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119 Table of contents 120 </a> 121 122 <div class="picker"><div class="toc"><ul> 123<li><span class="stability_1"><a href="#async-hooks">Async hooks</a></span> 124<ul> 125<li><a href="#terminology">Terminology</a></li> 126<li><a href="#overview">Overview</a></li> 127<li><a href="#async_hookscreatehookcallbacks"><code>async_hooks.createHook(callbacks)</code></a> 128<ul> 129<li><a href="#error-handling">Error handling</a></li> 130<li><a href="#printing-in-asynchook-callbacks">Printing in <code>AsyncHook</code> callbacks</a></li> 131</ul> 132</li> 133<li><a href="#class-asynchook">Class: <code>AsyncHook</code></a> 134<ul> 135<li><a href="#asynchookenable"><code>asyncHook.enable()</code></a></li> 136<li><a href="#asynchookdisable"><code>asyncHook.disable()</code></a></li> 137<li><a href="#hook-callbacks">Hook callbacks</a> 138<ul> 139<li><a href="#initasyncid-type-triggerasyncid-resource"><code>init(asyncId, type, triggerAsyncId, resource)</code></a> 140<ul> 141<li><a href="#type"><code>type</code></a></li> 142<li><a href="#triggerasyncid"><code>triggerAsyncId</code></a></li> 143<li><a href="#resource"><code>resource</code></a></li> 144<li><a href="#asynchronous-context-example">Asynchronous context example</a></li> 145</ul> 146</li> 147<li><a href="#beforeasyncid"><code>before(asyncId)</code></a></li> 148<li><a href="#afterasyncid"><code>after(asyncId)</code></a></li> 149<li><a href="#destroyasyncid"><code>destroy(asyncId)</code></a></li> 150<li><a href="#promiseresolveasyncid"><code>promiseResolve(asyncId)</code></a></li> 151</ul> 152</li> 153<li><a href="#async_hooksexecutionasyncresource"><code>async_hooks.executionAsyncResource()</code></a></li> 154<li><a href="#async_hooksexecutionasyncid"><code>async_hooks.executionAsyncId()</code></a></li> 155<li><a href="#async_hookstriggerasyncid"><code>async_hooks.triggerAsyncId()</code></a></li> 156<li><a href="#async_hooksasyncwrapproviders"><code>async_hooks.asyncWrapProviders</code></a></li> 157</ul> 158</li> 159<li><a href="#promise-execution-tracking">Promise execution tracking</a></li> 160<li><a href="#javascript-embedder-api">JavaScript embedder API</a> 161<ul> 162<li><a href="#class-asyncresource">Class: <code>AsyncResource</code></a></li> 163</ul> 164</li> 165<li><a href="#class-asynclocalstorage">Class: <code>AsyncLocalStorage</code></a></li> 166</ul> 167</li> 168</ul></div></div> 169 </li> 170 171 172 <li class="picker-header"> 173 <a href="#"> 174 <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> 175 Index 176 </a> 177 178 <div class="picker"><ul> 179<li><a href="documentation.html" class="nav-documentation">About this documentation</a></li> 180<li><a href="synopsis.html" class="nav-synopsis">Usage and example</a></li> 181 182 <li> 183 <a href="index.html">Index</a> 184 </li> 185 </ul> 186 187<hr class="line"> 188<ul> 189<li><a href="assert.html" class="nav-assert">Assertion testing</a></li> 190<li><a href="async_context.html" 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href="https://nodejs.org/docs/latest-v16.x/api/async_hooks.html">16.x</a></li> 267<li><a href="https://nodejs.org/docs/latest-v15.x/api/async_hooks.html">15.x</a></li> 268<li><a href="https://nodejs.org/docs/latest-v14.x/api/async_hooks.html">14.x</a></li> 269<li><a href="https://nodejs.org/docs/latest-v13.x/api/async_hooks.html">13.x</a></li> 270<li><a href="https://nodejs.org/docs/latest-v12.x/api/async_hooks.html">12.x</a></li> 271<li><a href="https://nodejs.org/docs/latest-v11.x/api/async_hooks.html">11.x</a></li> 272<li><a href="https://nodejs.org/docs/latest-v10.x/api/async_hooks.html">10.x</a></li> 273<li><a href="https://nodejs.org/docs/latest-v9.x/api/async_hooks.html">9.x</a></li> 274<li><a href="https://nodejs.org/docs/latest-v8.x/api/async_hooks.html">8.x</a></li></ol></div> 275 </li> 276 277 <li class="picker-header"> 278 <a href="#"> 279 <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> 280 Options 281 </a> 282 283 <div class="picker"> 284 <ul> 285 <li> 286 <a href="all.html">View on single page</a> 287 </li> 288 <li> 289 <a href="async_hooks.json">View as JSON</a> 290 </li> 291 <li class="edit_on_github"><a href="https://github.com/nodejs/node/edit/main/doc/api/async_hooks.md">Edit on GitHub</a></li> 292 </ul> 293 </div> 294 </li> 295 </ul> 296 </div> 297 <hr> 298 </header> 299 300 <details id="toc" open><summary>Table of contents</summary><ul> 301<li><span class="stability_1"><a href="#async-hooks">Async hooks</a></span> 302<ul> 303<li><a href="#terminology">Terminology</a></li> 304<li><a href="#overview">Overview</a></li> 305<li><a href="#async_hookscreatehookcallbacks"><code>async_hooks.createHook(callbacks)</code></a> 306<ul> 307<li><a href="#error-handling">Error handling</a></li> 308<li><a href="#printing-in-asynchook-callbacks">Printing in <code>AsyncHook</code> callbacks</a></li> 309</ul> 310</li> 311<li><a href="#class-asynchook">Class: <code>AsyncHook</code></a> 312<ul> 313<li><a href="#asynchookenable"><code>asyncHook.enable()</code></a></li> 314<li><a href="#asynchookdisable"><code>asyncHook.disable()</code></a></li> 315<li><a href="#hook-callbacks">Hook callbacks</a> 316<ul> 317<li><a href="#initasyncid-type-triggerasyncid-resource"><code>init(asyncId, type, triggerAsyncId, resource)</code></a> 318<ul> 319<li><a href="#type"><code>type</code></a></li> 320<li><a href="#triggerasyncid"><code>triggerAsyncId</code></a></li> 321<li><a href="#resource"><code>resource</code></a></li> 322<li><a href="#asynchronous-context-example">Asynchronous context example</a></li> 323</ul> 324</li> 325<li><a href="#beforeasyncid"><code>before(asyncId)</code></a></li> 326<li><a href="#afterasyncid"><code>after(asyncId)</code></a></li> 327<li><a href="#destroyasyncid"><code>destroy(asyncId)</code></a></li> 328<li><a href="#promiseresolveasyncid"><code>promiseResolve(asyncId)</code></a></li> 329</ul> 330</li> 331<li><a href="#async_hooksexecutionasyncresource"><code>async_hooks.executionAsyncResource()</code></a></li> 332<li><a href="#async_hooksexecutionasyncid"><code>async_hooks.executionAsyncId()</code></a></li> 333<li><a href="#async_hookstriggerasyncid"><code>async_hooks.triggerAsyncId()</code></a></li> 334<li><a href="#async_hooksasyncwrapproviders"><code>async_hooks.asyncWrapProviders</code></a></li> 335</ul> 336</li> 337<li><a href="#promise-execution-tracking">Promise execution tracking</a></li> 338<li><a href="#javascript-embedder-api">JavaScript embedder API</a> 339<ul> 340<li><a href="#class-asyncresource">Class: <code>AsyncResource</code></a></li> 341</ul> 342</li> 343<li><a href="#class-asynclocalstorage">Class: <code>AsyncLocalStorage</code></a></li> 344</ul> 345</li> 346</ul></details> 347 348 <div id="apicontent"> 349 <h2>Async hooks<span><a class="mark" href="#async-hooks" id="async-hooks">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_async_hooks"></a></h2> 350 351<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental. Please migrate away from this API, if you can. 352We do not recommend using the <a href="#async_hookscreatehookcallbacks"><code>createHook</code></a>, <a href="#class-asynchook"><code>AsyncHook</code></a>, and 353<a href="#async_hooksexecutionasyncresource"><code>executionAsyncResource</code></a> APIs as they have usability issues, safety risks, 354and performance implications. Async context tracking use cases are better 355served by the stable <a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a> API. If you have a use case for 356<code>createHook</code>, <code>AsyncHook</code>, or <code>executionAsyncResource</code> beyond the context 357tracking need solved by <a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a> or diagnostics data currently 358provided by <a href="diagnostics_channel.html">Diagnostics Channel</a>, please open an issue at 359<a href="https://github.com/nodejs/node/issues">https://github.com/nodejs/node/issues</a> describing your use case so we can 360create a more purpose-focused API.</div><p></p> 361<p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v18.18.2/lib/async_hooks.js">lib/async_hooks.js</a></p> 362<p>We strongly discourage the use of the <code>async_hooks</code> API. 363Other APIs that can cover most of its use cases include:</p> 364<ul> 365<li><a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a> tracks async context</li> 366<li><a href="process.html#processgetactiveresourcesinfo"><code>process.getActiveResourcesInfo()</code></a> tracks active resources</li> 367</ul> 368<p>The <code>node:async_hooks</code> module provides an API to track asynchronous resources. 