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class="stability_1"><a href="#async_hooks_async_hooks">Async hooks</a></span> 140<ul> 141<li><a href="#async_hooks_terminology">Terminology</a></li> 142<li><a href="#async_hooks_overview">Overview</a></li> 143<li><a href="#async_hooks_async_hooks_createhook_callbacks"><code>async_hooks.createHook(callbacks)</code></a> 144<ul> 145<li><a href="#async_hooks_error_handling">Error handling</a></li> 146<li><a href="#async_hooks_printing_in_asynchooks_callbacks">Printing in AsyncHooks callbacks</a></li> 147</ul> 148</li> 149<li><a href="#async_hooks_class_asynchook">Class: <code>AsyncHook</code></a> 150<ul> 151<li><a href="#async_hooks_asynchook_enable"><code>asyncHook.enable()</code></a></li> 152<li><a href="#async_hooks_asynchook_disable"><code>asyncHook.disable()</code></a></li> 153<li><a href="#async_hooks_hook_callbacks">Hook callbacks</a> 154<ul> 155<li><a href="#async_hooks_init_asyncid_type_triggerasyncid_resource"><code>init(asyncId, type, triggerAsyncId, resource)</code></a> 156<ul> 157<li><a href="#async_hooks_type"><code>type</code></a></li> 158<li><a href="#async_hooks_triggerasyncid"><code>triggerAsyncId</code></a></li> 159<li><a href="#async_hooks_resource"><code>resource</code></a></li> 160<li><a href="#async_hooks_asynchronous_context_example">Asynchronous context example</a></li> 161</ul> 162</li> 163<li><a href="#async_hooks_before_asyncid"><code>before(asyncId)</code></a></li> 164<li><a href="#async_hooks_after_asyncid"><code>after(asyncId)</code></a></li> 165<li><a href="#async_hooks_destroy_asyncid"><code>destroy(asyncId)</code></a></li> 166<li><a href="#async_hooks_promiseresolve_asyncid"><code>promiseResolve(asyncId)</code></a></li> 167</ul> 168</li> 169<li><a href="#async_hooks_async_hooks_executionasyncresource"><code>async_hooks.executionAsyncResource()</code></a></li> 170<li><a href="#async_hooks_async_hooks_executionasyncid"><code>async_hooks.executionAsyncId()</code></a></li> 171<li><a href="#async_hooks_async_hooks_triggerasyncid"><code>async_hooks.triggerAsyncId()</code></a></li> 172</ul> 173</li> 174<li><a href="#async_hooks_promise_execution_tracking">Promise execution tracking</a></li> 175<li><a href="#async_hooks_javascript_embedder_api">JavaScript embedder API</a> 176<ul> 177<li><a href="#async_hooks_class_asyncresource">Class: <code>AsyncResource</code></a> 178<ul> 179<li><a href="#async_hooks_new_asyncresource_type_options"><code>new AsyncResource(type[, options])</code></a></li> 180<li><a href="#async_hooks_static_method_asyncresource_bind_fn_type">Static method: <code>AsyncResource.bind(fn[, type])</code></a></li> 181<li><a href="#async_hooks_asyncresource_bind_fn"><code>asyncResource.bind(fn)</code></a></li> 182<li><a href="#async_hooks_asyncresource_runinasyncscope_fn_thisarg_args"><code>asyncResource.runInAsyncScope(fn[, thisArg, ...args])</code></a></li> 183<li><a href="#async_hooks_asyncresource_emitdestroy"><code>asyncResource.emitDestroy()</code></a></li> 184<li><a href="#async_hooks_asyncresource_asyncid"><code>asyncResource.asyncId()</code></a></li> 185<li><a href="#async_hooks_asyncresource_triggerasyncid"><code>asyncResource.triggerAsyncId()</code></a></li> 186</ul> 187</li> 188<li><a href="#async_hooks_using_asyncresource_for_a_worker_thread_pool">Using <code>AsyncResource</code> for a <code>Worker</code> thread pool</a></li> 189<li><a href="#async_hooks_integrating_asyncresource_with_eventemitter">Integrating <code>AsyncResource</code> with <code>EventEmitter</code></a></li> 190</ul> 191</li> 192<li><a href="#async_hooks_class_asynclocalstorage">Class: <code>AsyncLocalStorage</code></a> 193<ul> 194<li><a href="#async_hooks_new_asynclocalstorage"><code>new AsyncLocalStorage()</code></a></li> 195<li><a href="#async_hooks_asynclocalstorage_disable"><code>asyncLocalStorage.disable()</code></a></li> 196<li><a href="#async_hooks_asynclocalstorage_getstore"><code>asyncLocalStorage.getStore()</code></a></li> 197<li><a href="#async_hooks_asynclocalstorage_enterwith_store"><code>asyncLocalStorage.enterWith(store)</code></a></li> 198<li><a href="#async_hooks_asynclocalstorage_run_store_callback_args"><code>asyncLocalStorage.run(store, callback[, ...args])</code></a></li> 199<li><a href="#async_hooks_asynclocalstorage_exit_callback_args"><code>asyncLocalStorage.exit(callback[, ...args])</code></a></li> 200<li><a href="#async_hooks_usage_with_async_await">Usage with <code>async/await</code></a></li> 201<li><a href="#async_hooks_troubleshooting">Troubleshooting</a></li> 202</ul> 203</li> 204</ul> 205</li> 206</ul></details> 207 208 <div id="apicontent"> 209 <h2>Async hooks<span><a class="mark" href="#async_hooks_async_hooks" id="async_hooks_async_hooks">#</a></span></h2> 210 211<p></p><div class="api_stability api_stability_1"><a href="documentation.html#documentation_stability_index">Stability: 1</a> - Experimental</div><p></p> 212<p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v14.20.1/lib/async_hooks.js">lib/async_hooks.js</a></p> 213<p>The <code>async_hooks</code> module provides an API to track asynchronous resources. It 214can be accessed using:</p> 215<pre><code class="language-js"><span class="hljs-keyword">const</span> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'async_hooks'</span>);</code></pre> 216<section><h3>Terminology<span><a class="mark" href="#async_hooks_terminology" id="async_hooks_terminology">#</a></span></h3> 217<p>An asynchronous resource represents an object with an associated callback. 218This callback may be called multiple times, for example, the <code>'connection'</code> 219event in <code>net.createServer()</code>, or just a single time like in <code>fs.open()</code>. 220A resource can also be closed before the callback is called. <code>AsyncHook</code> does 221not explicitly distinguish between these different cases but will represent them 222as the abstract concept that is a resource.</p> 223<p>If <a href="worker_threads.html#worker_threads_class_worker"><code>Worker</code></a>s are used, each thread has an independent <code>async_hooks</code> 224interface, and each thread will use a new set of async IDs.</p> 225</section><section><h3>Overview<span><a class="mark" href="#async_hooks_overview" id="async_hooks_overview">#</a></span></h3> 226<p>Following is a simple overview of the public API.</p> 227<pre><code class="language-js"><span class="hljs-keyword">const</span> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'async_hooks'</span>); 228 229<span class="hljs-comment">// Return the ID of the current execution context.</span> 230<span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 231 232<span class="hljs-comment">// Return the ID of the handle responsible for triggering the callback of the</span> 233<span class="hljs-comment">// current execution scope to call.</span> 234<span class="hljs-keyword">const</span> tid = async_hooks.<span class="hljs-title function_">triggerAsyncId</span>(); 235 236<span class="hljs-comment">// Create a new AsyncHook instance. All of these callbacks are optional.