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<li><span class="stability_2"><a href="#asynchronous-context-tracking">Asynchronous context tracking</a></span>
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<li><a href="#introduction">Introduction</a></li>
<li><a href="#class-asynclocalstorage">Class: <code>AsyncLocalStorage</code></a>
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<li><a href="#new-asynclocalstorage"><code>new AsyncLocalStorage()</code></a></li>
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<li><span class="stability_1"><a href="#asynclocalstorageexitcallback-args"><code>asyncLocalStorage.exit(callback[, ...args])</code></a></span></li>
<li><a href="#usage-with-asyncawait">Usage with <code>async/await</code></a></li>
<li><a href="#troubleshooting-context-loss">Troubleshooting: Context loss</a></li>
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<li><a href="#class-asyncresource">Class: <code>AsyncResource</code></a>
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<li><a href="#new-asyncresourcetype-options"><code>new AsyncResource(type[, options])</code></a></li>
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      <details role="navigation" id="toc" open><summary>Table of contents</summary><ul>
<li><span class="stability_2"><a href="#asynchronous-context-tracking">Asynchronous context tracking</a></span>
<ul>
<li><a href="#introduction">Introduction</a></li>
<li><a href="#class-asynclocalstorage">Class: <code>AsyncLocalStorage</code></a>
<ul>
<li><a href="#new-asynclocalstorage"><code>new AsyncLocalStorage()</code></a></li>
<li><span class="stability_1"><a href="#static-method-asynclocalstoragebindfn">Static method: <code>AsyncLocalStorage.bind(fn)</code></a></span></li>
<li><span class="stability_1"><a href="#static-method-asynclocalstoragesnapshot">Static method: <code>AsyncLocalStorage.snapshot()</code></a></span></li>
<li><span class="stability_1"><a href="#asynclocalstoragedisable"><code>asyncLocalStorage.disable()</code></a></span></li>
<li><a href="#asynclocalstoragegetstore"><code>asyncLocalStorage.getStore()</code></a></li>
<li><span class="stability_1"><a href="#asynclocalstorageenterwithstore"><code>asyncLocalStorage.enterWith(store)</code></a></span></li>
<li><a href="#asynclocalstoragerunstore-callback-args"><code>asyncLocalStorage.run(store, callback[, ...args])</code></a></li>
<li><span class="stability_1"><a href="#asynclocalstorageexitcallback-args"><code>asyncLocalStorage.exit(callback[, ...args])</code></a></span></li>
<li><a href="#usage-with-asyncawait">Usage with <code>async/await</code></a></li>
<li><a href="#troubleshooting-context-loss">Troubleshooting: Context loss</a></li>
</ul>
</li>
<li><a href="#class-asyncresource">Class: <code>AsyncResource</code></a>
<ul>
<li><a href="#new-asyncresourcetype-options"><code>new AsyncResource(type[, options])</code></a></li>
<li><a href="#static-method-asyncresourcebindfn-type-thisarg">Static method: <code>AsyncResource.bind(fn[, type[, thisArg]])</code></a></li>
<li><a href="#asyncresourcebindfn-thisarg"><code>asyncResource.bind(fn[, thisArg])</code></a></li>
<li><a href="#asyncresourceruninasyncscopefn-thisarg-args"><code>asyncResource.runInAsyncScope(fn[, thisArg, ...args])</code></a></li>
<li><a href="#asyncresourceemitdestroy"><code>asyncResource.emitDestroy()</code></a></li>
<li><a href="#asyncresourceasyncid"><code>asyncResource.asyncId()</code></a></li>
<li><a href="#asyncresourcetriggerasyncid"><code>asyncResource.triggerAsyncId()</code></a></li>
<li><a href="#using-asyncresource-for-a-worker-thread-pool">Using <code>AsyncResource</code> for a <code>Worker</code> thread pool</a></li>
<li><a href="#integrating-asyncresource-with-eventemitter">Integrating <code>AsyncResource</code> with <code>EventEmitter</code></a></li>
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      <div role="main" id="apicontent">
        <h2>Asynchronous context tracking<span><a class="mark" href="#asynchronous-context-tracking" id="asynchronous-context-tracking">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asynchronous_context_tracking"></a></h2>

<p></p><div class="api_stability api_stability_2"><a href="documentation.html#stability-index">Stability: 2</a> - Stable</div><p></p>
<p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v20.18.0/lib/async_hooks.js">lib/async_hooks.js</a></p>
<section><h3>Introduction<span><a class="mark" href="#introduction" id="introduction">#</a></span><a aria-hidden="true" class="legacy" id="async_context_introduction"></a></h3>
<p>These classes are used to associate state and propagate it throughout
callbacks and promise chains.
They allow storing data throughout the lifetime of a web request
or any other asynchronous duration. It is similar to thread-local storage
in other languages.</p>
<p>The <code>AsyncLocalStorage</code> and <code>AsyncResource</code> classes are part of the
<code>node:async_hooks</code> module:</p>

<pre class="with-73-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { <span class="hljs-title class_">AsyncLocalStorage</span>, <span class="hljs-title class_">AsyncResource</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>;</code><code class="language-js cjs"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">AsyncLocalStorage</span>, <span class="hljs-title class_">AsyncResource</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);</code><button class="copy-button">copy</button></pre>
</section><section><h3>Class: <code>AsyncLocalStorage</code><span><a class="mark" href="#class-asynclocalstorage" id="class-asynclocalstorage">#</a></span><a aria-hidden="true" class="legacy" id="async_context_class_asynclocalstorage"></a></h3>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v16.4.0</td>
<td><p>AsyncLocalStorage is now Stable. Previously, it had been Experimental.</p></td></tr>
<tr><td>v13.10.0, v12.17.0</td>
<td><p><span>Added in: v13.10.0, v12.17.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<p>This class creates stores that stay coherent through asynchronous operations.</p>
<p>While you can create your own implementation on top of the <code>node:async_hooks</code>
module, <code>AsyncLocalStorage</code> should be preferred as it is a performant and memory
safe implementation that involves significant optimizations that are non-obvious
to implement.</p>
<p>The following example uses <code>AsyncLocalStorage</code> to build a simple logger
that assigns IDs to incoming HTTP requests and includes them in messages
logged within each request.</p>

<pre class="with-58-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> http <span class="hljs-keyword">from</span> <span class="hljs-string">'node:http'</span>;
<span class="hljs-keyword">import</span> { <span class="hljs-title class_">AsyncLocalStorage</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>;

<span class="hljs-keyword">const</span> asyncLocalStorage = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncLocalStorage</span>();