369It can be accessed using:</p> 370 371<pre class="with-48-chars"><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> async_hooks <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> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);</code><button class="copy-button">copy</button></pre> 372<section><h3>Terminology<span><a class="mark" href="#terminology" id="terminology">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_terminology"></a></h3> 373<p>An asynchronous resource represents an object with an associated callback. 374This callback may be called multiple times, such as the <code>'connection'</code> 375event in <code>net.createServer()</code>, or just a single time like in <code>fs.open()</code>. 376A resource can also be closed before the callback is called. <code>AsyncHook</code> does 377not explicitly distinguish between these different cases but will represent them 378as the abstract concept that is a resource.</p> 379<p>If <a href="worker_threads.html#class-worker"><code>Worker</code></a>s are used, each thread has an independent <code>async_hooks</code> 380interface, and each thread will use a new set of async IDs.</p> 381</section><section><h3>Overview<span><a class="mark" href="#overview" id="overview">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_overview"></a></h3> 382<p>Following is a simple overview of the public API.</p> 383 384<pre class="with-48-chars"><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> async_hooks <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>; 385 386<span class="hljs-comment">// Return the ID of the current execution context.</span> 387<span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 388 389<span class="hljs-comment">// Return the ID of the handle responsible for triggering the callback of the</span> 390<span class="hljs-comment">// current execution scope to call.</span> 391<span class="hljs-keyword">const</span> tid = async_hooks.<span class="hljs-title function_">triggerAsyncId</span>(); 392 393<span class="hljs-comment">// Create a new AsyncHook instance. All of these callbacks are optional.</span> 394<span class="hljs-keyword">const</span> asyncHook = 395 async_hooks.<span class="hljs-title function_">createHook</span>({ init, before, after, destroy, promiseResolve }); 396 397<span class="hljs-comment">// Allow callbacks of this AsyncHook instance to call. This is not an implicit</span> 398<span class="hljs-comment">// action after running the constructor, and must be explicitly run to begin</span> 399<span class="hljs-comment">// executing callbacks.</span> 400asyncHook.<span class="hljs-title function_">enable</span>(); 401 402<span class="hljs-comment">// Disable listening for new asynchronous events.</span> 403asyncHook.<span class="hljs-title function_">disable</span>(); 404 405<span class="hljs-comment">//</span> 406<span class="hljs-comment">// The following are the callbacks that can be passed to createHook().</span> 407<span class="hljs-comment">//</span> 408 409<span class="hljs-comment">// init() is called during object construction. The resource may not have</span> 410<span class="hljs-comment">// completed construction when this callback runs. Therefore, all fields of the</span> 411<span class="hljs-comment">// resource referenced by "asyncId" may not have been populated.</span> 412<span class="hljs-keyword">function</span> <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { } 413 414<span class="hljs-comment">// before() is called just before the resource's callback is called. It can be</span> 415<span class="hljs-comment">// called 0-N times for handles (such as TCPWrap), and will be called exactly 1</span> 416<span class="hljs-comment">// time for requests (such as FSReqCallback).</span> 417<span class="hljs-keyword">function</span> <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { } 418 419<span class="hljs-comment">// after() is called just after the resource's callback has finished.</span> 420<span class="hljs-keyword">function</span> <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { } 421 422<span class="hljs-comment">// destroy() is called when the resource is destroyed.</span> 423<span class="hljs-keyword">function</span> <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { } 424 425<span class="hljs-comment">// promiseResolve() is called only for promise resources, when the</span> 426<span class="hljs-comment">// resolve() function passed to the Promise constructor is invoked</span> 427<span class="hljs-comment">// (either directly or through other means of resolving a promise).</span> 428<span class="hljs-keyword">function</span> <span class="hljs-title function_">promiseResolve</span>(<span class="hljs-params">asyncId</span>) { }</code><code class="language-js cjs"><span class="hljs-keyword">const</span> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>); 429 430<span class="hljs-comment">// Return the ID of the current execution context.</span> 431<span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 432 433<span class="hljs-comment">// Return the ID of the handle responsible for triggering the callback of the</span> 434<span class="hljs-comment">// current execution scope to call.</span> 435<span class="hljs-keyword">const</span> tid = async_hooks.<span class="hljs-title function_">triggerAsyncId</span>(); 436 437<span class="hljs-comment">// Create a new AsyncHook instance. All of these callbacks are optional.</span> 438<span class="hljs-keyword">const</span> asyncHook = 439 async_hooks.<span class="hljs-title function_">createHook</span>({ init, before, after, destroy, promiseResolve }); 440 441<span class="hljs-comment">// Allow callbacks of this AsyncHook instance to call. This is not an implicit</span> 442<span class="hljs-comment">// action after running the constructor, and must be explicitly run to begin</span> 443<span class="hljs-comment">// executing callbacks.</span> 444asyncHook.<span class="hljs-title function_">enable</span>(); 445 446<span class="hljs-comment">// Disable listening for new asynchronous events.</span> 447asyncHook.<span class="hljs-title function_">disable</span>(); 448 449<span class="hljs-comment">//</span> 450<span class="hljs-comment">// The following are the callbacks that can be passed to createHook().</span> 451<span class="hljs-comment">//</span> 452 453<span class="hljs-comment">// init() is called during object construction. The resource may not have</span> 454<span class="hljs-comment">// completed construction when this callback runs. Therefore, all fields of the</span> 455<span class="hljs-comment">// resource referenced by "asyncId" may not have been populated.</span> 456<span class="hljs-keyword">function</span> <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { } 457 458<span class="hljs-comment">// before() is called just before the resource's callback is called. It can be</span> 459<span class="hljs-comment">// called 0-N times for handles (such as TCPWrap), and will be called exactly 1</span> 460<span class="hljs-comment">// time for requests (such as FSReqCallback).</span> 461<span class="hljs-keyword">function</span> <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { } 462 463<span class="hljs-comment">// after() is called just after the resource's callback has finished.</span> 464<span class="hljs-keyword">function</span> <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { } 465 466<span class="hljs-comment">// destroy() is called when the resource is destroyed.</span> 467<span class="hljs-keyword">function</span> <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { } 468 469<span class="hljs-comment">// promiseResolve() is called only for promise resources, when the</span> 470<span class="hljs-comment">// resolve() function passed to the Promise constructor is invoked</span> 471<span class="hljs-comment">// (either directly or through other means of resolving a promise).</span> 472<span class="hljs-keyword">function</span> <span class="hljs-title function_">promiseResolve</span>(<span class="hljs-params">asyncId</span>) { }</code><button class="copy-button">copy</button></pre> 473</section><section><h3><code>async_hooks.createHook(callbacks)</code><span><a class="mark" href="#async_hookscreatehookcallbacks" id="async_hookscreatehookcallbacks">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_async_hooks_createhook_callbacks"></a></h3> 474<div class="api_metadata"> 475<span>Added in: v8.1.0</span> 476</div> 477<ul> 478<li><code>callbacks</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> The <a href="#hook-callbacks">Hook Callbacks</a> to register 479<ul> 480<li><code>init</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#initasyncid-type-triggerasyncid-resource"><code>init</code> callback</a>.