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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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266<li><a href="https://nodejs.org/docs/latest-v17.x/api/async_hooks.html">17.x</a></li> 267<li><a href="https://nodejs.org/docs/latest-v16.x/api/async_hooks.html">16.x</a></li> 268<li><a href="https://nodejs.org/docs/latest-v15.x/api/async_hooks.html">15.x</a></li> 269<li><a href="https://nodejs.org/docs/latest-v14.x/api/async_hooks.html">14.x</a></li> 270<li><a href="https://nodejs.org/docs/latest-v13.x/api/async_hooks.html">13.x</a></li> 271<li><a href="https://nodejs.org/docs/latest-v12.x/api/async_hooks.html">12.x</a></li> 272<li><a href="https://nodejs.org/docs/latest-v11.x/api/async_hooks.html">11.x</a></li> 273<li><a href="https://nodejs.org/docs/latest-v10.x/api/async_hooks.html">10.x</a></li> 274<li><a href="https://nodejs.org/docs/latest-v9.x/api/async_hooks.html">9.x</a></li> 275<li><a href="https://nodejs.org/docs/latest-v8.x/api/async_hooks.html">8.x</a></li></ol></div> 276 </li> 277 278 <li class="picker-header"> 279 <a href="#"> 280 <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> 281 Options 282 </a> 283 284 <div class="picker"> 285 <ul> 286 <li> 287 <a href="all.html">View on single page</a> 288 </li> 289 <li> 290 <a href="async_hooks.json">View as JSON</a> 291 </li> 292 <li class="edit_on_github"><a href="https://github.com/nodejs/node/edit/main/doc/api/async_hooks.md">Edit on GitHub</a></li> 293 </ul> 294 </div> 295 </li> 296 </ul> 297 </div> 298 <hr> 299 </header> 300 301 <details id="toc" open><summary>Table of contents</summary><ul> 302<li><span class="stability_1"><a href="#async-hooks">Async hooks</a></span> 303<ul> 304<li><a href="#terminology">Terminology</a></li> 305<li><a href="#overview">Overview</a></li> 306<li><a href="#async_hookscreatehookcallbacks"><code>async_hooks.createHook(callbacks)</code></a> 307<ul> 308<li><a href="#error-handling">Error handling</a></li> 309<li><a href="#printing-in-asynchook-callbacks">Printing in <code>AsyncHook</code> callbacks</a></li> 310</ul> 311</li> 312<li><a href="#class-asynchook">Class: <code>AsyncHook</code></a> 313<ul> 314<li><a href="#asynchookenable"><code>asyncHook.enable()</code></a></li> 315<li><a href="#asynchookdisable"><code>asyncHook.disable()</code></a></li> 316<li><a href="#hook-callbacks">Hook callbacks</a> 317<ul> 318<li><a href="#initasyncid-type-triggerasyncid-resource"><code>init(asyncId, type, triggerAsyncId, resource)</code></a> 319<ul> 320<li><a href="#type"><code>type</code></a></li> 321<li><a href="#triggerasyncid"><code>triggerAsyncId</code></a></li> 322<li><a href="#resource"><code>resource</code></a></li> 323<li><a href="#asynchronous-context-example">Asynchronous context example</a></li> 324</ul> 325</li> 326<li><a href="#beforeasyncid"><code>before(asyncId)</code></a></li> 327<li><a href="#afterasyncid"><code>after(asyncId)</code></a></li> 328<li><a href="#destroyasyncid"><code>destroy(asyncId)</code></a></li> 329<li><a href="#promiseresolveasyncid"><code>promiseResolve(asyncId)</code></a></li> 330</ul> 331</li> 332<li><a href="#async_hooksexecutionasyncresource"><code>async_hooks.executionAsyncResource()</code></a></li> 333<li><a href="#async_hooksexecutionasyncid"><code>async_hooks.executionAsyncId()</code></a></li> 334<li><a href="#async_hookstriggerasyncid"><code>async_hooks.triggerAsyncId()</code></a></li> 335<li><a href="#async_hooksasyncwrapproviders"><code>async_hooks.asyncWrapProviders</code></a></li> 336</ul> 337</li> 338<li><a href="#promise-execution-tracking">Promise execution tracking</a></li> 339<li><a href="#javascript-embedder-api">JavaScript embedder API</a> 340<ul> 341<li><a href="#class-asyncresource">Class: <code>AsyncResource</code></a></li> 342</ul> 343</li> 344<li><a href="#class-asynclocalstorage">Class: <code>AsyncLocalStorage</code></a></li> 345</ul> 346</li> 347</ul></details> 348 349 <div id="apicontent"> 350 <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> 351 352<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. 353We do not recommend using the <a href="#async_hookscreatehookcallbacks"><code>createHook</code></a>, <a href="#class-asynchook"><code>AsyncHook</code></a>, and 354<a href="#async_hooksexecutionasyncresource"><code>executionAsyncResource</code></a> APIs as they have usability issues, safety risks, 355and performance implications. Async context tracking use cases are better 356served by the stable <a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a> API. If you have a use case for 357<code>createHook</code>, <code>AsyncHook</code>, or <code>executionAsyncResource</code> beyond the context 358tracking need solved by <a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a> or diagnostics data currently 359provided by <a href="diagnostics_channel.html">Diagnostics Channel</a>, please open an issue at 360<a href="https://github.com/nodejs/node/issues">https://github.com/nodejs/node/issues</a> describing your use case so we can 361create a more purpose-focused API.</div><p></p> 362<p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v18.20.1/lib/async_hooks.js">lib/async_hooks.js</a></p> 363<p>We strongly discourage the use of the <code>async_hooks</code> API. 364Other APIs that can cover most of its use cases include:</p> 365<ul> 366<li><a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a> tracks async context</li> 367<li><a href="process.html#processgetactiveresourcesinfo"><code>process.getActiveResourcesInfo()</code></a> tracks active resources</li> 368</ul> 369<p>The <code>node:async_hooks</code> module provides an API to track asynchronous resources. 370It can be accessed using:</p> 371 372<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> 373<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> 374<p>An asynchronous resource represents an object with an associated callback. 375This callback may be called multiple times, such as the <code>'connection'</code> 376event in <code>net.createServer()</code>, or just a single time like in <code>fs.open()</code>. 377A resource can also be closed before the callback is called. <code>AsyncHook</code> does 378not explicitly distinguish between these different cases but will represent them 379as the abstract concept that is a resource.</p> 380<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> 381interface, and each thread will use a new set of async IDs.