<span class="hljs-keyword">function</span> <span class="hljs-title function_">logWithId</span>(<span class="hljs-params">msg</span>) {
  <span class="hljs-keyword">const</span> id = asyncLocalStorage.<span class="hljs-title function_">getStore</span>();
  <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`<span class="hljs-subst">${id !== <span class="hljs-literal">undefined</span> ? id : <span class="hljs-string">'-'</span>}</span>:`</span>, msg);
}

<span class="hljs-keyword">let</span> idSeq = <span class="hljs-number">0</span>;
http.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> {
  asyncLocalStorage.<span class="hljs-title function_">run</span>(idSeq++, <span class="hljs-function">() =></span> {
    <span class="hljs-title function_">logWithId</span>(<span class="hljs-string">'start'</span>);
    <span class="hljs-comment">// Imagine any chain of async operations here</span>
    <span class="hljs-title function_">setImmediate</span>(<span class="hljs-function">() =></span> {
      <span class="hljs-title function_">logWithId</span>(<span class="hljs-string">'finish'</span>);
      res.<span class="hljs-title function_">end</span>();
    });
  });
}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">8080</span>);

http.<span class="hljs-title function_">get</span>(<span class="hljs-string">'http://localhost:8080'</span>);
http.<span class="hljs-title function_">get</span>(<span class="hljs-string">'http://localhost:8080'</span>);
<span class="hljs-comment">// Prints:</span>
<span class="hljs-comment">//   0: start</span>
<span class="hljs-comment">//   1: start</span>
<span class="hljs-comment">//   0: finish</span>
<span class="hljs-comment">//   1: finish</span></code><code class="language-js cjs"><span class="hljs-keyword">const</span> http = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:http'</span>);
<span class="hljs-keyword">const</span> { <span class="hljs-title class_">AsyncLocalStorage</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);

<span class="hljs-keyword">const</span> asyncLocalStorage = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncLocalStorage</span>();

<span class="hljs-keyword">function</span> <span class="hljs-title function_">logWithId</span>(<span class="hljs-params">msg</span>) {
  <span class="hljs-keyword">const</span> id = asyncLocalStorage.<span class="hljs-title function_">getStore</span>();
  <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`<span class="hljs-subst">${id !== <span class="hljs-literal">undefined</span> ? id : <span class="hljs-string">'-'</span>}</span>:`</span>, msg);
}

<span class="hljs-keyword">let</span> idSeq = <span class="hljs-number">0</span>;
http.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> {
  asyncLocalStorage.<span class="hljs-title function_">run</span>(idSeq++, <span class="hljs-function">() =></span> {
    <span class="hljs-title function_">logWithId</span>(<span class="hljs-string">'start'</span>);
    <span class="hljs-comment">// Imagine any chain of async operations here</span>
    <span class="hljs-title function_">setImmediate</span>(<span class="hljs-function">() =></span> {
      <span class="hljs-title function_">logWithId</span>(<span class="hljs-string">'finish'</span>);
      res.<span class="hljs-title function_">end</span>();
    });
  });
}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">8080</span>);

http.<span class="hljs-title function_">get</span>(<span class="hljs-string">'http://localhost:8080'</span>);
http.<span class="hljs-title function_">get</span>(<span class="hljs-string">'http://localhost:8080'</span>);
<span class="hljs-comment">// Prints:</span>
<span class="hljs-comment">//   0: start</span>
<span class="hljs-comment">//   1: start</span>
<span class="hljs-comment">//   0: finish</span>
<span class="hljs-comment">//   1: finish</span></code><button class="copy-button">copy</button></pre>
<p>Each instance of <code>AsyncLocalStorage</code> maintains an independent storage context.
Multiple instances can safely exist simultaneously without risk of interfering
with each other's data.</p>
<h4><code>new AsyncLocalStorage()</code><span><a class="mark" href="#new-asynclocalstorage" id="new-asynclocalstorage">#</a></span><a aria-hidden="true" class="legacy" id="async_context_new_asynclocalstorage"></a></h4>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v19.7.0</td>
<td><p>Removed experimental onPropagate option.</p></td></tr>
<tr><td>v19.2.0, v18.13.0</td>
<td><p>Add option onPropagate.</p></td></tr>
<tr><td>v13.10.0, v12.17.0</td>
<td><p><span>Added in: v13.10.0, v12.17.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<p>Creates a new instance of <code>AsyncLocalStorage</code>. Store is only provided within a
<code>run()</code> call or after an <code>enterWith()</code> call.</p>
<h4>Static method: <code>AsyncLocalStorage.bind(fn)</code><span><a class="mark" href="#static-method-asynclocalstoragebindfn" id="static-method-asynclocalstoragebindfn">#</a></span><a aria-hidden="true" class="legacy" id="async_context_static_method_asynclocalstorage_bind_fn"></a></h4>
<div class="api_metadata">
<span>Added in: v19.8.0</span>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<ul>
<li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a> The function to bind to the current execution context.</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a> A new function that calls <code>fn</code> within the captured
execution context.</li>
</ul>
<p>Binds the given function to the current execution context.</p>
<h4>Static method: <code>AsyncLocalStorage.snapshot()</code><span><a class="mark" href="#static-method-asynclocalstoragesnapshot" id="static-method-asynclocalstoragesnapshot">#</a></span><a aria-hidden="true" class="legacy" id="async_context_static_method_asynclocalstorage_snapshot"></a></h4>
<div class="api_metadata">
<span>Added in: v19.8.0</span>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a> A new function with the signature
<code>(fn: (...args) : R, ...args) : R</code>.</li>
</ul>
<p>Captures the current execution context and returns a function that accepts a
function as an argument. Whenever the returned function is called, it
calls the function passed to it within the captured context.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> asyncLocalStorage = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncLocalStorage</span>();
<span class="hljs-keyword">const</span> runInAsyncScope = asyncLocalStorage.<span class="hljs-title function_">run</span>(<span class="hljs-number">123</span>, <span class="hljs-function">() =></span> <span class="hljs-title class_">AsyncLocalStorage</span>.<span class="hljs-title function_">snapshot</span>());
<span class="hljs-keyword">const</span> result = asyncLocalStorage.<span class="hljs-title function_">run</span>(<span class="hljs-number">321</span>, <span class="hljs-function">() =></span> <span class="hljs-title function_">runInAsyncScope</span>(<span class="hljs-function">() =></span> asyncLocalStorage.<span class="hljs-title function_">getStore</span>()));
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(result);  <span class="hljs-comment">// returns 123</span></code> <button class="copy-button">copy</button></pre>
<p>AsyncLocalStorage.snapshot() can replace the use of AsyncResource for simple
async context tracking purposes, for example:</p>
<pre><code class="language-js"><span class="hljs-keyword">class</span> <span class="hljs-title class_">Foo</span> {
  #runInAsyncScope = <span class="hljs-title class_">AsyncLocalStorage</span>.<span class="hljs-title function_">snapshot</span>();