</li> 481<li><code>before</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#beforeasyncid"><code>before</code> callback</a>.</li> 482<li><code>after</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#afterasyncid"><code>after</code> callback</a>.</li> 483<li><code>destroy</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#destroyasyncid"><code>destroy</code> callback</a>.</li> 484<li><code>promiseResolve</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#promiseresolveasyncid"><code>promiseResolve</code> callback</a>.</li> 485</ul> 486</li> 487<li>Returns: <a href="async_hooks.html#async_hookscreatehookcallbacks" class="type"><AsyncHook></a> Instance used for disabling and enabling hooks</li> 488</ul> 489<p>Registers functions to be called for different lifetime events of each async 490operation.</p> 491<p>The callbacks <code>init()</code>/<code>before()</code>/<code>after()</code>/<code>destroy()</code> are called for the 492respective asynchronous event during a resource's lifetime.</p> 493<p>All callbacks are optional. For example, if only resource cleanup needs to 494be tracked, then only the <code>destroy</code> callback needs to be passed. The 495specifics of all functions that can be passed to <code>callbacks</code> is in the 496<a href="#hook-callbacks">Hook Callbacks</a> section.</p> 497 498<pre class="with-48-chars"><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> { createHook } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>; 499 500<span class="hljs-keyword">const</span> asyncHook = <span class="hljs-title function_">createHook</span>({ 501 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { }, 502 <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { }, 503});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>); 504 505<span class="hljs-keyword">const</span> asyncHook = async_hooks.<span class="hljs-title function_">createHook</span>({ 506 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { }, 507 <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { }, 508});</code><button class="copy-button">copy</button></pre> 509<p>The callbacks will be inherited via the prototype chain:</p> 510<pre><code class="language-js"><span class="hljs-keyword">class</span> <span class="hljs-title class_">MyAsyncCallbacks</span> { 511 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { } 512 <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) {} 513} 514 515<span class="hljs-keyword">class</span> <span class="hljs-title class_">MyAddedCallbacks</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">MyAsyncCallbacks</span> { 516 <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { } 517 <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { } 518} 519 520<span class="hljs-keyword">const</span> asyncHook = async_hooks.<span class="hljs-title function_">createHook</span>(<span class="hljs-keyword">new</span> <span class="hljs-title class_">MyAddedCallbacks</span>());</code> <button class="copy-button">copy</button></pre> 521<p>Because promises are asynchronous resources whose lifecycle is tracked 522via the async hooks mechanism, the <code>init()</code>, <code>before()</code>, <code>after()</code>, and 523<code>destroy()</code> callbacks <em>must not</em> be async functions that return promises.</p> 524<h4>Error handling<span><a class="mark" href="#error-handling" id="error-handling">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_error_handling"></a></h4> 525<p>If any <code>AsyncHook</code> callbacks throw, the application will print the stack trace 526and exit. The exit path does follow that of an uncaught exception, but 527all <code>'uncaughtException'</code> listeners are removed, thus forcing the process to 528exit. The <code>'exit'</code> callbacks will still be called unless the application is run 529with <code>--abort-on-uncaught-exception</code>, in which case a stack trace will be 530printed and the application exits, leaving a core file.</p> 531<p>The reason for this error handling behavior is that these callbacks are running 532at potentially volatile points in an object's lifetime, for example during 533class construction and destruction. Because of this, it is deemed necessary to 534bring down the process quickly in order to prevent an unintentional abort in the 535future. This is subject to change in the future if a comprehensive analysis is 536performed to ensure an exception can follow the normal control flow without 537unintentional side effects.</p> 538<h4>Printing in <code>AsyncHook</code> callbacks<span><a class="mark" href="#printing-in-asynchook-callbacks" id="printing-in-asynchook-callbacks">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_printing_in_asynchook_callbacks"></a></h4> 539<p>Because printing to the console is an asynchronous operation, <code>console.log()</code> 540will cause <code>AsyncHook</code> callbacks to be called. Using <code>console.log()</code> or 541similar asynchronous operations inside an <code>AsyncHook</code> callback function will 542cause an infinite recursion. An easy solution to this when debugging is to use a 543synchronous logging operation such as <code>fs.writeFileSync(file, msg, flag)</code>. 544This will print to the file and will not invoke <code>AsyncHook</code> recursively because 545it is synchronous.</p> 546 547<pre class="with-40-chars"><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> { writeFileSync } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>; 548<span class="hljs-keyword">import</span> { format } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:util'</span>; 549 550<span class="hljs-keyword">function</span> <span class="hljs-title function_">debug</span>(<span class="hljs-params">...args</span>) { 551 <span class="hljs-comment">// Use a function like this one when debugging inside an AsyncHook callback</span> 552 <span class="hljs-title function_">writeFileSync</span>(<span class="hljs-string">'log.out'</span>, <span class="hljs-string">`<span class="hljs-subst">${format(...args)}</span>\n`</span>, { <span class="hljs-attr">flag</span>: <span class="hljs-string">'a'</span> }); 553}</code><code class="language-js cjs"><span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>); 554<span class="hljs-keyword">const</span> util = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:util'</span>); 555 556<span class="hljs-keyword">function</span> <span class="hljs-title function_">debug</span>(<span class="hljs-params">...args</span>) { 557 <span class="hljs-comment">// Use a function like this one when debugging inside an AsyncHook callback</span> 558 fs.<span class="hljs-title function_">writeFileSync</span>(<span class="hljs-string">'log.out'</span>, <span class="hljs-string">`<span class="hljs-subst">${util.format(...args)}</span>\n`</span>, { <span class="hljs-attr">flag</span>: <span class="hljs-string">'a'</span> }); 559}</code><button class="copy-button">copy</button></pre> 560<p>If an asynchronous operation is needed for logging, it is possible to keep 561track of what caused the asynchronous operation using the information 562provided by <code>AsyncHook</code> itself. The logging should then be skipped when 563it was the logging itself that caused the <code>AsyncHook</code> callback to be called. By 564doing this, the otherwise infinite recursion is broken.</p> 565</section><section><h3>Class: <code>AsyncHook</code><span><a class="mark" href="#class-asynchook" id="class-asynchook">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_class_asynchook"></a></h3> 566<p>The class <code>AsyncHook</code> exposes an interface for tracking lifetime events 567of asynchronous operations.</p> 568<h4><code>asyncHook.enable()</code><span><a class="mark" href="#asynchookenable" id="asynchookenable">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_asynchook_enable"></a></h4> 569<ul> 570<li>Returns: <a href="async_hooks.html#async_hookscreatehookcallbacks" class="type"><AsyncHook></a> A reference to <code>asyncHook</code>.</li> 571</ul> 572<p>Enable the callbacks for a given <code>AsyncHook</code> instance. If no callbacks are 573provided, enabling is a no-op.