</p> 382</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> 383<p>Following is a simple overview of the public API.</p> 384 385<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>; 386 387<span class="hljs-comment">// Return the ID of the current execution context.</span> 388<span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 389 390<span class="hljs-comment">// Return the ID of the handle responsible for triggering the callback of the</span> 391<span class="hljs-comment">// current execution scope to call.</span> 392<span class="hljs-keyword">const</span> tid = async_hooks.<span class="hljs-title function_">triggerAsyncId</span>(); 393 394<span class="hljs-comment">// Create a new AsyncHook instance. All of these callbacks are optional.</span> 395<span class="hljs-keyword">const</span> asyncHook = 396 async_hooks.<span class="hljs-title function_">createHook</span>({ init, before, after, destroy, promiseResolve }); 397 398<span class="hljs-comment">// Allow callbacks of this AsyncHook instance to call. This is not an implicit</span> 399<span class="hljs-comment">// action after running the constructor, and must be explicitly run to begin</span> 400<span class="hljs-comment">// executing callbacks.</span> 401asyncHook.<span class="hljs-title function_">enable</span>(); 402 403<span class="hljs-comment">// Disable listening for new asynchronous events.</span> 404asyncHook.<span class="hljs-title function_">disable</span>(); 405 406<span class="hljs-comment">//</span> 407<span class="hljs-comment">// The following are the callbacks that can be passed to createHook().</span> 408<span class="hljs-comment">//</span> 409 410<span class="hljs-comment">// init() is called during object construction. The resource may not have</span> 411<span class="hljs-comment">// completed construction when this callback runs. Therefore, all fields of the</span> 412<span class="hljs-comment">// resource referenced by "asyncId" may not have been populated.</span> 413<span class="hljs-keyword">function</span> <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { } 414 415<span class="hljs-comment">// before() is called just before the resource's callback is called. It can be</span> 416<span class="hljs-comment">// called 0-N times for handles (such as TCPWrap), and will be called exactly 1</span> 417<span class="hljs-comment">// time for requests (such as FSReqCallback).</span> 418<span class="hljs-keyword">function</span> <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { } 419 420<span class="hljs-comment">// after() is called just after the resource's callback has finished.</span> 421<span class="hljs-keyword">function</span> <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { } 422 423<span class="hljs-comment">// destroy() is called when the resource is destroyed.</span> 424<span class="hljs-keyword">function</span> <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { } 425 426<span class="hljs-comment">// promiseResolve() is called only for promise resources, when the</span> 427<span class="hljs-comment">// resolve() function passed to the Promise constructor is invoked</span> 428<span class="hljs-comment">// (either directly or through other means of resolving a promise).</span> 429<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>); 430 431<span class="hljs-comment">// Return the ID of the current execution context.</span> 432<span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 433 434<span class="hljs-comment">// Return the ID of the handle responsible for triggering the callback of the</span> 435<span class="hljs-comment">// current execution scope to call.</span> 436<span class="hljs-keyword">const</span> tid = async_hooks.<span class="hljs-title function_">triggerAsyncId</span>(); 437 438<span class="hljs-comment">// Create a new AsyncHook instance. All of these callbacks are optional.</span> 439<span class="hljs-keyword">const</span> asyncHook = 440 async_hooks.<span class="hljs-title function_">createHook</span>({ init, before, after, destroy, promiseResolve }); 441 442<span class="hljs-comment">// Allow callbacks of this AsyncHook instance to call. This is not an implicit</span> 443<span class="hljs-comment">// action after running the constructor, and must be explicitly run to begin</span> 444<span class="hljs-comment">// executing callbacks.</span> 445asyncHook.<span class="hljs-title function_">enable</span>(); 446 447<span class="hljs-comment">// Disable listening for new asynchronous events.</span> 448asyncHook.<span class="hljs-title function_">disable</span>(); 449 450<span class="hljs-comment">//</span> 451<span class="hljs-comment">// The following are the callbacks that can be passed to createHook().</span> 452<span class="hljs-comment">//</span> 453 454<span class="hljs-comment">// init() is called during object construction. The resource may not have</span> 455<span class="hljs-comment">// completed construction when this callback runs. Therefore, all fields of the</span> 456<span class="hljs-comment">// resource referenced by "asyncId" may not have been populated.</span> 457<span class="hljs-keyword">function</span> <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { } 458 459<span class="hljs-comment">// before() is called just before the resource's callback is called. It can be</span> 460<span class="hljs-comment">// called 0-N times for handles (such as TCPWrap), and will be called exactly 1</span> 461<span class="hljs-comment">// time for requests (such as FSReqCallback).</span> 462<span class="hljs-keyword">function</span> <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { } 463 464<span class="hljs-comment">// after() is called just after the resource's callback has finished.</span> 465<span class="hljs-keyword">function</span> <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { } 466 467<span class="hljs-comment">// destroy() is called when the resource is destroyed.</span> 468<span class="hljs-keyword">function</span> <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { } 469 470<span class="hljs-comment">// promiseResolve() is called only for promise resources, when the</span> 471<span class="hljs-comment">// resolve() function passed to the Promise constructor is invoked</span> 472<span class="hljs-comment">// (either directly or through other means of resolving a promise).