  <span class="hljs-title function_">get</span>(<span class="hljs-params"></span>) { <span class="hljs-keyword">return</span> <span class="hljs-variable language_">this</span>.#<span class="hljs-title function_">runInAsyncScope</span>(<span class="hljs-function">() =></span> asyncLocalStorage.<span class="hljs-title function_">getStore</span>()); }
}

<span class="hljs-keyword">const</span> foo = asyncLocalStorage.<span class="hljs-title function_">run</span>(<span class="hljs-number">123</span>, <span class="hljs-function">() =></span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Foo</span>());
<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(asyncLocalStorage.<span class="hljs-title function_">run</span>(<span class="hljs-number">321</span>, <span class="hljs-function">() =></span> foo.<span class="hljs-title function_">get</span>())); <span class="hljs-comment">// returns 123</span></code> <button class="copy-button">copy</button></pre>
<h4><code>asyncLocalStorage.disable()</code><span><a class="mark" href="#asynclocalstoragedisable" id="asynclocalstoragedisable">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asynclocalstorage_disable"></a></h4>
<div class="api_metadata">
<span>Added in: v13.10.0, v12.17.0</span>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<p>Disables the instance of <code>AsyncLocalStorage</code>. All subsequent calls
to <code>asyncLocalStorage.getStore()</code> will return <code>undefined</code> until
<code>asyncLocalStorage.run()</code> or <code>asyncLocalStorage.enterWith()</code> is called again.</p>
<p>When calling <code>asyncLocalStorage.disable()</code>, all current contexts linked to the
instance will be exited.</p>
<p>Calling <code>asyncLocalStorage.disable()</code> is required before the
<code>asyncLocalStorage</code> can be garbage collected. This does not apply to stores
provided by the <code>asyncLocalStorage</code>, as those objects are garbage collected
along with the corresponding async resources.</p>
<p>Use this method when the <code>asyncLocalStorage</code> is not in use anymore
in the current process.</p>
<h4><code>asyncLocalStorage.getStore()</code><span><a class="mark" href="#asynclocalstoragegetstore" id="asynclocalstoragegetstore">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asynclocalstorage_getstore"></a></h4>
<div class="api_metadata">
<span>Added in: v13.10.0, v12.17.0</span>
</div>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type">&#x3C;any></a></li>
</ul>
<p>Returns the current store.
If called outside of an asynchronous context initialized by
calling <code>asyncLocalStorage.run()</code> or <code>asyncLocalStorage.enterWith()</code>, it
returns <code>undefined</code>.</p>
<h4><code>asyncLocalStorage.enterWith(store)</code><span><a class="mark" href="#asynclocalstorageenterwithstore" id="asynclocalstorageenterwithstore">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asynclocalstorage_enterwith_store"></a></h4>
<div class="api_metadata">
<span>Added in: v13.11.0, v12.17.0</span>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<ul>
<li><code>store</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type">&#x3C;any></a></li>
</ul>
<p>Transitions into the context for the remainder of the current
synchronous execution and then persists the store through any following
asynchronous calls.</p>
<p>Example:</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> store = { <span class="hljs-attr">id</span>: <span class="hljs-number">1</span> };
<span class="hljs-comment">// Replaces previous store with the given store object</span>
asyncLocalStorage.<span class="hljs-title function_">enterWith</span>(store);
asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the store object</span>
<span class="hljs-title function_">someAsyncOperation</span>(<span class="hljs-function">() =></span> {
  asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the same object</span>
});</code> <button class="copy-button">copy</button></pre>
<p>This transition will continue for the <em>entire</em> synchronous execution.
This means that if, for example, the context is entered within an event
handler subsequent event handlers will also run within that context unless
specifically bound to another context with an <code>AsyncResource</code>. That is why
<code>run()</code> should be preferred over <code>enterWith()</code> unless there are strong reasons
to use the latter method.</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> store = { <span class="hljs-attr">id</span>: <span class="hljs-number">1</span> };

emitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'my-event'</span>, <span class="hljs-function">() =></span> {
  asyncLocalStorage.<span class="hljs-title function_">enterWith</span>(store);
});
emitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'my-event'</span>, <span class="hljs-function">() =></span> {
  asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the same object</span>
});

asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns undefined</span>
emitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'my-event'</span>);
asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the same object</span></code> <button class="copy-button">copy</button></pre>
<h4><code>asyncLocalStorage.run(store, callback[, ...args])</code><span><a class="mark" href="#asynclocalstoragerunstore-callback-args" id="asynclocalstoragerunstore-callback-args">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asynclocalstorage_run_store_callback_args"></a></h4>
<div class="api_metadata">
<span>Added in: v13.10.0, v12.17.0</span>
</div>
<ul>
<li><code>store</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type">&#x3C;any></a></li>
<li><code>callback</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a></li>
<li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type">&#x3C;any></a></li>
</ul>
<p>Runs a function synchronously within a context and returns its
return value. The store is not accessible outside of the callback function.
The store is accessible to any asynchronous operations created within the
callback.</p>
<p>The optional <code>args</code> are passed to the callback function.</p>
<p>If the callback function throws an error, the error is thrown by <code>run()</code> too.
The stacktrace is not impacted by this call and the context is exited.</p>
<p>Example:</p>
<pre><code class="language-js"><span class="hljs-keyword">const</span> store = { <span class="hljs-attr">id</span>: <span class="hljs-number">2</span> };
<span class="hljs-keyword">try</span> {
  asyncLocalStorage.<span class="hljs-title function_">run</span>(store, <span class="hljs-function">() =></span> {
    asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the store object</span>
    <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> {
      asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the store object</span>
    }, <span class="hljs-number">200</span>);
    <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>();
  });
} <span class="hljs-keyword">catch</span> (e) {
  asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns undefined</span>
  <span class="hljs-comment">// The error will be caught here</span>
}</code> <button class="copy-button">copy</button></pre>
<h4><code>asyncLocalStorage.exit(callback[, ...args])</code><span><a class="mark" href="#asynclocalstorageexitcallback-args" id="asynclocalstorageexitcallback-args">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asynclocalstorage_exit_callback_args"></a></h4>
<div class="api_metadata">
<span>Added in: v13.10.0, v12.17.0</span>
</div>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<ul>
<li><code>callback</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a></li>
<li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type">&#x3C;any></a></li>
</ul>
<p>Runs a function synchronously outside of a context and returns its
return value. The store is not accessible within the callback function or
the asynchronous operations created within the callback. Any <code>getStore()</code>
call done within the callback function will always return <code>undefined</code>.</p>
<p>The optional <code>args</code> are passed to the callback function.</p>
<p>If the callback function throws an error, the error is thrown by <code>exit()</code> too.
The stacktrace is not impacted by this call and the context is re-entered.</p>
<p>Example:</p>
<pre><code class="language-js"><span class="hljs-comment">// Within a call to run</span>
<span class="hljs-keyword">try</span> {
  asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the store object or value</span>
  asyncLocalStorage.<span class="hljs-title function_">exit</span>(<span class="hljs-function">() =></span> {
    asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns undefined</span>
    <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>();
  });
} <span class="hljs-keyword">catch</span> (e) {
  asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the same object or value</span>
  <span class="hljs-comment">// The error will be caught here</span>
}</code> <button class="copy-button">copy</button></pre>
<h4>Usage with <code>async/await</code><span><a class="mark" href="#usage-with-asyncawait" id="usage-with-asyncawait">#</a></span><a aria-hidden="true" class="legacy" id="async_context_usage_with_async_await"></a></h4>
<p>If, within an async function, only one <code>await</code> call is to run within a context,
the following pattern should be used:</p>
<pre><code class="language-js"><span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">fn</span>(<span class="hljs-params"></span>) {
  <span class="hljs-keyword">await</span> asyncLocalStorage.<span class="hljs-title function_">run</span>(<span class="hljs-keyword">new</span> <span class="hljs-title class_">Map</span>(), <span class="hljs-function">() =></span> {
    asyncLocalStorage.<span class="hljs-title function_">getStore</span>().<span class="hljs-title function_">set</span>(<span class="hljs-string">'key'</span>, value);
    <span class="hljs-keyword">return</span> <span class="hljs-title function_">foo</span>(); <span class="hljs-comment">// The return value of foo will be awaited</span>
  });
}</code> <button class="copy-button">copy</button></pre>
<p>In this example, the store is only available in the callback function and the
functions called by <code>foo</code>. Outside of <code>run</code>, calling <code>getStore</code> will return
<code>undefined</code>.</p>
<h4>Troubleshooting: Context loss<span><a class="mark" href="#troubleshooting-context-loss" id="troubleshooting-context-loss">#</a></span><a aria-hidden="true" class="legacy" id="async_context_troubleshooting_context_loss"></a></h4>
<p>In most cases, <code>AsyncLocalStorage</code> works without issues. In rare situations, the
current store is lost in one of the asynchronous operations.</p>
<p>If your code is callback-based, it is enough to promisify it with
<a href="util.html#utilpromisifyoriginal"><code>util.promisify()</code></a> so it starts working with native promises.</p>
<p>If you need to use a callback-based API or your code assumes
a custom thenable implementation, use the <a href="#class-asyncresource"><code>AsyncResource</code></a> class
to associate the asynchronous operation with the correct execution context.
Find the function call responsible for the context loss by logging the content
of <code>asyncLocalStorage.getStore()</code> after the calls you suspect are responsible
for the loss. When the code logs <code>undefined</code>, the last callback called is
probably responsible for the context loss.</p>
</section><section><h3>Class: <code>AsyncResource</code><span><a class="mark" href="#class-asyncresource" id="class-asyncresource">#</a></span><a aria-hidden="true" class="legacy" id="async_context_class_asyncresource"></a></h3>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v16.4.0</td>
<td><p>AsyncResource is now Stable. Previously, it had been Experimental.</p></td></tr>
</tbody></table>
</details>
</div>
<p>The class <code>AsyncResource</code> is designed to be extended by the embedder's async
resources. Using this, users can easily trigger the lifetime events of their
own resources.</p>
<p>The <code>init</code> hook will trigger when an <code>AsyncResource</code> is instantiated.</p>
<p>The following is an overview of the <code>AsyncResource</code> API.</p>

<pre class="with-72-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { <span class="hljs-title class_">AsyncResource</span>, executionAsyncId } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>;

<span class="hljs-comment">// AsyncResource() is meant to be extended. Instantiating a</span>
<span class="hljs-comment">// new AsyncResource() also triggers init. If triggerAsyncId is omitted then</span>
<span class="hljs-comment">// async_hook.executionAsyncId() is used.</span>
<span class="hljs-keyword">const</span> asyncResource = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncResource</span>(
  type, { <span class="hljs-attr">triggerAsyncId</span>: <span class="hljs-title function_">executionAsyncId</span>(), <span class="hljs-attr">requireManualDestroy</span>: <span class="hljs-literal">false</span> },
);

<span class="hljs-comment">// Run a function in the execution context of the resource. This will</span>
<span class="hljs-comment">// * establish the context of the resource</span>
<span class="hljs-comment">// * trigger the AsyncHooks before callbacks</span>
<span class="hljs-comment">// * call the provided function `fn` with the supplied arguments</span>
<span class="hljs-comment">// * trigger the AsyncHooks after callbacks</span>
<span class="hljs-comment">// * restore the original execution context</span>
asyncResource.<span class="hljs-title function_">runInAsyncScope</span>(fn, thisArg, ...args);

<span class="hljs-comment">// Call AsyncHooks destroy callbacks.</span>
asyncResource.<span class="hljs-title function_">emitDestroy</span>();

<span class="hljs-comment">// Return the unique ID assigned to the AsyncResource instance.</span>
asyncResource.<span class="hljs-title function_">asyncId</span>();

<span class="hljs-comment">// Return the trigger ID for the AsyncResource instance.</span>
asyncResource.<span class="hljs-title function_">triggerAsyncId</span>();</code><code class="language-js cjs"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">AsyncResource</span>, executionAsyncId } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);

<span class="hljs-comment">// AsyncResource() is meant to be extended. Instantiating a</span>
<span class="hljs-comment">// new AsyncResource() also triggers init. If triggerAsyncId is omitted then</span>
<span class="hljs-comment">// async_hook.executionAsyncId() is used.</span>
<span class="hljs-keyword">const</span> asyncResource = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncResource</span>(
  type, { <span class="hljs-attr">triggerAsyncId</span>: <span class="hljs-title function_">executionAsyncId</span>(), <span class="hljs-attr">requireManualDestroy</span>: <span class="hljs-literal">false</span> },
);