</p> 574<p>The <code>AsyncHook</code> instance is disabled by default. If the <code>AsyncHook</code> instance 575should be enabled immediately after creation, the following pattern can be used.</p> 576 577<pre class="with-48-chars"><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> { createHook } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>; 578 579<span class="hljs-keyword">const</span> hook = <span class="hljs-title function_">createHook</span>(callbacks).<span class="hljs-title function_">enable</span>();</code><code class="language-js cjs"><span class="hljs-keyword">const</span> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>); 580 581<span class="hljs-keyword">const</span> hook = async_hooks.<span class="hljs-title function_">createHook</span>(callbacks).<span class="hljs-title function_">enable</span>();</code><button class="copy-button">copy</button></pre> 582<h4><code>asyncHook.disable()</code><span><a class="mark" href="#asynchookdisable" id="asynchookdisable">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_asynchook_disable"></a></h4> 583<ul> 584<li>Returns: <a href="async_hooks.html#async_hookscreatehookcallbacks" class="type"><AsyncHook></a> A reference to <code>asyncHook</code>.</li> 585</ul> 586<p>Disable the callbacks for a given <code>AsyncHook</code> instance from the global pool of 587<code>AsyncHook</code> callbacks to be executed. Once a hook has been disabled it will not 588be called again until enabled.</p> 589<p>For API consistency <code>disable()</code> also returns the <code>AsyncHook</code> instance.</p> 590<h4>Hook callbacks<span><a class="mark" href="#hook-callbacks" id="hook-callbacks">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_hook_callbacks"></a></h4> 591<p>Key events in the lifetime of asynchronous events have been categorized into 592four areas: instantiation, before/after the callback is called, and when the 593instance is destroyed.</p> 594<h5><code>init(asyncId, type, triggerAsyncId, resource)</code><span><a class="mark" href="#initasyncid-type-triggerasyncid-resource" id="initasyncid-type-triggerasyncid-resource">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_init_asyncid_type_triggerasyncid_resource"></a></h5> 595<ul> 596<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> A unique ID for the async resource.</li> 597<li><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The type of the async resource.</li> 598<li><code>triggerAsyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The unique ID of the async resource in whose 599execution context this async resource was created.</li> 600<li><code>resource</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> Reference to the resource representing the async 601operation, needs to be released during <em>destroy</em>.</li> 602</ul> 603<p>Called when a class is constructed that has the <em>possibility</em> to emit an 604asynchronous event. This <em>does not</em> mean the instance must call 605<code>before</code>/<code>after</code> before <code>destroy</code> is called, only that the possibility 606exists.</p> 607<p>This behavior can be observed by doing something like opening a resource then 608closing it before the resource can be used. The following snippet demonstrates 609this.</p> 610 611<pre class="with-72-chars"><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> { createServer } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:net'</span>; 612 613<span class="hljs-title function_">createServer</span>().<span class="hljs-title function_">listen</span>(<span class="hljs-keyword">function</span>(<span class="hljs-params"></span>) { <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">close</span>(); }); 614<span class="hljs-comment">// OR</span> 615<span class="hljs-built_in">clearTimeout</span>(<span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> {}, <span class="hljs-number">10</span>));</code><code class="language-js cjs"><span class="hljs-built_in">require</span>(<span class="hljs-string">'node:net'</span>).<span class="hljs-title function_">createServer</span>().<span class="hljs-title function_">listen</span>(<span class="hljs-keyword">function</span>(<span class="hljs-params"></span>) { <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">close</span>(); }); 616<span class="hljs-comment">// OR</span> 617<span class="hljs-built_in">clearTimeout</span>(<span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> {}, <span class="hljs-number">10</span>));</code><button class="copy-button">copy</button></pre> 618<p>Every new resource is assigned an ID that is unique within the scope of the 619current Node.js instance.</p> 620<h6><code>type</code><span><a class="mark" href="#type" id="type">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_type"></a></h6> 621<p>The <code>type</code> is a string identifying the type of resource that caused 622<code>init</code> to be called. Generally, it will correspond to the name of the 623resource's constructor.</p> 624<p>The <code>type</code> of resources created by Node.js itself can change in any Node.js 625release. Valid values include <code>TLSWRAP</code>, 626<code>TCPWRAP</code>, <code>TCPSERVERWRAP</code>, <code>GETADDRINFOREQWRAP</code>, <code>FSREQCALLBACK</code>, 627<code>Microtask</code>, and <code>Timeout</code>. Inspect the source code of the Node.js version used 628to get the full list.</p> 629<p>Furthermore users of <a href="async_context.html#class-asyncresource"><code>AsyncResource</code></a> create async resources independent 630of Node.js itself.</p> 631<p>There is also the <code>PROMISE</code> resource type, which is used to track <code>Promise</code> 632instances and asynchronous work scheduled by them.</p> 633<p>Users are able to define their own <code>type</code> when using the public embedder API.</p> 634<p>It is possible to have type name collisions. Embedders are encouraged to use 635unique prefixes, such as the npm package name, to prevent collisions when 636listening to the hooks.</p> 637<h6><code>triggerAsyncId</code><span><a class="mark" href="#triggerasyncid" id="triggerasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_triggerasyncid"></a></h6> 638<p><code>triggerAsyncId</code> is the <code>asyncId</code> of the resource that caused (or "triggered") 639the new resource to initialize and that caused <code>init</code> to call. This is different 640from <code>async_hooks.executionAsyncId()</code> that only shows <em>when</em> a resource was 641created, while <code>triggerAsyncId</code> shows <em>why</em> a resource was created.</p> 642<p>The following is a simple demonstration of <code>triggerAsyncId</code>:</p> 643 644<pre class="with-69-chars"><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> { createHook, executionAsyncId } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>; 645<span class="hljs-keyword">import</span> { stdout } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:process'</span>; 646<span class="hljs-keyword">import</span> net <span class="hljs-keyword">from</span> <span class="hljs-string">'node:net'</span>; 647<span class="hljs-keyword">import</span> fs <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>; 648 649<span class="hljs-title function_">createHook</span>({ 650 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId</span>) { 651 <span class="hljs-keyword">const</span> eid = <span class="hljs-title function_">executionAsyncId</span>(); 652 fs.<span class="hljs-title function_">writeSync</span>( 653 stdout.<span class="hljs-property">fd</span>, 654 <span class="hljs-string">`<span class="hljs-subst">${type}</span>(<span class="hljs-subst">${asyncId}</span>): trigger: <span class="hljs-subst">${triggerAsyncId}</span> execution: <span class="hljs-subst">${eid}</span>\n`</span>); 655 }, 656}).<span class="hljs-title function_">enable</span>(); 657 658net.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">conn</span>) =></span> {}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">8080</span>);</code><code class="language-js cjs"><span class="hljs-keyword">const</span> { createHook, executionAsyncId } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>); 659<span class="hljs-keyword">const</span> { stdout } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:process'</span>); 660<span class="hljs-keyword">const</span> net = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:net'</span>); 661<span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>); 662 663<span class="hljs-title function_">createHook</span>({ 664 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId</span>) { 665 <span class="hljs-keyword">const</span> eid = <span class="hljs-title function_">executionAsyncId</span>(); 666 fs.