</span> 473<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> 474</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> 475<div class="api_metadata"> 476<span>Added in: v8.1.0</span> 477</div> 478<ul> 479<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 480<ul> 481<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> 482<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> 483<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> 484<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> 485<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> 486</ul> 487</li> 488<li>Returns: <a href="async_hooks.html#async_hookscreatehookcallbacks" class="type"><AsyncHook></a> Instance used for disabling and enabling hooks</li> 489</ul> 490<p>Registers functions to be called for different lifetime events of each async 491operation.</p> 492<p>The callbacks <code>init()</code>/<code>before()</code>/<code>after()</code>/<code>destroy()</code> are called for the 493respective asynchronous event during a resource's lifetime.</p> 494<p>All callbacks are optional. For example, if only resource cleanup needs to 495be tracked, then only the <code>destroy</code> callback needs to be passed. The 496specifics of all functions that can be passed to <code>callbacks</code> is in the 497<a href="#hook-callbacks">Hook Callbacks</a> section.</p> 498 499<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>; 500 501<span class="hljs-keyword">const</span> asyncHook = <span class="hljs-title function_">createHook</span>({ 502 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { }, 503 <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { }, 504});</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>); 505 506<span class="hljs-keyword">const</span> asyncHook = async_hooks.<span class="hljs-title function_">createHook</span>({ 507 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { }, 508 <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { }, 509});</code><button class="copy-button">copy</button></pre> 510<p>The callbacks will be inherited via the prototype chain:</p> 511<pre><code class="language-js"><span class="hljs-keyword">class</span> <span class="hljs-title class_">MyAsyncCallbacks</span> { 512 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { } 513 <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) {} 514} 515 516<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> { 517 <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { } 518 <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { } 519} 520 521<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> 522<p>Because promises are asynchronous resources whose lifecycle is tracked 523via the async hooks mechanism, the <code>init()</code>, <code>before()</code>, <code>after()</code>, and 524<code>destroy()</code> callbacks <em>must not</em> be async functions that return promises.</p> 525<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> 526<p>If any <code>AsyncHook</code> callbacks throw, the application will print the stack trace 527and exit. The exit path does follow that of an uncaught exception, but 528all <code>'uncaughtException'</code> listeners are removed, thus forcing the process to 529exit. The <code>'exit'</code> callbacks will still be called unless the application is run 530with <code>--abort-on-uncaught-exception</code>, in which case a stack trace will be 531printed and the application exits, leaving a core file.</p> 532<p>The reason for this error handling behavior is that these callbacks are running 533at potentially volatile points in an object's lifetime, for example during 534class construction and destruction. Because of this, it is deemed necessary to 535bring down the process quickly in order to prevent an unintentional abort in the 536future. This is subject to change in the future if a comprehensive analysis is 537performed to ensure an exception can follow the normal control flow without 538unintentional side effects.</p> 539<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> 540<p>Because printing to the console is an asynchronous operation, <code>console.log()</code> 541will cause <code>AsyncHook</code> callbacks to be called. Using <code>console.log()</code> or 542similar asynchronous operations inside an <code>AsyncHook</code> callback function will 543cause an infinite recursion. An easy solution to this when debugging is to use a 544synchronous logging operation such as <code>fs.writeFileSync(file, msg, flag)</code>. 545This will print to the file and will not invoke <code>AsyncHook</code> recursively because 546it is synchronous.</p> 547 548<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>; 549<span class="hljs-keyword">import</span> { format } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:util'</span>; 550 551<span class="hljs-keyword">function</span> <span class="hljs-title function_">debug</span>(<span class="hljs-params">...args</span>) { 552 <span class="hljs-comment">// Use a function like this one when debugging inside an AsyncHook callback</span> 553 <span class="hljs-title function_">writeFileSync</span>(<span class="hljs-string">'log.out'</span>, <span class="hljs-string">`<span class="hljs-subst">${format(...args)}</span>\n`</span>, { <span class="hljs-attr">flag</span>: <span class="hljs-string">'a'</span> }); 554}</code><code class="language-js cjs"><span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>); 555<span class="hljs-keyword">const</span> util = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:util'</span>); 556 557<span class="hljs-keyword">function</span> <span class="hljs-title function_">debug</span>(<span class="hljs-params">...args</span>) { 558 <span class="hljs-comment">// Use a function like this one when debugging inside an AsyncHook callback</span> 559 fs.<span class="hljs-title function_">writeFileSync</span>(<span class="hljs-string">'log.out'</span>, <span class="hljs-string">`<span class="hljs-subst">${util.format(...args)}</span>\n`</span>, { <span class="hljs-attr">flag</span>: <span class="hljs-string">'a'</span> }); 560}</code><button class="copy-button">copy</button></pre> 561<p>If an asynchronous operation is needed for logging, it is possible to keep 562track of what caused the asynchronous operation using the information 563provided by <code>AsyncHook</code> itself. The logging should then be skipped when 564it was the logging itself that caused the <code>AsyncHook</code> callback to be called. By 565doing this, the otherwise infinite recursion is broken.