<span class="hljs-comment">// Run a function in the execution context of the resource. This will</span>
<span class="hljs-comment">// * establish the context of the resource</span>
<span class="hljs-comment">// * trigger the AsyncHooks before callbacks</span>
<span class="hljs-comment">// * call the provided function `fn` with the supplied arguments</span>
<span class="hljs-comment">// * trigger the AsyncHooks after callbacks</span>
<span class="hljs-comment">// * restore the original execution context</span>
asyncResource.<span class="hljs-title function_">runInAsyncScope</span>(fn, thisArg, ...args);

<span class="hljs-comment">// Call AsyncHooks destroy callbacks.</span>
asyncResource.<span class="hljs-title function_">emitDestroy</span>();

<span class="hljs-comment">// Return the unique ID assigned to the AsyncResource instance.</span>
asyncResource.<span class="hljs-title function_">asyncId</span>();

<span class="hljs-comment">// Return the trigger ID for the AsyncResource instance.</span>
asyncResource.<span class="hljs-title function_">triggerAsyncId</span>();</code><button class="copy-button">copy</button></pre>
<h4><code>new AsyncResource(type[, options])</code><span><a class="mark" href="#new-asyncresourcetype-options" id="new-asyncresourcetype-options">#</a></span><a aria-hidden="true" class="legacy" id="async_context_new_asyncresource_type_options"></a></h4>
<ul>
<li><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&#x3C;string></a> The type of async event.</li>
<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type">&#x3C;Object></a>
<ul>
<li><code>triggerAsyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a> The ID of the execution context that created this
async event. <strong>Default:</strong> <code>executionAsyncId()</code>.</li>
<li><code>requireManualDestroy</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type">&#x3C;boolean></a> If set to <code>true</code>, disables <code>emitDestroy</code>
when the object is garbage collected. This usually does not need to be set
(even if <code>emitDestroy</code> is called manually), unless the resource's <code>asyncId</code>
is retrieved and the sensitive API's <code>emitDestroy</code> is called with it.
When set to <code>false</code>, the <code>emitDestroy</code> call on garbage collection
will only take place if there is at least one active <code>destroy</code> hook.
<strong>Default:</strong> <code>false</code>.</li>
</ul>
</li>
</ul>
<p>Example usage:</p>
<pre><code class="language-js"><span class="hljs-keyword">class</span> <span class="hljs-title class_">DBQuery</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">AsyncResource</span> {
  <span class="hljs-title function_">constructor</span>(<span class="hljs-params">db</span>) {
    <span class="hljs-variable language_">super</span>(<span class="hljs-string">'DBQuery'</span>);
    <span class="hljs-variable language_">this</span>.<span class="hljs-property">db</span> = db;
  }

  <span class="hljs-title function_">getInfo</span>(<span class="hljs-params">query, callback</span>) {
    <span class="hljs-variable language_">this</span>.<span class="hljs-property">db</span>.<span class="hljs-title function_">get</span>(query, <span class="hljs-function">(<span class="hljs-params">err, data</span>) =></span> {
      <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">runInAsyncScope</span>(callback, <span class="hljs-literal">null</span>, err, data);
    });
  }

  <span class="hljs-title function_">close</span>(<span class="hljs-params"></span>) {
    <span class="hljs-variable language_">this</span>.<span class="hljs-property">db</span> = <span class="hljs-literal">null</span>;
    <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emitDestroy</span>();
  }
}</code> <button class="copy-button">copy</button></pre>
<h4>Static method: <code>AsyncResource.bind(fn[, type[, thisArg]])</code><span><a class="mark" href="#static-method-asyncresourcebindfn-type-thisarg" id="static-method-asyncresourcebindfn-type-thisarg">#</a></span><a aria-hidden="true" class="legacy" id="async_context_static_method_asyncresource_bind_fn_type_thisarg"></a></h4>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v20.0.0</td>
<td><p>The <code>asyncResource</code> property added to the bound function has been deprecated and will be removed in a future version.</p></td></tr>
<tr><td>v17.8.0, v16.15.0</td>
<td><p>Changed the default when <code>thisArg</code> is undefined to use <code>this</code> from the caller.</p></td></tr>
<tr><td>v16.0.0</td>
<td><p>Added optional thisArg.</p></td></tr>
<tr><td>v14.8.0, v12.19.0</td>
<td><p><span>Added in: v14.8.0, v12.19.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<ul>
<li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a> The function to bind to the current execution context.</li>
<li><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&#x3C;string></a> An optional name to associate with the underlying
<code>AsyncResource</code>.</li>
<li><code>thisArg</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type">&#x3C;any></a></li>
</ul>
<p>Binds the given function to the current execution context.</p>
<h4><code>asyncResource.bind(fn[, thisArg])</code><span><a class="mark" href="#asyncresourcebindfn-thisarg" id="asyncresourcebindfn-thisarg">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asyncresource_bind_fn_thisarg"></a></h4>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v20.0.0</td>
<td><p>The <code>asyncResource</code> property added to the bound function has been deprecated and will be removed in a future version.</p></td></tr>
<tr><td>v17.8.0, v16.15.0</td>
<td><p>Changed the default when <code>thisArg</code> is undefined to use <code>this</code> from the caller.</p></td></tr>
<tr><td>v16.0.0</td>
<td><p>Added optional thisArg.</p></td></tr>
<tr><td>v14.8.0, v12.19.0</td>
<td><p><span>Added in: v14.8.0, v12.19.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<ul>
<li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a> The function to bind to the current <code>AsyncResource</code>.</li>
<li><code>thisArg</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type">&#x3C;any></a></li>
</ul>
<p>Binds the given function to execute to this <code>AsyncResource</code>'s scope.</p>
<h4><code>asyncResource.runInAsyncScope(fn[, thisArg, ...args])</code><span><a class="mark" href="#asyncresourceruninasyncscopefn-thisarg-args" id="asyncresourceruninasyncscopefn-thisarg-args">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asyncresource_runinasyncscope_fn_thisarg_args"></a></h4>
<div class="api_metadata">
<span>Added in: v9.6.0</span>
</div>
<ul>
<li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a> The function to call in the execution context of this async
resource.</li>
<li><code>thisArg</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type">&#x3C;any></a> The receiver to be used for the function call.</li>
<li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type">&#x3C;any></a> Optional arguments to pass to the function.</li>
</ul>
<p>Call the provided function with the provided arguments in the execution context
of the async resource. This will establish the context, trigger the AsyncHooks
before callbacks, call the function, trigger the AsyncHooks after callbacks, and
then restore the original execution context.</p>
<h4><code>asyncResource.emitDestroy()</code><span><a class="mark" href="#asyncresourceemitdestroy" id="asyncresourceemitdestroy">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asyncresource_emitdestroy"></a></h4>
<ul>
<li>Returns: <a href="async_hooks.html#class-asyncresource" class="type">&#x3C;AsyncResource></a> A reference to <code>asyncResource</code>.</li>
</ul>
<p>Call all <code>destroy</code> hooks. This should only ever be called once. An error will
be thrown if it is called more than once. This <strong>must</strong> be manually called. If
the resource is left to be collected by the GC then the <code>destroy</code> hooks will
never be called.</p>
<h4><code>asyncResource.asyncId()</code><span><a class="mark" href="#asyncresourceasyncid" id="asyncresourceasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asyncresource_asyncid"></a></h4>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a> The unique <code>asyncId</code> assigned to the resource.</li>
</ul>
<h4><code>asyncResource.triggerAsyncId()</code><span><a class="mark" href="#asyncresourcetriggerasyncid" id="asyncresourcetriggerasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asyncresource_triggerasyncid"></a></h4>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a> The same <code>triggerAsyncId</code> that is passed to the
<code>AsyncResource</code> constructor.</li>
</ul>
<p><a id="async-resource-worker-pool"></a></p>
<h4>Using <code>AsyncResource</code> for a <code>Worker</code> thread pool<span><a class="mark" href="#using-asyncresource-for-a-worker-thread-pool" id="using-asyncresource-for-a-worker-thread-pool">#</a></span><a aria-hidden="true" class="legacy" id="async_context_using_asyncresource_for_a_worker_thread_pool"></a></h4>
<p>The following example shows how to use the <code>AsyncResource</code> class to properly
provide async tracking for a <a href="worker_threads.html#class-worker"><code>Worker</code></a> pool. Other resource pools, such as
database connection pools, can follow a similar model.</p>
<p>Assuming that the task is adding two numbers, using a file named
<code>task_processor.js</code> with the following content:</p>