</span> 237<span class="hljs-keyword">const</span> asyncHook = 238 async_hooks.<span class="hljs-title function_">createHook</span>({ init, before, after, destroy, promiseResolve }); 239 240<span class="hljs-comment">// Allow callbacks of this AsyncHook instance to call. This is not an implicit</span> 241<span class="hljs-comment">// action after running the constructor, and must be explicitly run to begin</span> 242<span class="hljs-comment">// executing callbacks.</span> 243asyncHook.<span class="hljs-title function_">enable</span>(); 244 245<span class="hljs-comment">// Disable listening for new asynchronous events.</span> 246asyncHook.<span class="hljs-title function_">disable</span>(); 247 248<span class="hljs-comment">//</span> 249<span class="hljs-comment">// The following are the callbacks that can be passed to createHook().</span> 250<span class="hljs-comment">//</span> 251 252<span class="hljs-comment">// init is called during object construction. The resource may not have</span> 253<span class="hljs-comment">// completed construction when this callback runs, therefore all fields of the</span> 254<span class="hljs-comment">// resource referenced by "asyncId" may not have been populated.</span> 255<span class="hljs-keyword">function</span> <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { } 256 257<span class="hljs-comment">// Before is called just before the resource's callback is called. It can be</span> 258<span class="hljs-comment">// called 0-N times for handles (such as TCPWrap), and will be called exactly 1</span> 259<span class="hljs-comment">// time for requests (such as FSReqCallback).</span> 260<span class="hljs-keyword">function</span> <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { } 261 262<span class="hljs-comment">// After is called just after the resource's callback has finished.</span> 263<span class="hljs-keyword">function</span> <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { } 264 265<span class="hljs-comment">// Destroy is called when the resource is destroyed.</span> 266<span class="hljs-keyword">function</span> <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { } 267 268<span class="hljs-comment">// promiseResolve is called only for promise resources, when the</span> 269<span class="hljs-comment">// `resolve` function passed to the `Promise` constructor is invoked</span> 270<span class="hljs-comment">// (either directly or through other means of resolving a promise).</span> 271<span class="hljs-keyword">function</span> <span class="hljs-title function_">promiseResolve</span>(<span class="hljs-params">asyncId</span>) { }</code></pre> 272</section><section><h3><code>async_hooks.createHook(callbacks)</code><span><a class="mark" href="#async_hooks_async_hooks_createhook_callbacks" id="async_hooks_async_hooks_createhook_callbacks">#</a></span></h3> 273<div class="api_metadata"> 274<span>Added in: v8.1.0</span> 275</div> 276<ul> 277<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="#async_hooks_hook_callbacks">Hook Callbacks</a> to register 278<ul> 279<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="#async_hooks_init_asyncid_type_triggerasyncid_resource"><code>init</code> callback</a>.</li> 280<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="#async_hooks_before_asyncid"><code>before</code> callback</a>.</li> 281<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="#async_hooks_after_asyncid"><code>after</code> callback</a>.</li> 282<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="#async_hooks_destroy_asyncid"><code>destroy</code> callback</a>.</li> 283<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="#async_hooks_promiseresolve_asyncid"><code>promiseResolve</code> callback</a>.</li> 284</ul> 285</li> 286<li>Returns: <a href="async_hooks.html#async_hooks_async_hooks_createhook_callbacks" class="type"><AsyncHook></a> Instance used for disabling and enabling hooks</li> 287</ul> 288<p>Registers functions to be called for different lifetime events of each async 289operation.</p> 290<p>The callbacks <code>init()</code>/<code>before()</code>/<code>after()</code>/<code>destroy()</code> are called for the 291respective asynchronous event during a resource's lifetime.</p> 292<p>All callbacks are optional. For example, if only resource cleanup needs to 293be tracked, then only the <code>destroy</code> callback needs to be passed. The 294specifics of all functions that can be passed to <code>callbacks</code> is in the 295<a href="#async_hooks_hook_callbacks">Hook Callbacks</a> section.</p> 296<pre><code class="language-js"><span class="hljs-keyword">const</span> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'async_hooks'</span>); 297 298<span class="hljs-keyword">const</span> asyncHook = async_hooks.<span class="hljs-title function_">createHook</span>({ 299 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { }, 300 <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { } 301});</code></pre> 302<p>The callbacks will be inherited via the prototype chain:</p> 303<pre><code class="language-js"><span class="hljs-keyword">class</span> <span class="hljs-title class_">MyAsyncCallbacks</span> { 304 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { } 305 <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) {} 306} 307 308<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> { 309 <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { } 310 <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { } 311} 312 313<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></pre> 314<p>Because promises are asynchronous resources whose lifecycle is tracked 315via the async hooks mechanism, the <code>init()</code>, <code>before()</code>, <code>after()</code>, and 316<code>destroy()</code> callbacks <em>must not</em> be async functions that return promises.</p> 317<h4>Error handling<span><a class="mark" href="#async_hooks_error_handling" id="async_hooks_error_handling">#</a></span></h4> 318<p>If any <code>AsyncHook</code> callbacks throw, the application will print the stack trace 319and exit. The exit path does follow that of an uncaught exception, but 320all <code>'uncaughtException'</code> listeners are removed, thus forcing the process to 321exit. The <code>'exit'</code> callbacks will still be called unless the application is run 322with <code>--abort-on-uncaught-exception</code>, in which case a stack trace will be 323printed and the application exits, leaving a core file.</p> 324<p>The reason for this error handling behavior is that these callbacks are running 325at potentially volatile points in an object's lifetime, for example during 326class construction and destruction. Because of this, it is deemed necessary to 327bring down the process quickly in order to prevent an unintentional abort in the 328future. This is subject to change in the future if a comprehensive analysis is 329performed to ensure an exception can follow the normal control flow without 330unintentional side effects.