</p> 566</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> 567<p>The class <code>AsyncHook</code> exposes an interface for tracking lifetime events 568of asynchronous operations.</p> 569<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> 570<ul> 571<li>Returns: <a href="async_hooks.html#async_hookscreatehookcallbacks" class="type"><AsyncHook></a> A reference to <code>asyncHook</code>.</li> 572</ul> 573<p>Enable the callbacks for a given <code>AsyncHook</code> instance. If no callbacks are 574provided, enabling is a no-op.</p> 575<p>The <code>AsyncHook</code> instance is disabled by default. If the <code>AsyncHook</code> instance 576should be enabled immediately after creation, the following pattern can be used.</p> 577 578<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>; 579 580<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>); 581 582<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> 583<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> 584<ul> 585<li>Returns: <a href="async_hooks.html#async_hookscreatehookcallbacks" class="type"><AsyncHook></a> A reference to <code>asyncHook</code>.</li> 586</ul> 587<p>Disable the callbacks for a given <code>AsyncHook</code> instance from the global pool of 588<code>AsyncHook</code> callbacks to be executed. Once a hook has been disabled it will not 589be called again until enabled.</p> 590<p>For API consistency <code>disable()</code> also returns the <code>AsyncHook</code> instance.</p> 591<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> 592<p>Key events in the lifetime of asynchronous events have been categorized into 593four areas: instantiation, before/after the callback is called, and when the 594instance is destroyed.</p> 595<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> 596<ul> 597<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> 598<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> 599<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 600execution context this async resource was created.</li> 601<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 602operation, needs to be released during <em>destroy</em>.</li> 603</ul> 604<p>Called when a class is constructed that has the <em>possibility</em> to emit an 605asynchronous event. This <em>does not</em> mean the instance must call 606<code>before</code>/<code>after</code> before <code>destroy</code> is called, only that the possibility 607exists.</p> 608<p>This behavior can be observed by doing something like opening a resource then 609closing it before the resource can be used. The following snippet demonstrates 610this.</p> 611 612<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>; 613 614<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>(); }); 615<span class="hljs-comment">// OR</span> 616<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>(); }); 617<span class="hljs-comment">// OR</span> 618<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> 619<p>Every new resource is assigned an ID that is unique within the scope of the 620current Node.js instance.</p> 621<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> 622<p>The <code>type</code> is a string identifying the type of resource that caused 623<code>init</code> to be called. Generally, it will correspond to the name of the 624resource's constructor.</p> 625<p>The <code>type</code> of resources created by Node.js itself can change in any Node.js 626release. Valid values include <code>TLSWRAP</code>, 627<code>TCPWRAP</code>, <code>TCPSERVERWRAP</code>, <code>GETADDRINFOREQWRAP</code>, <code>FSREQCALLBACK</code>, 628<code>Microtask</code>, and <code>Timeout</code>. Inspect the source code of the Node.js version used 629to get the full list.</p> 630<p>Furthermore users of <a href="async_context.html#class-asyncresource"><code>AsyncResource</code></a> create async resources independent 631of Node.js itself.</p> 632<p>There is also the <code>PROMISE</code> resource type, which is used to track <code>Promise</code> 633instances and asynchronous work scheduled by them.</p> 634<p>Users are able to define their own <code>type</code> when using the public embedder API.</p> 635<p>It is possible to have type name collisions. Embedders are encouraged to use 636unique prefixes, such as the npm package name, to prevent collisions when 637listening to the hooks.</p> 638<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> 639<p><code>triggerAsyncId</code> is the <code>asyncId</code> of the resource that caused (or "triggered") 640the new resource to initialize and that caused <code>init</code> to call. This is different 641from <code>async_hooks.executionAsyncId()</code> that only shows <em>when</em> a resource was 642created, while <code>triggerAsyncId</code> shows <em>why</em> a resource was created.</p> 643<p>The following is a simple demonstration of <code>triggerAsyncId</code>:</p> 644 645<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>; 646<span class="hljs-keyword">import</span> { stdout } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:process'</span>; 647<span class="hljs-keyword">import</span> net <span class="hljs-keyword">from</span> <span class="hljs-string">'node:net'</span>; 648<span class="hljs-keyword">import</span> fs <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>; 649 650<span class="hljs-title function_">createHook</span>({ 651 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId</span>) { 652 <span class="hljs-keyword">const</span> eid = <span class="hljs-title function_">executionAsyncId</span>(); 653 fs.<span class="hljs-title function_">writeSync</span>( 654 stdout.<span class="hljs-property">fd</span>, 655 <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>); 656 }, 657}).<span class="hljs-title function_">enable</span>(); 658 659net.<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>); 660<span class="hljs-keyword">const</span> { stdout } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:process'</span>); 661<span class="hljs-keyword">const</span> net = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:net'</span>); 662<span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>); 663 664<span class="hljs-title function_">createHook</span>({ 665 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId</span>) { 666 <span class="hljs-keyword">const</span> eid = <span class="hljs-title function_">executionAsyncId</span>(); 667 fs.