<pre class="with-54-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { parentPort } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:worker_threads'</span>;
parentPort.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">task</span>) =></span> {
  parentPort.<span class="hljs-title function_">postMessage</span>(task.<span class="hljs-property">a</span> + task.<span class="hljs-property">b</span>);
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> { parentPort } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:worker_threads'</span>);
parentPort.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">task</span>) =></span> {
  parentPort.<span class="hljs-title function_">postMessage</span>(task.<span class="hljs-property">a</span> + task.<span class="hljs-property">b</span>);
});</code><button class="copy-button">copy</button></pre>
<p>a Worker pool around it could use the following structure:</p>

<pre class="with-54-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { <span class="hljs-title class_">AsyncResource</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>;
<span class="hljs-keyword">import</span> { <span class="hljs-title class_">EventEmitter</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:events'</span>;
<span class="hljs-keyword">import</span> path <span class="hljs-keyword">from</span> <span class="hljs-string">'node:path'</span>;
<span class="hljs-keyword">import</span> { <span class="hljs-title class_">Worker</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:worker_threads'</span>;

<span class="hljs-keyword">const</span> kTaskInfo = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'kTaskInfo'</span>);
<span class="hljs-keyword">const</span> kWorkerFreedEvent = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'kWorkerFreedEvent'</span>);

<span class="hljs-keyword">class</span> <span class="hljs-title class_">WorkerPoolTaskInfo</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">AsyncResource</span> {
  <span class="hljs-title function_">constructor</span>(<span class="hljs-params">callback</span>) {
    <span class="hljs-variable language_">super</span>(<span class="hljs-string">'WorkerPoolTaskInfo'</span>);
    <span class="hljs-variable language_">this</span>.<span class="hljs-property">callback</span> = callback;
  }

  <span class="hljs-title function_">done</span>(<span class="hljs-params">err, result</span>) {
    <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">runInAsyncScope</span>(<span class="hljs-variable language_">this</span>.<span class="hljs-property">callback</span>, <span class="hljs-literal">null</span>, err, result);
    <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emitDestroy</span>();  <span class="hljs-comment">// `TaskInfo`s are used only once.</span>
  }
}

<span class="hljs-keyword">export</span> <span class="hljs-keyword">default</span> <span class="hljs-keyword">class</span> <span class="hljs-title class_">WorkerPool</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">EventEmitter</span> {
  <span class="hljs-title function_">constructor</span>(<span class="hljs-params">numThreads</span>) {
    <span class="hljs-variable language_">super</span>();
    <span class="hljs-variable language_">this</span>.<span class="hljs-property">numThreads</span> = numThreads;
    <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span> = [];
    <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span> = [];
    <span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span> = [];

    <span class="hljs-keyword">for</span> (<span class="hljs-keyword">let</span> i = <span class="hljs-number">0</span>; i &#x3C; numThreads; i++)
      <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">addNewWorker</span>();

    <span class="hljs-comment">// Any time the kWorkerFreedEvent is emitted, dispatch</span>
    <span class="hljs-comment">// the next task pending in the queue, if any.</span>
    <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">on</span>(kWorkerFreedEvent, <span class="hljs-function">() =></span> {
      <span class="hljs-keyword">if</span> (<span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span>.<span class="hljs-property">length</span> > <span class="hljs-number">0</span>) {
        <span class="hljs-keyword">const</span> { task, callback } = <span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span>.<span class="hljs-title function_">shift</span>();
        <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">runTask</span>(task, callback);
      }
    });
  }

  <span class="hljs-title function_">addNewWorker</span>(<span class="hljs-params"></span>) {
    <span class="hljs-keyword">const</span> worker = <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-string">'task_processor.js'</span>, <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>));
    worker.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">result</span>) =></span> {
      <span class="hljs-comment">// In case of success: Call the callback that was passed to `runTask`,</span>
      <span class="hljs-comment">// remove the `TaskInfo` associated with the Worker, and mark it as free</span>
      <span class="hljs-comment">// again.</span>
      worker[kTaskInfo].<span class="hljs-title function_">done</span>(<span class="hljs-literal">null</span>, result);
      worker[kTaskInfo] = <span class="hljs-literal">null</span>;
      <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-title function_">push</span>(worker);
      <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(kWorkerFreedEvent);
    });
    worker.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">err</span>) =></span> {
      <span class="hljs-comment">// In case of an uncaught exception: Call the callback that was passed to</span>
      <span class="hljs-comment">// `runTask` with the error.</span>
      <span class="hljs-keyword">if</span> (worker[kTaskInfo])
        worker[kTaskInfo].<span class="hljs-title function_">done</span>(err, <span class="hljs-literal">null</span>);
      <span class="hljs-keyword">else</span>
        <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'error'</span>, err);
      <span class="hljs-comment">// Remove the worker from the list and start a new Worker to replace the</span>
      <span class="hljs-comment">// current one.</span>
      <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>.<span class="hljs-title function_">splice</span>(<span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>.<span class="hljs-title function_">indexOf</span>(worker), <span class="hljs-number">1</span>);
      <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">addNewWorker</span>();
    });
    <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>.<span class="hljs-title function_">push</span>(worker);
    <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-title function_">push</span>(worker);
    <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(kWorkerFreedEvent);
  }