<span class="hljs-title function_">writeSync</span>( 667 stdout.<span class="hljs-property">fd</span>, 668 <span class="hljs-string">`<span class="hljs-subst">${type}</span>(<span class="hljs-subst">${asyncId}</span>): trigger: <span class="hljs-subst">${triggerAsyncId}</span> execution: <span class="hljs-subst">${eid}</span>\n`</span>); 669 }, 670}).<span class="hljs-title function_">enable</span>(); 671 672net.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">conn</span>) =></span> {}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">8080</span>);</code><button class="copy-button">copy</button></pre> 673<p>Output when hitting the server with <code>nc localhost 8080</code>:</p> 674<pre><code class="language-console">TCPSERVERWRAP(5): trigger: 1 execution: 1 675TCPWRAP(7): trigger: 5 execution: 0</code> <button class="copy-button">copy</button></pre> 676<p>The <code>TCPSERVERWRAP</code> is the server which receives the connections.</p> 677<p>The <code>TCPWRAP</code> is the new connection from the client. When a new 678connection is made, the <code>TCPWrap</code> instance is immediately constructed. This 679happens outside of any JavaScript stack. (An <code>executionAsyncId()</code> of <code>0</code> means 680that it is being executed from C++ with no JavaScript stack above it.) With only 681that information, it would be impossible to link resources together in 682terms of what caused them to be created, so <code>triggerAsyncId</code> is given the task 683of propagating what resource is responsible for the new resource's existence.</p> 684<h6><code>resource</code><span><a class="mark" href="#resource" id="resource">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_resource"></a></h6> 685<p><code>resource</code> is an object that represents the actual async resource that has 686been initialized. The API to access the object may be specified by the 687creator of the resource. Resources created by Node.js itself are internal 688and may change at any time. Therefore no API is specified for these.</p> 689<p>In some cases the resource object is reused for performance reasons, it is 690thus not safe to use it as a key in a <code>WeakMap</code> or add properties to it.</p> 691<h6>Asynchronous context example<span><a class="mark" href="#asynchronous-context-example" id="asynchronous-context-example">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_asynchronous_context_example"></a></h6> 692<p>The context tracking use case is covered by the stable API <a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a>. 693This example only illustrates async hooks operation but <a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a> 694fits better to this use case.</p> 695<p>The following is an example with additional information about the calls to 696<code>init</code> between the <code>before</code> and <code>after</code> calls, specifically what the 697callback to <code>listen()</code> will look like. The output formatting is slightly more 698elaborate to make calling context easier to see.</p> 699 700<pre class="with-48-chars"><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> async_hooks <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>; 701<span class="hljs-keyword">import</span> fs <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>; 702<span class="hljs-keyword">import</span> net <span class="hljs-keyword">from</span> <span class="hljs-string">'node:net'</span>; 703<span class="hljs-keyword">import</span> { stdout } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:process'</span>; 704<span class="hljs-keyword">const</span> { fd } = stdout; 705 706<span class="hljs-keyword">let</span> indent = <span class="hljs-number">0</span>; 707async_hooks.<span class="hljs-title function_">createHook</span>({ 708 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId</span>) { 709 <span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 710 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 711 fs.<span class="hljs-title function_">writeSync</span>( 712 fd, 713 <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span><span class="hljs-subst">${type}</span>(<span class="hljs-subst">${asyncId}</span>):`</span> + 714 <span class="hljs-string">` trigger: <span class="hljs-subst">${triggerAsyncId}</span> execution: <span class="hljs-subst">${eid}</span>\n`</span>); 715 }, 716 <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { 717 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 718 fs.<span class="hljs-title function_">writeSync</span>(fd, <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span>before: <span class="hljs-subst">${asyncId}</span>\n`</span>); 719 indent += <span class="hljs-number">2</span>; 720 }, 721 <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { 722 indent -= <span class="hljs-number">2</span>; 723 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 724 fs.<span class="hljs-title function_">writeSync</span>(fd, <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span>after: <span class="hljs-subst">${asyncId}</span>\n`</span>); 725 }, 726 <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { 727 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 728 fs.<span class="hljs-title function_">writeSync</span>(fd, <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span>destroy: <span class="hljs-subst">${asyncId}</span>\n`</span>); 729 }, 730}).<span class="hljs-title function_">enable</span>(); 731 732net.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">() =></span> {}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">8080</span>, <span class="hljs-function">() =></span> { 733 <span class="hljs-comment">// Let's wait 10ms before logging the server started.</span> 734 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { 735 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'>>>'</span>, async_hooks.<span class="hljs-title function_">executionAsyncId</span>()); 736 }, <span class="hljs-number">10</span>); 737});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>); 738<span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>); 739<span class="hljs-keyword">const</span> net = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:net'</span>); 740<span class="hljs-keyword">const</span> { fd } = process.<span class="hljs-property">stdout</span>; 741 742<span class="hljs-keyword">let</span> indent = <span class="hljs-number">0</span>; 743async_hooks.<span class="hljs-title function_">createHook</span>({ 744 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId</span>) { 745 <span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 746 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 747 fs.<span class="hljs-title function_">writeSync</span>( 748 fd, 749 <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span><span class="hljs-subst">${type}</span>(<span class="hljs-subst">${asyncId}</span>):`</span> + 750 <span class="hljs-string">` trigger: <span class="hljs-subst">${triggerAsyncId}</span> execution: <span class="hljs-subst">${eid}</span>\n`</span>); 751 }, 752 <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { 753 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 754 fs.<span class="hljs-title function_">writeSync</span>(fd, <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span>before: <span class="hljs-subst">${asyncId}</span>\n`</span>); 755 indent += <span class="hljs-number">2</span>; 756 }, 757 <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { 758 indent -= <span class="hljs-number">2</span>; 759 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 760 fs.<span class="hljs-title function_">writeSync</span>(fd, <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span>after: <span class="hljs-subst">${asyncId}</span>\n`</span>); 761 }, 762 <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { 763 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 764 fs.<span class="hljs-title function_">writeSync</span>(fd, <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span>destroy: <span class="hljs-subst">${asyncId}</span>\n`</span>); 765 }, 766}).<span class="hljs-title function_">enable</span>(); 767 768net.