<span class="hljs-title function_">writeSync</span>( 668 stdout.<span class="hljs-property">fd</span>, 669 <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>); 670 }, 671}).<span class="hljs-title function_">enable</span>(); 672 673net.<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> 674<p>Output when hitting the server with <code>nc localhost 8080</code>:</p> 675<pre><code class="language-console">TCPSERVERWRAP(5): trigger: 1 execution: 1 676TCPWRAP(7): trigger: 5 execution: 0</code> <button class="copy-button">copy</button></pre> 677<p>The <code>TCPSERVERWRAP</code> is the server which receives the connections.</p> 678<p>The <code>TCPWRAP</code> is the new connection from the client. When a new 679connection is made, the <code>TCPWrap</code> instance is immediately constructed. This 680happens outside of any JavaScript stack. (An <code>executionAsyncId()</code> of <code>0</code> means 681that it is being executed from C++ with no JavaScript stack above it.) With only 682that information, it would be impossible to link resources together in 683terms of what caused them to be created, so <code>triggerAsyncId</code> is given the task 684of propagating what resource is responsible for the new resource's existence.</p> 685<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> 686<p><code>resource</code> is an object that represents the actual async resource that has 687been initialized. The API to access the object may be specified by the 688creator of the resource. Resources created by Node.js itself are internal 689and may change at any time. Therefore no API is specified for these.</p> 690<p>In some cases the resource object is reused for performance reasons, it is 691thus not safe to use it as a key in a <code>WeakMap</code> or add properties to it.</p> 692<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> 693<p>The context tracking use case is covered by the stable API <a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a>. 694This example only illustrates async hooks operation but <a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a> 695fits better to this use case.</p> 696<p>The following is an example with additional information about the calls to 697<code>init</code> between the <code>before</code> and <code>after</code> calls, specifically what the 698callback to <code>listen()</code> will look like. The output formatting is slightly more 699elaborate to make calling context easier to see.</p> 700 701<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>; 702<span class="hljs-keyword">import</span> fs <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>; 703<span class="hljs-keyword">import</span> net <span class="hljs-keyword">from</span> <span class="hljs-string">'node:net'</span>; 704<span class="hljs-keyword">import</span> { stdout } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:process'</span>; 705<span class="hljs-keyword">const</span> { fd } = stdout; 706 707<span class="hljs-keyword">let</span> indent = <span class="hljs-number">0</span>; 708async_hooks.<span class="hljs-title function_">createHook</span>({ 709 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId</span>) { 710 <span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 711 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 712 fs.<span class="hljs-title function_">writeSync</span>( 713 fd, 714 <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span><span class="hljs-subst">${type}</span>(<span class="hljs-subst">${asyncId}</span>):`</span> + 715 <span class="hljs-string">` trigger: <span class="hljs-subst">${triggerAsyncId}</span> execution: <span class="hljs-subst">${eid}</span>\n`</span>); 716 }, 717 <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { 718 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 719 fs.<span class="hljs-title function_">writeSync</span>(fd, <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span>before: <span class="hljs-subst">${asyncId}</span>\n`</span>); 720 indent += <span class="hljs-number">2</span>; 721 }, 722 <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { 723 indent -= <span class="hljs-number">2</span>; 724 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 725 fs.<span class="hljs-title function_">writeSync</span>(fd, <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span>after: <span class="hljs-subst">${asyncId}</span>\n`</span>); 726 }, 727 <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { 728 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 729 fs.<span class="hljs-title function_">writeSync</span>(fd, <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span>destroy: <span class="hljs-subst">${asyncId}</span>\n`</span>); 730 }, 731}).<span class="hljs-title function_">enable</span>(); 732 733net.<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> { 734 <span class="hljs-comment">// Let's wait 10ms before logging the server started.</span> 735 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { 736 <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>()); 737 }, <span class="hljs-number">10</span>); 738});</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>); 739<span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>); 740<span class="hljs-keyword">const</span> net = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:net'</span>); 741<span class="hljs-keyword">const</span> { fd } = process.<span class="hljs-property">stdout</span>; 742 743<span class="hljs-keyword">let</span> indent = <span class="hljs-number">0</span>; 744async_hooks.<span class="hljs-title function_">createHook</span>({ 745 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId</span>) { 746 <span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 747 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 748 fs.