</p> 331<h4>Printing in AsyncHooks callbacks<span><a class="mark" href="#async_hooks_printing_in_asynchooks_callbacks" id="async_hooks_printing_in_asynchooks_callbacks">#</a></span></h4> 332<p>Because printing to the console is an asynchronous operation, <code>console.log()</code> 333will cause the AsyncHooks callbacks to be called. Using <code>console.log()</code> or 334similar asynchronous operations inside an AsyncHooks callback function will thus 335cause an infinite recursion. An easy solution to this when debugging is to use a 336synchronous logging operation such as <code>fs.writeFileSync(file, msg, flag)</code>. 337This will print to the file and will not invoke AsyncHooks recursively because 338it is synchronous.</p> 339<pre><code class="language-js"><span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'fs'</span>); 340<span class="hljs-keyword">const</span> util = <span class="hljs-built_in">require</span>(<span class="hljs-string">'util'</span>); 341 342<span class="hljs-keyword">function</span> <span class="hljs-title function_">debug</span>(<span class="hljs-params">...args</span>) { 343 <span class="hljs-comment">// Use a function like this one when debugging inside an AsyncHooks callback</span> 344 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> }); 345}</code></pre> 346<p>If an asynchronous operation is needed for logging, it is possible to keep 347track of what caused the asynchronous operation using the information 348provided by AsyncHooks itself. The logging should then be skipped when 349it was the logging itself that caused AsyncHooks callback to call. By 350doing this the otherwise infinite recursion is broken.</p> 351</section><section><h3>Class: <code>AsyncHook</code><span><a class="mark" href="#async_hooks_class_asynchook" id="async_hooks_class_asynchook">#</a></span></h3> 352<p>The class <code>AsyncHook</code> exposes an interface for tracking lifetime events 353of asynchronous operations.</p> 354<h4><code>asyncHook.enable()</code><span><a class="mark" href="#async_hooks_asynchook_enable" id="async_hooks_asynchook_enable">#</a></span></h4> 355<ul> 356<li>Returns: <a href="async_hooks.html#async_hooks_async_hooks_createhook_callbacks" class="type"><AsyncHook></a> A reference to <code>asyncHook</code>.</li> 357</ul> 358<p>Enable the callbacks for a given <code>AsyncHook</code> instance. If no callbacks are 359provided, enabling is a no-op.</p> 360<p>The <code>AsyncHook</code> instance is disabled by default. If the <code>AsyncHook</code> instance 361should be enabled immediately after creation, the following pattern can be used.</p> 362<pre><code class="language-js"><span class="hljs-keyword">const</span> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'async_hooks'</span>); 363 364<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></pre> 365<h4><code>asyncHook.disable()</code><span><a class="mark" href="#async_hooks_asynchook_disable" id="async_hooks_asynchook_disable">#</a></span></h4> 366<ul> 367<li>Returns: <a href="async_hooks.html#async_hooks_async_hooks_createhook_callbacks" class="type"><AsyncHook></a> A reference to <code>asyncHook</code>.</li> 368</ul> 369<p>Disable the callbacks for a given <code>AsyncHook</code> instance from the global pool of 370<code>AsyncHook</code> callbacks to be executed. Once a hook has been disabled it will not 371be called again until enabled.</p> 372<p>For API consistency <code>disable()</code> also returns the <code>AsyncHook</code> instance.</p> 373<h4>Hook callbacks<span><a class="mark" href="#async_hooks_hook_callbacks" id="async_hooks_hook_callbacks">#</a></span></h4> 374<p>Key events in the lifetime of asynchronous events have been categorized into 375four areas: instantiation, before/after the callback is called, and when the 376instance is destroyed.</p> 377<h5><code>init(asyncId, type, triggerAsyncId, resource)</code><span><a class="mark" href="#async_hooks_init_asyncid_type_triggerasyncid_resource" id="async_hooks_init_asyncid_type_triggerasyncid_resource">#</a></span></h5> 378<ul> 379<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> 380<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> 381<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 382execution context this async resource was created.</li> 383<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 384operation, needs to be released during <em>destroy</em>.</li> 385</ul> 386<p>Called when a class is constructed that has the <em>possibility</em> to emit an 387asynchronous event. This <em>does not</em> mean the instance must call 388<code>before</code>/<code>after</code> before <code>destroy</code> is called, only that the possibility 389exists.</p> 390<p>This behavior can be observed by doing something like opening a resource then 391closing it before the resource can be used. The following snippet demonstrates 392this.</p> 393<pre><code class="language-js"><span class="hljs-built_in">require</span>(<span class="hljs-string">'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>(); }); 394<span class="hljs-comment">// OR</span> 395<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></pre> 396<p>Every new resource is assigned an ID that is unique within the scope of the 397current Node.js instance.</p> 398<h6><code>type</code><span><a class="mark" href="#async_hooks_type" id="async_hooks_type">#</a></span></h6> 399<p>The <code>type</code> is a string identifying the type of resource that caused 400<code>init</code> to be called. Generally, it will correspond to the name of the 401resource's constructor.</p> 402<pre><code class="language-text">FSEVENTWRAP, FSREQCALLBACK, GETADDRINFOREQWRAP, GETNAMEINFOREQWRAP, HTTPINCOMINGMESSAGE, 403HTTPCLIENTREQUEST, JSSTREAM, PIPECONNECTWRAP, PIPEWRAP, PROCESSWRAP, QUERYWRAP, 404SHUTDOWNWRAP, SIGNALWRAP, STATWATCHER, TCPCONNECTWRAP, TCPSERVERWRAP, TCPWRAP, 405TTYWRAP, UDPSENDWRAP, UDPWRAP, WRITEWRAP, ZLIB, SSLCONNECTION, PBKDF2REQUEST, 406RANDOMBYTESREQUEST, TLSWRAP, Microtask, Timeout, Immediate, TickObject</code></pre> 407<p>There is also the <code>PROMISE</code> resource type, which is used to track <code>Promise</code> 408instances and asynchronous work scheduled by them.</p> 409<p>Users are able to define their own <code>type</code> when using the public embedder API.</p> 410<p>It is possible to have type name collisions. Embedders are encouraged to use 411unique prefixes, such as the npm package name, to prevent collisions when 412listening to the hooks.</p> 413<h6><code>triggerAsyncId</code><span><a class="mark" href="#async_hooks_triggerasyncid" id="async_hooks_triggerasyncid">#</a></span></h6> 414<p><code>triggerAsyncId</code> is the <code>asyncId</code> of the resource that caused (or "triggered") 415the new resource to initialize and that caused <code>init</code> to call. This is different 416from <code>async_hooks.executionAsyncId()</code> that only shows <em>when</em> a resource was 417created, while <code>triggerAsyncId</code> shows <em>why</em> a resource was created.