  <span class="hljs-title function_">runTask</span>(<span class="hljs-params">task, callback</span>) {
    <span class="hljs-keyword">if</span> (<span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-property">length</span> === <span class="hljs-number">0</span>) {
      <span class="hljs-comment">// No free threads, wait until a worker thread becomes free.</span>
      <span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span>.<span class="hljs-title function_">push</span>({ task, callback });
      <span class="hljs-keyword">return</span>;
    }

    <span class="hljs-keyword">const</span> worker = <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-title function_">pop</span>();
    worker[kTaskInfo] = <span class="hljs-keyword">new</span> <span class="hljs-title class_">WorkerPoolTaskInfo</span>(callback);
    worker.<span class="hljs-title function_">postMessage</span>(task);
  }

  <span class="hljs-title function_">close</span>(<span class="hljs-params"></span>) {
    <span class="hljs-keyword">for</span> (<span class="hljs-keyword">const</span> worker <span class="hljs-keyword">of</span> <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>) worker.<span class="hljs-title function_">terminate</span>();
  }
}</code><code class="language-js cjs"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">AsyncResource</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);
<span class="hljs-keyword">const</span> { <span class="hljs-title class_">EventEmitter</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:events'</span>);
<span class="hljs-keyword">const</span> path = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:path'</span>);
<span class="hljs-keyword">const</span> { <span class="hljs-title class_">Worker</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:worker_threads'</span>);

<span class="hljs-keyword">const</span> kTaskInfo = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'kTaskInfo'</span>);
<span class="hljs-keyword">const</span> kWorkerFreedEvent = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'kWorkerFreedEvent'</span>);

<span class="hljs-keyword">class</span> <span class="hljs-title class_">WorkerPoolTaskInfo</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">AsyncResource</span> {
  <span class="hljs-title function_">constructor</span>(<span class="hljs-params">callback</span>) {
    <span class="hljs-variable language_">super</span>(<span class="hljs-string">'WorkerPoolTaskInfo'</span>);
    <span class="hljs-variable language_">this</span>.<span class="hljs-property">callback</span> = callback;
  }

  <span class="hljs-title function_">done</span>(<span class="hljs-params">err, result</span>) {
    <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">runInAsyncScope</span>(<span class="hljs-variable language_">this</span>.<span class="hljs-property">callback</span>, <span class="hljs-literal">null</span>, err, result);
    <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emitDestroy</span>();  <span class="hljs-comment">// `TaskInfo`s are used only once.</span>
  }
}

<span class="hljs-keyword">class</span> <span class="hljs-title class_">WorkerPool</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">EventEmitter</span> {
  <span class="hljs-title function_">constructor</span>(<span class="hljs-params">numThreads</span>) {
    <span class="hljs-variable language_">super</span>();
    <span class="hljs-variable language_">this</span>.<span class="hljs-property">numThreads</span> = numThreads;
    <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span> = [];
    <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span> = [];
    <span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span> = [];

    <span class="hljs-keyword">for</span> (<span class="hljs-keyword">let</span> i = <span class="hljs-number">0</span>; i &#x3C; numThreads; i++)
      <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">addNewWorker</span>();

    <span class="hljs-comment">// Any time the kWorkerFreedEvent is emitted, dispatch</span>
    <span class="hljs-comment">// the next task pending in the queue, if any.</span>
    <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">on</span>(kWorkerFreedEvent, <span class="hljs-function">() =></span> {
      <span class="hljs-keyword">if</span> (<span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span>.<span class="hljs-property">length</span> > <span class="hljs-number">0</span>) {
        <span class="hljs-keyword">const</span> { task, callback } = <span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span>.<span class="hljs-title function_">shift</span>();
        <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">runTask</span>(task, callback);
      }
    });
  }

  <span class="hljs-title function_">addNewWorker</span>(<span class="hljs-params"></span>) {
    <span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(path.<span class="hljs-title function_">resolve</span>(__dirname, <span class="hljs-string">'task_processor.js'</span>));
    worker.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">result</span>) =></span> {
      <span class="hljs-comment">// In case of success: Call the callback that was passed to `runTask`,</span>
      <span class="hljs-comment">// remove the `TaskInfo` associated with the Worker, and mark it as free</span>
      <span class="hljs-comment">// again.</span>
      worker[kTaskInfo].<span class="hljs-title function_">done</span>(<span class="hljs-literal">null</span>, result);
      worker[kTaskInfo] = <span class="hljs-literal">null</span>;
      <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-title function_">push</span>(worker);
      <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(kWorkerFreedEvent);
    });
    worker.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">err</span>) =></span> {
      <span class="hljs-comment">// In case of an uncaught exception: Call the callback that was passed to</span>
      <span class="hljs-comment">// `runTask` with the error.</span>
      <span class="hljs-keyword">if</span> (worker[kTaskInfo])
        worker[kTaskInfo].<span class="hljs-title function_">done</span>(err, <span class="hljs-literal">null</span>);
      <span class="hljs-keyword">else</span>
        <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'error'</span>, err);
      <span class="hljs-comment">// Remove the worker from the list and start a new Worker to replace the</span>
      <span class="hljs-comment">// current one.</span>
      <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>.<span class="hljs-title function_">splice</span>(<span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>.<span class="hljs-title function_">indexOf</span>(worker), <span class="hljs-number">1</span>);
      <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">addNewWorker</span>();
    });
    <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>.<span class="hljs-title function_">push</span>(worker);
    <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-title function_">push</span>(worker);
    <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(kWorkerFreedEvent);
  }

  <span class="hljs-title function_">runTask</span>(<span class="hljs-params">task, callback</span>) {
    <span class="hljs-keyword">if</span> (<span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-property">length</span> === <span class="hljs-number">0</span>) {
      <span class="hljs-comment">// No free threads, wait until a worker thread becomes free.</span>
      <span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span>.<span class="hljs-title function_">push</span>({ task, callback });
      <span class="hljs-keyword">return</span>;
    }