<span class="hljs-title function_">writeSync</span>( 749 fd, 750 <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span><span class="hljs-subst">${type}</span>(<span class="hljs-subst">${asyncId}</span>):`</span> + 751 <span class="hljs-string">` trigger: <span class="hljs-subst">${triggerAsyncId}</span> execution: <span class="hljs-subst">${eid}</span>\n`</span>); 752 }, 753 <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { 754 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 755 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>); 756 indent += <span class="hljs-number">2</span>; 757 }, 758 <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { 759 indent -= <span class="hljs-number">2</span>; 760 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 761 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>); 762 }, 763 <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { 764 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 765 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>); 766 }, 767}).<span class="hljs-title function_">enable</span>(); 768 769net.<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> { 770 <span class="hljs-comment">// Let's wait 10ms before logging the server started.</span> 771 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { 772 <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>()); 773 }, <span class="hljs-number">10</span>); 774});</code><button class="copy-button">copy</button></pre> 775<p>Output from only starting the server:</p> 776<pre><code class="language-console">TCPSERVERWRAP(5): trigger: 1 execution: 1 777TickObject(6): trigger: 5 execution: 1 778before: 6 779 Timeout(7): trigger: 6 execution: 6 780after: 6 781destroy: 6 782before: 7 783<span class="hljs-meta prompt_">></span><span class="language-bash">>> 7</span> 784 TickObject(8): trigger: 7 execution: 7 785after: 7 786before: 8 787after: 8</code> <button class="copy-button">copy</button></pre> 788<p>As illustrated in the example, <code>executionAsyncId()</code> and <code>execution</code> each specify 789the value of the current execution context; which is delineated by calls to 790<code>before</code> and <code>after</code>.</p> 791<p>Only using <code>execution</code> to graph resource allocation results in the following:</p> 792<pre><code class="language-console"> root(1) 793 ^ 794 | 795TickObject(6) 796 ^ 797 | 798 Timeout(7)</code> <button class="copy-button">copy</button></pre> 799<p>The <code>TCPSERVERWRAP</code> is not part of this graph, even though it was the reason for 800<code>console.log()</code> being called. This is because binding to a port without a host 801name is a <em>synchronous</em> operation, but to maintain a completely asynchronous 802API the user's callback is placed in a <code>process.nextTick()</code>. Which is why 803<code>TickObject</code> is present in the output and is a 'parent' for <code>.listen()</code> 804callback.</p> 805<p>The graph only shows <em>when</em> a resource was created, not <em>why</em>, so to track 806the <em>why</em> use <code>triggerAsyncId</code>. Which can be represented with the following 807graph:</p> 808<pre><code class="language-console"> bootstrap(1) 809 | 810 ˅ 811TCPSERVERWRAP(5) 812 | 813 ˅ 814 TickObject(6) 815 | 816 ˅ 817 Timeout(7)</code> <button class="copy-button">copy</button></pre> 818<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> 819<ul> 820<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 821</ul> 822<p>When an asynchronous operation is initiated (such as a TCP server receiving a 823new connection) or completes (such as writing data to disk) a callback is 824called to notify the user. The <code>before</code> callback is called just before said 825callback is executed. <code>asyncId</code> is the unique identifier assigned to the 826resource about to execute the callback.</p> 827<p>The <code>before</code> callback will be called 0 to N times. The <code>before</code> callback 828will typically be called 0 times if the asynchronous operation was cancelled 829or, for example, if no connections are received by a TCP server. Persistent 830asynchronous resources like a TCP server will typically call the <code>before</code> 831callback multiple times, while other operations like <code>fs.open()</code> will call 832it only once.</p> 833<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> 834<ul> 835<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 836</ul> 837<p>Called immediately after the callback specified in <code>before</code> is completed.</p> 838<p>If an uncaught exception occurs during execution of the callback, then <code>after</code> 839will run <em>after</em> the <code>'uncaughtException'</code> event is emitted or a <code>domain</code>'s 840handler runs.</p> 841<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> 842<ul> 843<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 844</ul> 845<p>Called after the resource corresponding to <code>asyncId</code> is destroyed. It is also 846called asynchronously from the embedder API <code>emitDestroy()</code>.</p> 847<p>Some resources depend on garbage collection for cleanup, so if a reference is 848made to the <code>resource</code> object passed to <code>init</code> it is possible that <code>destroy</code> 849will never be called, causing a memory leak in the application. If the resource 850does not depend on garbage collection, then this will not be an issue.</p> 851<p>Using the destroy hook results in additional overhead because it enables 852tracking of <code>Promise</code> instances via the garbage collector.</p> 853<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> 854<div class="api_metadata"> 855<span>Added in: v8.6.0</span> 856</div> 857<ul> 858<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 859</ul> 860<p>Called when the <code>resolve</code> function passed to the <code>Promise</code> constructor is 861invoked (either directly or through other means of resolving a promise).</p> 862<p><code>resolve()</code> does not do any observable synchronous work.</p> 863<p>The <code>Promise</code> is not necessarily fulfilled or rejected at this point if the 864<code>Promise</code> was resolved by assuming the state of another <code>Promise</code>.</p> 865<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> 866<p>calls the following callbacks:</p> 867<pre><code class="language-text">init for PROMISE with id 5, trigger id: 1 868 promise resolve 5 # corresponds to resolve(true) 869init for PROMISE with id 6, trigger id: 5 # the Promise returned by then() 870 before 6 # the then() callback is entered 871 promise resolve 6 # the then() callback resolves the promise by returning 872 after 6</code> <button class="copy-button">copy</button></pre> 873<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> 874<div class="api_metadata"> 875<span>Added in: v13.9.0, v12.17.0</span> 876</div> 877<ul> 878<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. 