</p> 418<p>The following is a simple demonstration of <code>triggerAsyncId</code>:</p> 419<pre><code class="language-js"><span class="hljs-keyword">const</span> { fd } = process.<span class="hljs-property">stdout</span>; 420 421async_hooks.<span class="hljs-title function_">createHook</span>({ 422 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId</span>) { 423 <span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 424 fs.<span class="hljs-title function_">writeSync</span>( 425 fd, 426 <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>); 427 } 428}).<span class="hljs-title function_">enable</span>(); 429 430net.<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></pre> 431<p>Output when hitting the server with <code>nc localhost 8080</code>:</p> 432<pre><code class="language-console">TCPSERVERWRAP(5): trigger: 1 execution: 1 433TCPWRAP(7): trigger: 5 execution: 0</code></pre> 434<p>The <code>TCPSERVERWRAP</code> is the server which receives the connections.</p> 435<p>The <code>TCPWRAP</code> is the new connection from the client. When a new 436connection is made, the <code>TCPWrap</code> instance is immediately constructed. This 437happens outside of any JavaScript stack. (An <code>executionAsyncId()</code> of <code>0</code> means 438that it is being executed from C++ with no JavaScript stack above it.) With only 439that information, it would be impossible to link resources together in 440terms of what caused them to be created, so <code>triggerAsyncId</code> is given the task 441of propagating what resource is responsible for the new resource's existence.</p> 442<h6><code>resource</code><span><a class="mark" href="#async_hooks_resource" id="async_hooks_resource">#</a></span></h6> 443<p><code>resource</code> is an object that represents the actual async resource that has 444been initialized. This can contain useful information that can vary based on 445the value of <code>type</code>. For instance, for the <code>GETADDRINFOREQWRAP</code> resource type, 446<code>resource</code> provides the host name used when looking up the IP address for the 447host in <code>net.Server.listen()</code>. The API for accessing this information is 448not supported, but using the Embedder API, users can provide 449and document their own resource objects. For example, such a resource object 450could contain the SQL query being executed.</p> 451<p>In some cases the resource object is reused for performance reasons, it is 452thus not safe to use it as a key in a <code>WeakMap</code> or add properties to it.</p> 453<h6>Asynchronous context example<span><a class="mark" href="#async_hooks_asynchronous_context_example" id="async_hooks_asynchronous_context_example">#</a></span></h6> 454<p>The following is an example with additional information about the calls to 455<code>init</code> between the <code>before</code> and <code>after</code> calls, specifically what the 456callback to <code>listen()</code> will look like. The output formatting is slightly more 457elaborate to make calling context easier to see.</p> 458<pre><code class="language-js"><span class="hljs-keyword">const</span> { fd } = process.<span class="hljs-property">stdout</span>; 459 460<span class="hljs-keyword">let</span> indent = <span class="hljs-number">0</span>; 461async_hooks.<span class="hljs-title function_">createHook</span>({ 462 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId</span>) { 463 <span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 464 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 465 fs.<span class="hljs-title function_">writeSync</span>( 466 fd, 467 <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span><span class="hljs-subst">${type}</span>(<span class="hljs-subst">${asyncId}</span>):`</span> + 468 <span class="hljs-string">` trigger: <span class="hljs-subst">${triggerAsyncId}</span> execution: <span class="hljs-subst">${eid}</span>\n`</span>); 469 }, 470 <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { 471 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 472 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>); 473 indent += <span class="hljs-number">2</span>; 474 }, 475 <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { 476 indent -= <span class="hljs-number">2</span>; 477 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 478 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>); 479 }, 480 <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { 481 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 482 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>); 483 }, 484}).<span class="hljs-title function_">enable</span>(); 485 486net.<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> { 487 <span class="hljs-comment">// Let's wait 10ms before logging the server started.</span> 488 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { 489 <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>()); 490 }, <span class="hljs-number">10</span>); 491});</code></pre> 492<p>Output from only starting the server:</p> 493<pre><code class="language-console">TCPSERVERWRAP(5): trigger: 1 execution: 1 494TickObject(6): trigger: 5 execution: 1 495before: 6 496 Timeout(7): trigger: 6 execution: 6 497after: 6 498destroy: 6 499before: 7 500<span class="hljs-meta">></span><span class="language-bash">>> 7</span> 501 TickObject(8): trigger: 7 execution: 7 502after: 7 503before: 8 504after: 8</code></pre> 505<p>As illustrated in the example, <code>executionAsyncId()</code> and <code>execution</code> each specify 506the value of the current execution context; which is delineated by calls to 507<code>before</code> and <code>after</code>.</p> 508<p>Only using <code>execution</code> to graph resource allocation results in the following:</p> 509<pre><code class="language-console"> root(1) 510 ^ 511 | 512TickObject(6) 513 ^ 514 | 515 Timeout(7)</code></pre> 516<p>The <code>TCPSERVERWRAP</code> is not part of this graph, even though it was the reason for 517<code>console.log()</code> being called. This is because binding to a port without a host 518name is a <em>synchronous</em> operation, but to maintain a completely asynchronous 519API the user's callback is placed in a <code>process.nextTick()</code>. Which is why 520<code>TickObject</code> is present in the output and is a 'parent' for <code>.listen()</code> 521callback.</p> 522<p>The graph only shows <em>when</em> a resource was created, not <em>why</em>, so to track 523the <em>why</em> use <code>triggerAsyncId</code>. Which can be represented with the following 524graph:</p> 525<pre><code class="language-console"> bootstrap(1) 526 | 527 ˅ 528TCPSERVERWRAP(5) 529 | 530 ˅ 531 TickObject(6) 532 | 533 ˅ 534 Timeout(7)</code></pre> 535<h5><code>before(asyncId)</code><span><a class="mark" href="#async_hooks_before_asyncid" id="async_hooks_before_asyncid">#</a></span></h5> 536<ul> 537<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 538</ul> 539<p>When an asynchronous operation is initiated (such as a TCP server receiving a 540new connection) or completes (such as writing data to disk) a callback is 541called to notify the user. The <code>before</code> callback is called just before said 542callback is executed. <code>asyncId</code> is the unique identifier assigned to the 543resource about to execute the callback.