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">() =></span> {}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">8080</span>, <span class="hljs-function">() =></span> { 769 <span class="hljs-comment">// Let's wait 10ms before logging the server started.</span> 770 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { 771 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">'>>>'</span>, async_hooks.<span class="hljs-title function_">executionAsyncId</span>()); 772 }, <span class="hljs-number">10</span>); 773});</code><button class="copy-button">copy</button></pre> 774<p>Output from only starting the server:</p> 775<pre><code class="language-console">TCPSERVERWRAP(5): trigger: 1 execution: 1 776TickObject(6): trigger: 5 execution: 1 777before: 6 778 Timeout(7): trigger: 6 execution: 6 779after: 6 780destroy: 6 781before: 7 782<span class="hljs-meta prompt_">></span><span class="language-bash">>> 7</span> 783 TickObject(8): trigger: 7 execution: 7 784after: 7 785before: 8 786after: 8</code> <button class="copy-button">copy</button></pre> 787<p>As illustrated in the example, <code>executionAsyncId()</code> and <code>execution</code> each specify 788the value of the current execution context; which is delineated by calls to 789<code>before</code> and <code>after</code>.</p> 790<p>Only using <code>execution</code> to graph resource allocation results in the following:</p> 791<pre><code class="language-console"> root(1) 792 ^ 793 | 794TickObject(6) 795 ^ 796 | 797 Timeout(7)</code> <button class="copy-button">copy</button></pre> 798<p>The <code>TCPSERVERWRAP</code> is not part of this graph, even though it was the reason for 799<code>console.log()</code> being called. This is because binding to a port without a host 800name is a <em>synchronous</em> operation, but to maintain a completely asynchronous 801API the user's callback is placed in a <code>process.nextTick()</code>. Which is why 802<code>TickObject</code> is present in the output and is a 'parent' for <code>.listen()</code> 803callback.</p> 804<p>The graph only shows <em>when</em> a resource was created, not <em>why</em>, so to track 805the <em>why</em> use <code>triggerAsyncId</code>. Which can be represented with the following 806graph:</p> 807<pre><code class="language-console"> bootstrap(1) 808 | 809 ˅ 810TCPSERVERWRAP(5) 811 | 812 ˅ 813 TickObject(6) 814 | 815 ˅ 816 Timeout(7)</code> <button class="copy-button">copy</button></pre> 817<h5><code>before(asyncId)</code><span><a class="mark" href="#beforeasyncid" id="beforeasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_before_asyncid"></a></h5> 818<ul> 819<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 820</ul> 821<p>When an asynchronous operation is initiated (such as a TCP server receiving a 822new connection) or completes (such as writing data to disk) a callback is 823called to notify the user. The <code>before</code> callback is called just before said 824callback is executed. <code>asyncId</code> is the unique identifier assigned to the 825resource about to execute the callback.</p> 826<p>The <code>before</code> callback will be called 0 to N times. The <code>before</code> callback 827will typically be called 0 times if the asynchronous operation was cancelled 828or, for example, if no connections are received by a TCP server. Persistent 829asynchronous resources like a TCP server will typically call the <code>before</code> 830callback multiple times, while other operations like <code>fs.open()</code> will call 831it only once.</p> 832<h5><code>after(asyncId)</code><span><a class="mark" href="#afterasyncid" id="afterasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_after_asyncid"></a></h5> 833<ul> 834<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 835</ul> 836<p>Called immediately after the callback specified in <code>before</code> is completed.</p> 837<p>If an uncaught exception occurs during execution of the callback, then <code>after</code> 838will run <em>after</em> the <code>'uncaughtException'</code> event is emitted or a <code>domain</code>'s 839handler runs.</p> 840<h5><code>destroy(asyncId)</code><span><a class="mark" href="#destroyasyncid" id="destroyasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_destroy_asyncid"></a></h5> 841<ul> 842<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 843</ul> 844<p>Called after the resource corresponding to <code>asyncId</code> is destroyed. It is also 845called asynchronously from the embedder API <code>emitDestroy()</code>.</p> 846<p>Some resources depend on garbage collection for cleanup, so if a reference is 847made to the <code>resource</code> object passed to <code>init</code> it is possible that <code>destroy</code> 848will never be called, causing a memory leak in the application. If the resource 849does not depend on garbage collection, then this will not be an issue.</p> 850<p>Using the destroy hook results in additional overhead because it enables 851tracking of <code>Promise</code> instances via the garbage collector.</p> 852<h5><code>promiseResolve(asyncId)</code><span><a class="mark" href="#promiseresolveasyncid" id="promiseresolveasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_promiseresolve_asyncid"></a></h5> 853<div class="api_metadata"> 854<span>Added in: v8.6.0</span> 855</div> 856<ul> 857<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 858</ul> 859<p>Called when the <code>resolve</code> function passed to the <code>Promise</code> constructor is 860invoked (either directly or through other means of resolving a promise).</p> 861<p><code>resolve()</code> does not do any observable synchronous work.</p> 862<p>The <code>Promise</code> is not necessarily fulfilled or rejected at this point if the 863<code>Promise</code> was resolved by assuming the state of another <code>Promise</code>.</p> 864<pre><code class="language-js"><span class="hljs-keyword">new</span> <span class="hljs-title class_">Promise</span>(<span class="hljs-function">(<span class="hljs-params">resolve</span>) =></span> <span class="hljs-title function_">resolve</span>(<span class="hljs-literal">true</span>)).<span class="hljs-title function_">then</span>(<span class="hljs-function">(<span class="hljs-params">a</span>) =></span> {});</code> <button class="copy-button">copy</button></pre> 865<p>calls the following callbacks:</p> 866<pre><code class="language-text">init for PROMISE with id 5, trigger id: 1 867 promise resolve 5 # corresponds to resolve(true) 868init for PROMISE with id 6, trigger id: 5 # the Promise returned by then() 869 before 6 # the then() callback is entered 870 promise resolve 6 # the then() callback resolves the promise by returning 871 after 6</code> <button class="copy-button">copy</button></pre> 872<h4><code>async_hooks.executionAsyncResource()</code><span><a class="mark" href="#async_hooksexecutionasyncresource" id="async_hooksexecutionasyncresource">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_async_hooks_executionasyncresource"></a></h4> 873<div class="api_metadata"> 874<span>Added in: v13.9.0, v12.17.0</span> 875</div> 876<ul> 877<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> The resource representing the current execution. 878Useful to store data within the resource.</li> 879</ul> 880<p>Resource objects returned by <code>executionAsyncResource()</code> are most often internal 881Node.js handle objects with undocumented APIs. Using any functions or properties 882on the object is likely to crash your application and should be avoided.</p> 883<p>Using <code>executionAsyncResource()</code> in the top-level execution context will 884return an empty object as there is no handle or request object to use, 885but having an object representing the top-level can be helpful.