879Useful to store data within the resource.</li> 880</ul> 881<p>Resource objects returned by <code>executionAsyncResource()</code> are most often internal 882Node.js handle objects with undocumented APIs. Using any functions or properties 883on the object is likely to crash your application and should be avoided.</p> 884<p>Using <code>executionAsyncResource()</code> in the top-level execution context will 885return an empty object as there is no handle or request object to use, 886but having an object representing the top-level can be helpful.</p> 887 888<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>; 889<span class="hljs-keyword">import</span> { executionAsyncId, executionAsyncResource } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>; 890 891<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> 892<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> { 893 <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> 894});</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>); 895<span class="hljs-keyword">const</span> { executionAsyncId, executionAsyncResource } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>); 896 897<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> 898<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> { 899 <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> 900});</code><button class="copy-button">copy</button></pre> 901<p>This can be used to implement continuation local storage without the 902use of a tracking <code>Map</code> to store the metadata:</p> 903 904<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>; 905<span class="hljs-keyword">import</span> { 906 executionAsyncId, 907 executionAsyncResource, 908 createHook, 909} <span class="hljs-keyword">from</span> <span class="hljs-string">'async_hooks'</span>; 910<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> 911 912<span class="hljs-title function_">createHook</span>({ 913 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { 914 <span class="hljs-keyword">const</span> cr = <span class="hljs-title function_">executionAsyncResource</span>(); 915 <span class="hljs-keyword">if</span> (cr) { 916 resource[sym] = cr[sym]; 917 } 918 }, 919}).<span class="hljs-title function_">enable</span>(); 920 921<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> { 922 <span class="hljs-title function_">executionAsyncResource</span>()[sym] = { <span class="hljs-attr">state</span>: req.<span class="hljs-property">url</span> }; 923 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-keyword">function</span>(<span class="hljs-params"></span>) { 924 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])); 925 }, <span class="hljs-number">100</span>); 926}).<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>); 927<span class="hljs-keyword">const</span> { 928 executionAsyncId, 929 executionAsyncResource, 930 createHook, 931} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>); 932<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> 933 934<span class="hljs-title function_">createHook</span>({ 935 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { 936 <span class="hljs-keyword">const</span> cr = <span class="hljs-title function_">executionAsyncResource</span>(); 937 <span class="hljs-keyword">if</span> (cr) { 938 resource[sym] = cr[sym]; 939 } 940 }, 941}).<span class="hljs-title function_">enable</span>(); 942 943<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> { 944 <span class="hljs-title function_">executionAsyncResource</span>()[sym] = { <span class="hljs-attr">state</span>: req.<span class="hljs-property">url</span> }; 945 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-keyword">function</span>(<span class="hljs-params"></span>) { 946 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])); 947 }, <span class="hljs-number">100</span>); 948}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">3000</span>);</code><button class="copy-button">copy</button></pre> 949<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> 950<div class="api_metadata"> 951<details class="changelog"><summary>History</summary> 952<table> 953<tbody><tr><th>Version</th><th>Changes</th></tr> 954<tr><td>v8.2.0</td> 955<td><p>Renamed from <code>currentId</code>.</p></td></tr> 956<tr><td>v8.1.0</td> 957<td><p><span>Added in: v8.1.0</span></p></td></tr> 958</tbody></table> 959</details> 960</div> 961<ul> 962<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 963track when something calls.</li> 964</ul> 965 966<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>; 967<span class="hljs-keyword">import</span> fs <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs'</span>; 968 969<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> 970<span class="hljs-keyword">const</span> path = <span class="hljs-string">'.'</span>; 971fs.<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> { 972 <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> 973});</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>); 974<span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs'</span>); 975 976<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">// 1 - bootstrap</span> 977<span class="hljs-keyword">const</span> path = <span class="hljs-string">'.'</span>; 978fs.<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> { 979 <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> 980});</code><button class="copy-button">copy</button></pre> 981<p>The ID returned from <code>executionAsyncId()</code> is related to execution timing, not 982causality (which is covered by <code>triggerAsyncId()</code>):</p> 983<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> { 984 <span class="hljs-comment">// Returns the ID of the server, not of the new connection, because the</span> 985 <span class="hljs-comment">// callback runs in the execution scope of the server's MakeCallback().</span> 986 async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 987 988}).<span class="hljs-title function_">listen</span>(port, <span class="hljs-function">() =></span> { 989 <span class="hljs-comment">// Returns the ID of a TickObject (process.nextTick()) because all</span> 990 <span class="hljs-comment">// callbacks passed to .listen() are wrapped in a nextTick().