    <span class="hljs-keyword">const</span> worker = <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-title function_">pop</span>();
    worker[kTaskInfo] = <span class="hljs-keyword">new</span> <span class="hljs-title class_">WorkerPoolTaskInfo</span>(callback);
    worker.<span class="hljs-title function_">postMessage</span>(task);
  }

  <span class="hljs-title function_">close</span>(<span class="hljs-params"></span>) {
    <span class="hljs-keyword">for</span> (<span class="hljs-keyword">const</span> worker <span class="hljs-keyword">of</span> <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>) worker.<span class="hljs-title function_">terminate</span>();
  }
}

<span class="hljs-variable language_">module</span>.<span class="hljs-property">exports</span> = <span class="hljs-title class_">WorkerPool</span>;</code><button class="copy-button">copy</button></pre>
<p>Without the explicit tracking added by the <code>WorkerPoolTaskInfo</code> objects,
it would appear that the callbacks are associated with the individual <code>Worker</code>
objects. However, the creation of the <code>Worker</code>s is not associated with the
creation of the tasks and does not provide information about when tasks
were scheduled.</p>
<p>This pool could be used as follows:</p>

<pre class="with-47-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> <span class="hljs-title class_">WorkerPool</span> <span class="hljs-keyword">from</span> <span class="hljs-string">'./worker_pool.js'</span>;
<span class="hljs-keyword">import</span> os <span class="hljs-keyword">from</span> <span class="hljs-string">'node:os'</span>;

<span class="hljs-keyword">const</span> pool = <span class="hljs-keyword">new</span> <span class="hljs-title class_">WorkerPool</span>(os.<span class="hljs-title function_">availableParallelism</span>());

<span class="hljs-keyword">let</span> finished = <span class="hljs-number">0</span>;
<span class="hljs-keyword">for</span> (<span class="hljs-keyword">let</span> i = <span class="hljs-number">0</span>; i &#x3C; <span class="hljs-number">10</span>; i++) {
  pool.<span class="hljs-title function_">runTask</span>({ <span class="hljs-attr">a</span>: <span class="hljs-number">42</span>, <span class="hljs-attr">b</span>: <span class="hljs-number">100</span> }, <span class="hljs-function">(<span class="hljs-params">err, result</span>) =></span> {
    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(i, err, result);
    <span class="hljs-keyword">if</span> (++finished === <span class="hljs-number">10</span>)
      pool.<span class="hljs-title function_">close</span>();
  });
}</code><code class="language-js cjs"><span class="hljs-keyword">const</span> <span class="hljs-title class_">WorkerPool</span> = <span class="hljs-built_in">require</span>(<span class="hljs-string">'./worker_pool.js'</span>);
<span class="hljs-keyword">const</span> os = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:os'</span>);

<span class="hljs-keyword">const</span> pool = <span class="hljs-keyword">new</span> <span class="hljs-title class_">WorkerPool</span>(os.<span class="hljs-title function_">availableParallelism</span>());

<span class="hljs-keyword">let</span> finished = <span class="hljs-number">0</span>;
<span class="hljs-keyword">for</span> (<span class="hljs-keyword">let</span> i = <span class="hljs-number">0</span>; i &#x3C; <span class="hljs-number">10</span>; i++) {
  pool.<span class="hljs-title function_">runTask</span>({ <span class="hljs-attr">a</span>: <span class="hljs-number">42</span>, <span class="hljs-attr">b</span>: <span class="hljs-number">100</span> }, <span class="hljs-function">(<span class="hljs-params">err, result</span>) =></span> {
    <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(i, err, result);
    <span class="hljs-keyword">if</span> (++finished === <span class="hljs-number">10</span>)
      pool.<span class="hljs-title function_">close</span>();
  });
}</code><button class="copy-button">copy</button></pre>
<h4>Integrating <code>AsyncResource</code> with <code>EventEmitter</code><span><a class="mark" href="#integrating-asyncresource-with-eventemitter" id="integrating-asyncresource-with-eventemitter">#</a></span><a aria-hidden="true" class="legacy" id="async_context_integrating_asyncresource_with_eventemitter"></a></h4>
<p>Event listeners triggered by an <a href="events.html#class-eventemitter"><code>EventEmitter</code></a> may be run in a different
execution context than the one that was active when <code>eventEmitter.on()</code> was
called.</p>
<p>The following example shows how to use the <code>AsyncResource</code> class to properly
associate an event listener with the correct execution context. The same
approach can be applied to a <a href="stream.html#stream"><code>Stream</code></a> or a similar event-driven class.</p>

<pre class="with-72-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { createServer } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:http'</span>;
<span class="hljs-keyword">import</span> { <span class="hljs-title class_">AsyncResource</span>, executionAsyncId } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>;

<span class="hljs-keyword">const</span> server = <span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> {
  req.<span class="hljs-title function_">on</span>(<span class="hljs-string">'close'</span>, <span class="hljs-title class_">AsyncResource</span>.<span class="hljs-title function_">bind</span>(<span class="hljs-function">() =></span> {
    <span class="hljs-comment">// Execution context is bound to the current outer scope.</span>
  }));
  req.<span class="hljs-title function_">on</span>(<span class="hljs-string">'close'</span>, <span class="hljs-function">() =></span> {
    <span class="hljs-comment">// Execution context is bound to the scope that caused 'close' to emit.</span>
  });
  res.<span class="hljs-title function_">end</span>();
}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">3000</span>);</code><code class="language-js cjs"><span class="hljs-keyword">const</span> { createServer } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:http'</span>);
<span class="hljs-keyword">const</span> { <span class="hljs-title class_">AsyncResource</span>, executionAsyncId } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);

<span class="hljs-keyword">const</span> server = <span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> {
  req.<span class="hljs-title function_">on</span>(<span class="hljs-string">'close'</span>, <span class="hljs-title class_">AsyncResource</span>.<span class="hljs-title function_">bind</span>(<span class="hljs-function">() =></span> {
    <span class="hljs-comment">// Execution context is bound to the current outer scope.</span>
  }));
  req.<span class="hljs-title function_">on</span>(<span class="hljs-string">'close'</span>, <span class="hljs-function">() =></span> {
    <span class="hljs-comment">// Execution context is bound to the scope that caused 'close' to emit.</span>
  });
  res.<span class="hljs-title function_">end</span>();
}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">3000</span>);</code><button class="copy-button">copy</button></pre></section>
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