</p> 544<p>The <code>before</code> callback will be called 0 to N times. The <code>before</code> callback 545will typically be called 0 times if the asynchronous operation was cancelled 546or, for example, if no connections are received by a TCP server. Persistent 547asynchronous resources like a TCP server will typically call the <code>before</code> 548callback multiple times, while other operations like <code>fs.open()</code> will call 549it only once.</p> 550<h5><code>after(asyncId)</code><span><a class="mark" href="#async_hooks_after_asyncid" id="async_hooks_after_asyncid">#</a></span></h5> 551<ul> 552<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 553</ul> 554<p>Called immediately after the callback specified in <code>before</code> is completed.</p> 555<p>If an uncaught exception occurs during execution of the callback, then <code>after</code> 556will run <em>after</em> the <code>'uncaughtException'</code> event is emitted or a <code>domain</code>'s 557handler runs.</p> 558<h5><code>destroy(asyncId)</code><span><a class="mark" href="#async_hooks_destroy_asyncid" id="async_hooks_destroy_asyncid">#</a></span></h5> 559<ul> 560<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 561</ul> 562<p>Called after the resource corresponding to <code>asyncId</code> is destroyed. It is also 563called asynchronously from the embedder API <code>emitDestroy()</code>.</p> 564<p>Some resources depend on garbage collection for cleanup, so if a reference is 565made to the <code>resource</code> object passed to <code>init</code> it is possible that <code>destroy</code> 566will never be called, causing a memory leak in the application. If the resource 567does not depend on garbage collection, then this will not be an issue.</p> 568<h5><code>promiseResolve(asyncId)</code><span><a class="mark" href="#async_hooks_promiseresolve_asyncid" id="async_hooks_promiseresolve_asyncid">#</a></span></h5> 569<div class="api_metadata"> 570<span>Added in: v8.6.0</span> 571</div> 572<ul> 573<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 574</ul> 575<p>Called when the <code>resolve</code> function passed to the <code>Promise</code> constructor is 576invoked (either directly or through other means of resolving a promise).</p> 577<p><code>resolve()</code> does not do any observable synchronous work.</p> 578<p>The <code>Promise</code> is not necessarily fulfilled or rejected at this point if the 579<code>Promise</code> was resolved by assuming the state of another <code>Promise</code>.</p> 580<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></pre> 581<p>calls the following callbacks:</p> 582<pre><code class="language-text">init for PROMISE with id 5, trigger id: 1 583 promise resolve 5 # corresponds to resolve(true) 584init for PROMISE with id 6, trigger id: 5 # the Promise returned by then() 585 before 6 # the then() callback is entered 586 promise resolve 6 # the then() callback resolves the promise by returning 587 after 6</code></pre> 588<h4><code>async_hooks.executionAsyncResource()</code><span><a class="mark" href="#async_hooks_async_hooks_executionasyncresource" id="async_hooks_async_hooks_executionasyncresource">#</a></span></h4> 589<div class="api_metadata"> 590<span>Added in: v13.9.0</span> 591</div> 592<ul> 593<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. 594Useful to store data within the resource.</li> 595</ul> 596<p>Resource objects returned by <code>executionAsyncResource()</code> are most often internal 597Node.js handle objects with undocumented APIs. Using any functions or properties 598on the object is likely to crash your application and should be avoided.</p> 599<p>Using <code>executionAsyncResource()</code> in the top-level execution context will 600return an empty object as there is no handle or request object to use, 601but having an object representing the top-level can be helpful.</p> 602<pre><code class="language-js"><span class="hljs-keyword">const</span> { open } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'fs'</span>); 603<span class="hljs-keyword">const</span> { executionAsyncId, executionAsyncResource } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'async_hooks'</span>); 604 605<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> 606<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> { 607 <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> 608});</code></pre> 609<p>This can be used to implement continuation local storage without the 610use of a tracking <code>Map</code> to store the metadata:</p> 611<pre><code class="language-js"><span class="hljs-keyword">const</span> { createServer } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'http'</span>); 612<span class="hljs-keyword">const</span> { 613 executionAsyncId, 614 executionAsyncResource, 615 createHook 616} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'async_hooks'</span>); 617<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> 618 619<span class="hljs-title function_">createHook</span>({ 620 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { 621 <span class="hljs-keyword">const</span> cr = <span class="hljs-title function_">executionAsyncResource</span>(); 622 <span class="hljs-keyword">if</span> (cr) { 623 resource[sym] = cr[sym]; 624 } 625 } 626}).<span class="hljs-title function_">enable</span>(); 627 628<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> { 629 <span class="hljs-title function_">executionAsyncResource</span>()[sym] = { <span class="hljs-attr">state</span>: req.<span class="hljs-property">url</span> }; 630 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-keyword">function</span>(<span class="hljs-params"></span>) { 631 res.<span class="hljs-title function_">end</span>(<span class="hljs-variable constant_">JSON</span>.<span class="hljs-title function_">stringify</span>(<span class="hljs-title function_">executionAsyncResource</span>()[sym])); 632 }, <span class="hljs-number">100</span>); 633}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">3000</span>);</code></pre> 634<h4><code>async_hooks.executionAsyncId()</code><span><a class="mark" href="#async_hooks_async_hooks_executionasyncid" id="async_hooks_async_hooks_executionasyncid">#</a></span></h4> 635<div class="api_metadata"> 636<details class="changelog"><summary>History</summary> 637<table> 638<tbody><tr><th>Version</th><th>Changes</th></tr> 639<tr><td>v8.2.0</td> 640<td><p>Renamed from <code>currentId</code>.</p></td></tr> 641<tr><td>v8.1.0</td> 642<td><p><span>Added in: v8.1.0</span></p></td></tr> 643</tbody></table> 644</details> 645</div> 646<ul> 647<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 648track when something calls.