</p> 886 887<pre class="with-81-chars"><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> { open } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>; 888<span class="hljs-keyword">import</span> { executionAsyncId, executionAsyncResource } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>; 889 890<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">executionAsyncId</span>(), <span class="hljs-title function_">executionAsyncResource</span>()); <span class="hljs-comment">// 1 {}</span> 891<span class="hljs-title function_">open</span>(<span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(<span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>), <span class="hljs-string">'r'</span>, <span class="hljs-function">(<span class="hljs-params">err, fd</span>) =></span> { 892 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">executionAsyncId</span>(), <span class="hljs-title function_">executionAsyncResource</span>()); <span class="hljs-comment">// 7 FSReqWrap</span> 893});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> { open } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>); 894<span class="hljs-keyword">const</span> { executionAsyncId, executionAsyncResource } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>); 895 896<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">executionAsyncId</span>(), <span class="hljs-title function_">executionAsyncResource</span>()); <span class="hljs-comment">// 1 {}</span> 897<span class="hljs-title function_">open</span>(__filename, <span class="hljs-string">'r'</span>, <span class="hljs-function">(<span class="hljs-params">err, fd</span>) =></span> { 898 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">executionAsyncId</span>(), <span class="hljs-title function_">executionAsyncResource</span>()); <span class="hljs-comment">// 7 FSReqWrap</span> 899});</code><button class="copy-button">copy</button></pre> 900<p>This can be used to implement continuation local storage without the 901use of a tracking <code>Map</code> to store the metadata:</p> 902 903<pre class="with-46-chars"><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> { createServer } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:http'</span>; 904<span class="hljs-keyword">import</span> { 905 executionAsyncId, 906 executionAsyncResource, 907 createHook, 908} <span class="hljs-keyword">from</span> <span class="hljs-string">'async_hooks'</span>; 909<span class="hljs-keyword">const</span> sym = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'state'</span>); <span class="hljs-comment">// Private symbol to avoid pollution</span> 910 911<span class="hljs-title function_">createHook</span>({ 912 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { 913 <span class="hljs-keyword">const</span> cr = <span class="hljs-title function_">executionAsyncResource</span>(); 914 <span class="hljs-keyword">if</span> (cr) { 915 resource[sym] = cr[sym]; 916 } 917 }, 918}).<span class="hljs-title function_">enable</span>(); 919 920<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> { 921 <span class="hljs-title function_">executionAsyncResource</span>()[sym] = { <span class="hljs-attr">state</span>: req.<span class="hljs-property">url</span> }; 922 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-keyword">function</span>(<span class="hljs-params"></span>) { 923 res.<span class="hljs-title function_">end</span>(<span class="hljs-title class_">JSON</span>.<span class="hljs-title function_">stringify</span>(<span class="hljs-title function_">executionAsyncResource</span>()[sym])); 924 }, <span class="hljs-number">100</span>); 925}).<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>); 926<span class="hljs-keyword">const</span> { 927 executionAsyncId, 928 executionAsyncResource, 929 createHook, 930} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>); 931<span class="hljs-keyword">const</span> sym = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'state'</span>); <span class="hljs-comment">// Private symbol to avoid pollution</span> 932 933<span class="hljs-title function_">createHook</span>({ 934 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { 935 <span class="hljs-keyword">const</span> cr = <span class="hljs-title function_">executionAsyncResource</span>(); 936 <span class="hljs-keyword">if</span> (cr) { 937 resource[sym] = cr[sym]; 938 } 939 }, 940}).<span class="hljs-title function_">enable</span>(); 941 942<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> { 943 <span class="hljs-title function_">executionAsyncResource</span>()[sym] = { <span class="hljs-attr">state</span>: req.<span class="hljs-property">url</span> }; 944 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-keyword">function</span>(<span class="hljs-params"></span>) { 945 res.<span class="hljs-title function_">end</span>(<span class="hljs-title class_">JSON</span>.<span class="hljs-title function_">stringify</span>(<span class="hljs-title function_">executionAsyncResource</span>()[sym])); 946 }, <span class="hljs-number">100</span>); 947}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">3000</span>);</code><button class="copy-button">copy</button></pre> 948<h4><code>async_hooks.executionAsyncId()</code><span><a class="mark" href="#async_hooksexecutionasyncid" id="async_hooksexecutionasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_async_hooks_executionasyncid"></a></h4> 949<div class="api_metadata"> 950<details class="changelog"><summary>History</summary> 951<table> 952<tbody><tr><th>Version</th><th>Changes</th></tr> 953<tr><td>v8.2.0</td> 954<td><p>Renamed from <code>currentId</code>.</p></td></tr> 955<tr><td>v8.1.0</td> 956<td><p><span>Added in: v8.1.0</span></p></td></tr> 957</tbody></table> 958</details> 959</div> 960<ul> 961<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The <code>asyncId</code> of the current execution context. Useful to 962track when something calls.</li> 963</ul> 964 965<pre class="with-52-chars"><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> { executionAsyncId } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>; 966<span class="hljs-keyword">import</span> fs <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>; 967 968<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">executionAsyncId</span>()); <span class="hljs-comment">// 1 - bootstrap</span> 969fs.<span class="hljs-title function_">open</span>(path, <span class="hljs-string">'r'</span>, <span class="hljs-function">(<span class="hljs-params">err, fd</span>) =></span> { 970 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-title function_">executionAsyncId</span>()); <span class="hljs-comment">// 6 - open()</span> 971});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>); 972<span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>); 973 974<span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(async_hooks.<span class="hljs-title function_">executionAsyncId</span>()); <span class="hljs-comment">// 1 - bootstrap</span> 975fs.<span class="hljs-title function_">open</span>(path, <span class="hljs-string">'r'</span>, <span class="hljs-function">(<span class="hljs-params">err, fd</span>) =></span> { 976 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(async_hooks.<span class="hljs-title function_">executionAsyncId</span>()); <span class="hljs-comment">// 6 - open()</span> 977});</code><button class="copy-button">copy</button></pre> 978<p>The ID returned from <code>executionAsyncId()</code> is related to execution timing, not 979causality (which is covered by <code>triggerAsyncId()</code>):</p> 980<pre><code class="language-js"><span class="hljs-keyword">const</span> server = net.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">conn</span>) =></span> { 981 <span class="hljs-comment">// Returns the ID of the server, not of the new connection, because the</span> 982 <span class="hljs-comment">// callback runs in the execution scope of the server's MakeCallback().</span> 983 async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 984 985}).<span class="hljs-title function_">listen</span>(port, <span class="hljs-function">() =></span> { 986 <span class="hljs-comment">// Returns the ID of a TickObject (process.nextTick()) because all</span> 987 <span class="hljs-comment">// callbacks passed to .listen() are wrapped in a nextTick().</span> 988 async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 989});</code> <button class="copy-button">copy</button></pre> 990<p>Promise contexts may not get precise <code>executionAsyncIds</code> by default. 991See the section on <a href="#promise-execution-tracking">promise execution tracking</a>.</p> 992<h4><code>async_hooks.triggerAsyncId()</code><span><a class="mark" href="#async_hookstriggerasyncid" id="async_hookstriggerasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_async_hooks_triggerasyncid"></a></h4> 993<ul> 994<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The ID of the resource responsible for calling the callback 995that is currently being executed.</li> 996</ul> 997<pre><code class="language-js"><span class="hljs-keyword">const</span> server = net.