 1169 themeToggleButton.addEventListener('click', function() { 1170 sessionStorage.setItem( 1171 kCustomPreference, 1172 document.body.classList.toggle('dark-mode') 1173 ); 1174 }); 1175 } 1176 } 1177 </script> 1178</body> 1179</html> 1180