</li> 649</ul> 650<pre><code class="language-js"><span class="hljs-keyword">const</span> async_hooks = <span class="hljs-built_in">require</span>(<span class="hljs-string">'async_hooks'</span>); 651 652<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> 653fs.<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> { 654 <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> 655});</code></pre> 656<p>The ID returned from <code>executionAsyncId()</code> is related to execution timing, not 657causality (which is covered by <code>triggerAsyncId()</code>):</p> 658<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> { 659 <span class="hljs-comment">// Returns the ID of the server, not of the new connection, because the</span> 660 <span class="hljs-comment">// callback runs in the execution scope of the server's MakeCallback().</span> 661 async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 662 663}).<span class="hljs-title function_">listen</span>(port, <span class="hljs-function">() =></span> { 664 <span class="hljs-comment">// Returns the ID of a TickObject (process.nextTick()) because all</span> 665 <span class="hljs-comment">// callbacks passed to .listen() are wrapped in a nextTick().</span> 666 async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 667});</code></pre> 668<p>Promise contexts may not get precise <code>executionAsyncIds</code> by default. 669See the section on <a href="#async_hooks_promise_execution_tracking">promise execution tracking</a>.</p> 670<h4><code>async_hooks.triggerAsyncId()</code><span><a class="mark" href="#async_hooks_async_hooks_triggerasyncid" id="async_hooks_async_hooks_triggerasyncid">#</a></span></h4> 671<ul> 672<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 673that is currently being executed.</li> 674</ul> 675<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> { 676 <span class="hljs-comment">// The resource that caused (or triggered) this callback to be called</span> 677 <span class="hljs-comment">// was that of the new connection. Thus the return value of triggerAsyncId()</span> 678 <span class="hljs-comment">// is the asyncId of "conn".</span> 679 async_hooks.<span class="hljs-title function_">triggerAsyncId</span>(); 680 681}).<span class="hljs-title function_">listen</span>(port, <span class="hljs-function">() =></span> { 682 <span class="hljs-comment">// Even though all callbacks passed to .listen() are wrapped in a nextTick()</span> 683 <span class="hljs-comment">// the callback itself exists because the call to the server's .listen()</span> 684 <span class="hljs-comment">// was made. So the return value would be the ID of the server.</span> 685 async_hooks.<span class="hljs-title function_">triggerAsyncId</span>(); 686});</code></pre> 687<p>Promise contexts may not get valid <code>triggerAsyncId</code>s by default. See 688the section on <a href="#async_hooks_promise_execution_tracking">promise execution tracking</a>.</p> 689</section><section><h3>Promise execution tracking<span><a class="mark" href="#async_hooks_promise_execution_tracking" id="async_hooks_promise_execution_tracking">#</a></span></h3> 690<p>By default, promise executions are not assigned <code>asyncId</code>s due to the relatively 691expensive nature of the <a href="https://docs.google.com/document/d/1rda3yKGHimKIhg5YeoAmCOtyURgsbTH_qaYR79FELlk/edit">promise introspection API</a> provided by 692V8. This means that programs using promises or <code>async</code>/<code>await</code> will not get 693correct execution and trigger ids for promise callback contexts by default.</p> 694<pre><code class="language-js"><span class="hljs-keyword">const</span> ah = <span class="hljs-built_in">require</span>(<span class="hljs-string">'async_hooks'</span>); 695<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> { 696 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`eid <span class="hljs-subst">${ah.executionAsyncId()}</span> tid <span class="hljs-subst">${ah.triggerAsyncId()}</span>`</span>); 697}); 698<span class="hljs-comment">// produces:</span> 699<span class="hljs-comment">// eid 1 tid 0</span></code></pre> 700<p>Observe that the <code>then()</code> callback claims to have executed in the context of the 701outer scope even though there was an asynchronous hop involved. Also, 702the <code>triggerAsyncId</code> value is <code>0</code>, which means that we are missing context about 703the resource that caused (triggered) the <code>then()</code> callback to be executed.</p> 704<p>Installing async hooks via <code>async_hooks.createHook</code> enables promise execution 705tracking:</p> 706<pre><code class="language-js"><span class="hljs-keyword">const</span> ah = <span class="hljs-built_in">require</span>(<span class="hljs-string">'async_hooks'</span>); 707ah.<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> 708<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> { 709 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`eid <span class="hljs-subst">${ah.executionAsyncId()}</span> tid <span class="hljs-subst">${ah.triggerAsyncId()}</span>`</span>); 710}); 711<span class="hljs-comment">// produces:</span> 712<span class="hljs-comment">// eid 7 tid 6</span></code></pre> 713<p>In this example, adding any actual hook function enabled the tracking of 714promises. There are two promises in the example above; the promise created by 715<code>Promise.resolve()</code> and the promise returned by the call to <code>then()</code>. In the 716example above, the first promise got the <code>asyncId</code> <code>6</code> and the latter got 717<code>asyncId</code> <code>7</code>. During the execution of the <code>then()</code> callback, we are executing 718in the context of promise with <code>asyncId</code> <code>7</code>. This promise was triggered by 719async resource <code>6</code>.</p> 720<p>Another subtlety with promises is that <code>before</code> and <code>after</code> callbacks are run 721only on chained promises. That means promises not created by <code>then()</code>/<code>catch()</code> 722will not have the <code>before</code> and <code>after</code> callbacks fired on them. For more details 723see the details of the V8 <a href="https://docs.google.com/document/d/1rda3yKGHimKIhg5YeoAmCOtyURgsbTH_qaYR79FELlk/edit">PromiseHooks</a> API.</p> 724</section><section><h3>JavaScript embedder API<span><a class="mark" href="#async_hooks_javascript_embedder_api" id="async_hooks_javascript_embedder_api">#</a></span></h3> 725<p>Library developers that handle their own asynchronous resources performing tasks 726like I/O, connection pooling, or managing callback queues may use the 727<code>AsyncResource</code> JavaScript API so that all the appropriate callbacks are called.</p> 728<h4>Class: <code>AsyncResource</code><span><a class="mark" href="#async_hooks_class_asyncresource" id="async_hooks_class_asyncresource">#</a></span></h4> 729<p>The class <code>AsyncResource</code> is designed to be extended by the embedder's async 730resources. Using this, users can easily trigger the lifetime events of their 731own resources.