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">conn</span>) =></span> { 998 <span class="hljs-comment">// The resource that caused (or triggered) this callback to be called</span> 999 <span class="hljs-comment">// was that of the new connection. Thus the return value of triggerAsyncId()</span> 1000 <span class="hljs-comment">// is the asyncId of "conn".</span> 1001 async_hooks.<span class="hljs-title function_">triggerAsyncId</span>(); 1002 1003}).<span class="hljs-title function_">listen</span>(port, <span class="hljs-function">() =></span> { 1004 <span class="hljs-comment">// Even though all callbacks passed to .listen() are wrapped in a nextTick()</span> 1005 <span class="hljs-comment">// the callback itself exists because the call to the server's .listen()</span> 1006 <span class="hljs-comment">// was made. So the return value would be the ID of the server.</span> 1007 async_hooks.<span class="hljs-title function_">triggerAsyncId</span>(); 1008});</code> <button class="copy-button">copy</button></pre> 1009<p>Promise contexts may not get valid <code>triggerAsyncId</code>s by default. See 1010the section on <a href="#promise-execution-tracking">promise execution tracking</a>.</p> 1011<h4><code>async_hooks.asyncWrapProviders</code><span><a class="mark" href="#async_hooksasyncwrapproviders" id="async_hooksasyncwrapproviders">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_async_hooks_asyncwrapproviders"></a></h4> 1012<div class="api_metadata"> 1013<span>Added in: v17.2.0, v16.14.0</span> 1014</div> 1015<ul> 1016<li>Returns: A map of provider types to the corresponding numeric id. 1017This map contains all the event types that might be emitted by the <code>async_hooks.init()</code> event.</li> 1018</ul> 1019<p>This feature suppresses the deprecated usage of <code>process.binding('async_wrap').Providers</code>. 1020See: <a href="deprecations.html#dep0111-processbinding">DEP0111</a></p> 1021</section><section><h3>Promise execution tracking<span><a class="mark" href="#promise-execution-tracking" id="promise-execution-tracking">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_promise_execution_tracking"></a></h3> 1022<p>By default, promise executions are not assigned <code>asyncId</code>s due to the relatively 1023expensive nature of the <a href="https://docs.google.com/document/d/1rda3yKGHimKIhg5YeoAmCOtyURgsbTH_qaYR79FELlk/edit">promise introspection API</a> provided by 1024V8. This means that programs using promises or <code>async</code>/<code>await</code> will not get 1025correct execution and trigger ids for promise callback contexts by default.</p> 1026 1027<pre class="with-73-chars"><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> { executionAsyncId, triggerAsyncId } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>; 1028 1029<span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-number">1729</span>).<span class="hljs-title function_">then</span>(<span class="hljs-function">() =></span> { 1030 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`eid <span class="hljs-subst">${executionAsyncId()}</span> tid <span class="hljs-subst">${triggerAsyncId()}</span>`</span>); 1031}); 1032<span class="hljs-comment">// produces:</span> 1033<span class="hljs-comment">// eid 1 tid 0</span></code><code class="language-js cjs"><span class="hljs-keyword">const</span> { executionAsyncId, triggerAsyncId } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>); 1034 1035<span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-number">1729</span>).<span class="hljs-title function_">then</span>(<span class="hljs-function">() =></span> { 1036 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`eid <span class="hljs-subst">${executionAsyncId()}</span> tid <span class="hljs-subst">${triggerAsyncId()}</span>`</span>); 1037}); 1038<span class="hljs-comment">// produces:</span> 1039<span class="hljs-comment">// eid 1 tid 0</span></code><button class="copy-button">copy</button></pre> 1040<p>Observe that the <code>then()</code> callback claims to have executed in the context of the 1041outer scope even though there was an asynchronous hop involved. Also, 1042the <code>triggerAsyncId</code> value is <code>0</code>, which means that we are missing context about 1043the resource that caused (triggered) the <code>then()</code> callback to be executed.</p> 1044<p>Installing async hooks via <code>async_hooks.createHook</code> enables promise execution 1045tracking:</p> 1046 1047<pre class="with-85-chars"><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> { createHook, executionAsyncId, triggerAsyncId } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>; 1048<span class="hljs-title function_">createHook</span>({ <span class="hljs-title function_">init</span>(<span class="hljs-params"></span>) {} }).<span class="hljs-title function_">enable</span>(); <span class="hljs-comment">// forces PromiseHooks to be enabled.</span> 1049<span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-number">1729</span>).<span class="hljs-title function_">then</span>(<span class="hljs-function">() =></span> { 1050 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`eid <span class="hljs-subst">${executionAsyncId()}</span> tid <span class="hljs-subst">${triggerAsyncId()}</span>`</span>); 1051}); 1052<span class="hljs-comment">// produces:</span> 1053<span class="hljs-comment">// eid 7 tid 6</span></code><code class="language-js cjs"><span class="hljs-keyword">const</span> { createHook, executionAsyncId, triggerAsyncId } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>); 1054 1055<span class="hljs-title function_">createHook</span>({ <span class="hljs-title function_">init</span>(<span class="hljs-params"></span>) {} }).<span class="hljs-title function_">enable</span>(); <span class="hljs-comment">// forces PromiseHooks to be enabled.</span> 1056<span class="hljs-title class_">Promise</span>.<span class="hljs-title function_">resolve</span>(<span class="hljs-number">1729</span>).<span class="hljs-title function_">then</span>(<span class="hljs-function">() =></span> { 1057 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`eid <span class="hljs-subst">${executionAsyncId()}</span> tid <span class="hljs-subst">${triggerAsyncId()}</span>`</span>); 1058}); 1059<span class="hljs-comment">// produces:</span> 1060<span class="hljs-comment">// eid 7 tid 6</span></code><button class="copy-button">copy</button></pre> 1061<p>In this example, adding any actual hook function enabled the tracking of 1062promises. There are two promises in the example above; the promise created by 1063<code>Promise.resolve()</code> and the promise returned by the call to <code>then()</code>. In the 1064example above, the first promise got the <code>asyncId</code> <code>6</code> and the latter got 1065<code>asyncId</code> <code>7</code>. During the execution of the <code>then()</code> callback, we are executing 1066in the context of promise with <code>asyncId</code> <code>7</code>. This promise was triggered by 1067async resource <code>6</code>.</p> 1068<p>Another subtlety with promises is that <code>before</code> and <code>after</code> callbacks are run 1069only on chained promises. That means promises not created by <code>then()</code>/<code>catch()</code> 1070will not have the <code>before</code> and <code>after</code> callbacks fired on them. For more details 1071see the details of the V8 <a href="https://docs.google.com/document/d/1rda3yKGHimKIhg5YeoAmCOtyURgsbTH_qaYR79FELlk/edit">PromiseHooks</a> API.</p> 1072</section><section><h3>JavaScript embedder API<span><a class="mark" href="#javascript-embedder-api" id="javascript-embedder-api">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_javascript_embedder_api"></a></h3> 1073<p>Library developers that handle their own asynchronous resources performing tasks 1074like I/O, connection pooling, or managing callback queues may use the 1075<code>AsyncResource</code> JavaScript API so that all the appropriate callbacks are called.</p> 1076<h4>Class: <code>AsyncResource</code><span><a class="mark" href="#class-asyncresource" id="class-asyncresource">#</a></span><a aria-hidden="true" class="legacy" id="async_hooks_class_asyncresource"></a></h4> 1077<p>The documentation for this class has moved <a href="async_context.html#class-asyncresource"><code>AsyncResource</code></a>.</p> 1078</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_hooks_class_asynclocalstorage"></a></h3> 1079<p>The documentation for this class has moved <a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a>.</p></section> 1080 <!-- API END --> 1081 </div> 1082 </div> 1083 </div> 1084</body> 1085</html> 1086