</span> 991 async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 992});</code> <button class="copy-button">copy</button></pre> 993<p>Promise contexts may not get precise <code>executionAsyncIds</code> by default. 994See the section on <a href="#promise-execution-tracking">promise execution tracking</a>.</p> 995<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> 996<ul> 997<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 998that is currently being executed.</li> 999</ul> 1000<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> { 1001 <span class="hljs-comment">// The resource that caused (or triggered) this callback to be called</span> 1002 <span class="hljs-comment">// was that of the new connection. Thus the return value of triggerAsyncId()</span> 1003 <span class="hljs-comment">// is the asyncId of "conn".</span> 1004 async_hooks.<span class="hljs-title function_">triggerAsyncId</span>(); 1005 1006}).<span class="hljs-title function_">listen</span>(port, <span class="hljs-function">() =></span> { 1007 <span class="hljs-comment">// Even though all callbacks passed to .listen() are wrapped in a nextTick()</span> 1008 <span class="hljs-comment">// the callback itself exists because the call to the server's .listen()</span> 1009 <span class="hljs-comment">// was made. So the return value would be the ID of the server.</span> 1010 async_hooks.<span class="hljs-title function_">triggerAsyncId</span>(); 1011});</code> <button class="copy-button">copy</button></pre> 1012<p>Promise contexts may not get valid <code>triggerAsyncId</code>s by default. See 1013the section on <a href="#promise-execution-tracking">promise execution tracking</a>.</p> 1014<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> 1015<div class="api_metadata"> 1016<span>Added in: v17.2.0, v16.14.0</span> 1017</div> 1018<ul> 1019<li>Returns: A map of provider types to the corresponding numeric id. 1020This map contains all the event types that might be emitted by the <code>async_hooks.init()</code> event.</li> 1021</ul> 1022<p>This feature suppresses the deprecated usage of <code>process.binding('async_wrap').Providers</code>. 1023See: <a href="deprecations.html#dep0111-processbinding">DEP0111</a></p> 1024</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> 1025<p>By default, promise executions are not assigned <code>asyncId</code>s due to the relatively 1026expensive nature of the <a href="https://docs.google.com/document/d/1rda3yKGHimKIhg5YeoAmCOtyURgsbTH_qaYR79FELlk/edit">promise introspection API</a> provided by 1027V8. This means that programs using promises or <code>async</code>/<code>await</code> will not get 1028correct execution and trigger ids for promise callback contexts by default.</p> 1029 1030<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>; 1031 1032<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> { 1033 <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>); 1034}); 1035<span class="hljs-comment">// produces:</span> 1036<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>); 1037 1038<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> { 1039 <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>); 1040}); 1041<span class="hljs-comment">// produces:</span> 1042<span class="hljs-comment">// eid 1 tid 0</span></code><button class="copy-button">copy</button></pre> 1043<p>Observe that the <code>then()</code> callback claims to have executed in the context of the 1044outer scope even though there was an asynchronous hop involved. Also, 1045the <code>triggerAsyncId</code> value is <code>0</code>, which means that we are missing context about 1046the resource that caused (triggered) the <code>then()</code> callback to be executed.</p> 1047<p>Installing async hooks via <code>async_hooks.createHook</code> enables promise execution 1048tracking:</p> 1049 1050<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>; 1051<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> 1052<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> { 1053 <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>); 1054}); 1055<span class="hljs-comment">// produces:</span> 1056<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>); 1057 1058<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> 1059<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> { 1060 <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>); 1061}); 1062<span class="hljs-comment">// produces:</span> 1063<span class="hljs-comment">// eid 7 tid 6</span></code><button class="copy-button">copy</button></pre> 1064<p>In this example, adding any actual hook function enabled the tracking of 1065promises. There are two promises in the example above; the promise created by 1066<code>Promise.resolve()</code> and the promise returned by the call to <code>then()</code>. In the 1067example above, the first promise got the <code>asyncId</code> <code>6</code> and the latter got 1068<code>asyncId</code> <code>7</code>. During the execution of the <code>then()</code> callback, we are executing 1069in the context of promise with <code>asyncId</code> <code>7</code>. This promise was triggered by 1070async resource <code>6</code>.</p> 1071<p>Another subtlety with promises is that <code>before</code> and <code>after</code> callbacks are run 1072only on chained promises. That means promises not created by <code>then()</code>/<code>catch()</code> 1073will not have the <code>before</code> and <code>after</code> callbacks fired on them. For more details 1074see the details of the V8 <a href="https://docs.google.com/document/d/1rda3yKGHimKIhg5YeoAmCOtyURgsbTH_qaYR79FELlk/edit">PromiseHooks</a> API.</p> 1075</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> 1076<p>Library developers that handle their own asynchronous resources performing tasks 1077like I/O, connection pooling, or managing callback queues may use the 1078<code>AsyncResource</code> JavaScript API so that all the appropriate callbacks are called.</p> 1079<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> 1080<p>The documentation for this class has moved <a href="async_context.html#class-asyncresource"><code>AsyncResource</code></a>.</p> 1081</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> 1082<p>The documentation for this class has moved <a href="async_context.html#class-asynclocalstorage"><code>AsyncLocalStorage</code></a>.</p></section> 1083 <!-- API END --> 1084 </div> 1085 </div> 1086 </div> 1087</body> 1088</html> 1089