</p> 732<p>The <code>init</code> hook will trigger when an <code>AsyncResource</code> is instantiated.</p> 733<p>The following is an overview of the <code>AsyncResource</code> API.</p> 734<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">AsyncResource</span>, executionAsyncId } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'async_hooks'</span>); 735 736<span class="hljs-comment">// AsyncResource() is meant to be extended. Instantiating a</span> 737<span class="hljs-comment">// new AsyncResource() also triggers init. If triggerAsyncId is omitted then</span> 738<span class="hljs-comment">// async_hook.executionAsyncId() is used.</span> 739<span class="hljs-keyword">const</span> asyncResource = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncResource</span>( 740 type, { <span class="hljs-attr">triggerAsyncId</span>: <span class="hljs-title function_">executionAsyncId</span>(), <span class="hljs-attr">requireManualDestroy</span>: <span class="hljs-literal">false</span> } 741); 742 743<span class="hljs-comment">// Run a function in the execution context of the resource. This will</span> 744<span class="hljs-comment">// * establish the context of the resource</span> 745<span class="hljs-comment">// * trigger the AsyncHooks before callbacks</span> 746<span class="hljs-comment">// * call the provided function `fn` with the supplied arguments</span> 747<span class="hljs-comment">// * trigger the AsyncHooks after callbacks</span> 748<span class="hljs-comment">// * restore the original execution context</span> 749asyncResource.<span class="hljs-title function_">runInAsyncScope</span>(fn, thisArg, ...args); 750 751<span class="hljs-comment">// Call AsyncHooks destroy callbacks.</span> 752asyncResource.<span class="hljs-title function_">emitDestroy</span>(); 753 754<span class="hljs-comment">// Return the unique ID assigned to the AsyncResource instance.</span> 755asyncResource.<span class="hljs-title function_">asyncId</span>(); 756 757<span class="hljs-comment">// Return the trigger ID for the AsyncResource instance.</span> 758asyncResource.<span class="hljs-title function_">triggerAsyncId</span>();</code></pre> 759<h5><code>new AsyncResource(type[, options])</code><span><a class="mark" href="#async_hooks_new_asyncresource_type_options" id="async_hooks_new_asyncresource_type_options">#</a></span></h5> 760<ul> 761<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 async event.</li> 762<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 763<ul> 764<li><code>triggerAsyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The ID of the execution context that created this 765async event. <strong>Default:</strong> <code>executionAsyncId()</code>.</li> 766<li><code>requireManualDestroy</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> If set to <code>true</code>, disables <code>emitDestroy</code> 767when the object is garbage collected. This usually does not need to be set 768(even if <code>emitDestroy</code> is called manually), unless the resource's <code>asyncId</code> 769is retrieved and the sensitive API's <code>emitDestroy</code> is called with it. 770When set to <code>false</code>, the <code>emitDestroy</code> call on garbage collection 771will only take place if there is at least one active <code>destroy</code> hook. 772<strong>Default:</strong> <code>false</code>.</li> 773</ul> 774</li> 775</ul> 776<p>Example usage:</p> 777<pre><code class="language-js"><span class="hljs-keyword">class</span> <span class="hljs-title class_">DBQuery</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">AsyncResource</span> { 778 <span class="hljs-title function_">constructor</span>(<span class="hljs-params">db</span>) { 779 <span class="hljs-variable language_">super</span>(<span class="hljs-string">'DBQuery'</span>); 780 <span class="hljs-variable language_">this</span>.<span class="hljs-property">db</span> = db; 781 } 782 783 <span class="hljs-title function_">getInfo</span>(<span class="hljs-params">query, callback</span>) { 784 <span class="hljs-variable language_">this</span>.<span class="hljs-property">db</span>.<span class="hljs-title function_">get</span>(query, <span class="hljs-function">(<span class="hljs-params">err, data</span>) =></span> { 785 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">runInAsyncScope</span>(callback, <span class="hljs-literal">null</span>, err, data); 786 }); 787 } 788 789 <span class="hljs-title function_">close</span>(<span class="hljs-params"></span>) { 790 <span class="hljs-variable language_">this</span>.<span class="hljs-property">db</span> = <span class="hljs-literal">null</span>; 791 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emitDestroy</span>(); 792 } 793}</code></pre> 794<h5>Static method: <code>AsyncResource.bind(fn[, type])</code><span><a class="mark" href="#async_hooks_static_method_asyncresource_bind_fn_type" id="async_hooks_static_method_asyncresource_bind_fn_type">#</a></span></h5> 795<div class="api_metadata"> 796<span>Added in: v14.8.0</span> 797</div> 798<ul> 799<li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The function to bind to the current execution context.</li> 800<li><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> An optional name to associate with the underlying 801<code>AsyncResource</code>.</li> 802</ul> 803<p>Binds the given function to the current execution context.</p> 804<p>The returned function will have an <code>asyncResource</code> property referencing 805the <code>AsyncResource</code> to which the function is bound.</p> 806<h5><code>asyncResource.bind(fn)</code><span><a class="mark" href="#async_hooks_asyncresource_bind_fn" id="async_hooks_asyncresource_bind_fn">#</a></span></h5> 807<div class="api_metadata"> 808<span>Added in: v14.8.0</span> 809</div> 810<ul> 811<li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The function to bind to the current <code>AsyncResource</code>.</li> 812</ul> 813<p>Binds the given function to execute to this <code>AsyncResource</code>'s scope.</p> 814<p>The returned function will have an <code>asyncResource</code> property referencing 815the <code>AsyncResource</code> to which the function is bound.</p> 816<h5><code>asyncResource.runInAsyncScope(fn[, thisArg, ...args])</code><span><a class="mark" href="#async_hooks_asyncresource_runinasyncscope_fn_thisarg_args" id="async_hooks_asyncresource_runinasyncscope_fn_thisarg_args">#</a></span></h5> 817<div class="api_metadata"> 818<span>Added in: v9.6.0</span> 819</div> 820<ul> 821<li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The function to call in the execution context of this async 822resource.</li> 823<li><code>thisArg</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> The receiver to be used for the function call.</li> 824<li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> Optional arguments to pass to the function.</li> 825</ul> 826<p>Call the provided function with the provided arguments in the execution context 827of the async resource. This will establish the context, trigger the AsyncHooks 828before callbacks, call the function, trigger the AsyncHooks after callbacks, and 829then restore the original execution context.</p> 830<h5><code>asyncResource.emitDestroy()</code><span><a class="mark" href="#async_hooks_asyncresource_emitdestroy" id="async_hooks_asyncresource_emitdestroy">#</a></span></h5> 831<ul> 832<li>Returns: <a href="async_hooks.html#async_hooks_class_asyncresource" class="type"><AsyncResource></a> A reference to <code>asyncResource</code>.</li> 833</ul> 834<p>Call all <code>destroy</code> hooks. This should only ever be called once. An error will 835be thrown if it is called more than once. This <strong>must</strong> be manually called. If 836the resource is left to be collected by the GC then the <code>destroy</code> hooks will 837never be called.</p> 838<h5><code>asyncResource.asyncId()</code><span><a class="mark" href="#async_hooks_asyncresource_asyncid" id="async_hooks_asyncresource_asyncid">#</a></span></h5> 839<ul> 840<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The unique <code>asyncId</code> assigned to the resource.</li> 841</ul> 842<h5><code>asyncResource.triggerAsyncId()</code><span><a class="mark" href="#async_hooks_asyncresource_triggerasyncid" id="async_hooks_asyncresource_triggerasyncid">#</a></span></h5> 843<ul> 844<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The same <code>triggerAsyncId</code> that is passed to the 845<code>AsyncResource</code> constructor.</li> 846</ul> 847<p><a id="async-resource-worker-pool"></a></p> 848<h4>Using <code>AsyncResource</code> for a <code>Worker</code> thread pool<span><a class="mark" href="#async_hooks_using_asyncresource_for_a_worker_thread_pool" id="async_hooks_using_asyncresource_for_a_worker_thread_pool">#</a></span></h4> 849<p>The following example shows how to use the <code>AsyncResource</code> class to properly 850provide async tracking for a <a href="worker_threads.html#worker_threads_class_worker"><code>Worker</code></a> pool. Other resource pools, such as 851database connection pools, can follow a similar model.</p> 852<p>Assuming that the task is adding two numbers, using a file named 853<code>task_processor.js</code> with the following content:</p> 854<pre><code class="language-js"><span class="hljs-keyword">const</span> { parentPort } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>); 855parentPort.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">task</span>) =></span> { 856 parentPort.<span class="hljs-title function_">postMessage</span>(task.<span class="hljs-property">a</span> + task.<span class="hljs-property">b</span>); 857});</code></pre> 858<p>a Worker pool around it could use the following structure:</p> 859<pre><code class="language-js"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">AsyncResource</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'async_hooks'</span>); 860<span class="hljs-keyword">const</span> { <span class="hljs-title class_">EventEmitter</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'events'</span>); 861<span class="hljs-keyword">const</span> path = <span class="hljs-built_in">require</span>(<span class="hljs-string">'path'</span>); 862<span class="hljs-keyword">const</span> { <span class="hljs-title class_">Worker</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'worker_threads'</span>); 863 864<span class="hljs-keyword">const</span> kTaskInfo = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'kTaskInfo'</span>); 865<span class="hljs-keyword">const</span> kWorkerFreedEvent = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'kWorkerFreedEvent'</span>); 866 867<span class="hljs-keyword">class</span> <span class="hljs-title class_">WorkerPoolTaskInfo</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">AsyncResource</span> { 868 <span class="hljs-title function_">constructor</span>(<span class="hljs-params">callback</span>) { 869 <span class="hljs-variable language_">super</span>(<span class="hljs-string">'WorkerPoolTaskInfo'</span>); 870 <span class="hljs-variable language_">this</span>.<span class="hljs-property">callback</span> = callback; 871 } 872 873 <span class="hljs-title function_">done</span>(<span class="hljs-params">err, result</span>) { 874 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">runInAsyncScope</span>(<span class="hljs-variable language_">this</span>.<span class="hljs-property">callback</span>, <span class="hljs-literal">null</span>, err, result); 875 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emitDestroy</span>(); <span class="hljs-comment">// `TaskInfo`s are used only once.</span> 876 } 877} 878 879<span class="hljs-keyword">class</span> <span class="hljs-title class_">WorkerPool</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">EventEmitter</span> { 880 <span class="hljs-title function_">constructor</span>(<span class="hljs-params">numThreads</span>) { 881 <span class="hljs-variable language_">super</span>(); 882 <span class="hljs-variable language_">this</span>.<span class="hljs-property">numThreads</span> = numThreads; 883 <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span> = []; 884 <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span> = []; 885 <span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span> = []; 886 887 <span class="hljs-keyword">for</span> (<span class="hljs-keyword">let</span> i = <span class="hljs-number">0</span>; i < numThreads; i++) 888 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">addNewWorker</span>(); 889 890 <span class="hljs-comment">// Any time the kWorkerFreedEvent is emitted, dispatch</span> 891 <span class="hljs-comment">// the next task pending in the queue, if any.</span> 892 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">on</span>(kWorkerFreedEvent, <span class="hljs-function">() =></span> { 893 <span class="hljs-keyword">if</span> (<span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span>.<span class="hljs-property">length</span> > <span class="hljs-number">0</span>) { 894 <span class="hljs-keyword">const</span> { task, callback } = <span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span>.<span class="hljs-title function_">shift</span>(); 895 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">runTask</span>(task, callback); 896 } 897 }); 898 } 899 900 <span class="hljs-title function_">addNewWorker</span>(<span class="hljs-params"></span>) { 901 <span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(path.<span class="hljs-title function_">resolve</span>(__dirname, <span class="hljs-string">'task_processor.js'</span>)); 902 worker.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">result</span>) =></span> { 903 <span class="hljs-comment">// In case of success: Call the callback that was passed to `runTask`,</span> 904 <span class="hljs-comment">// remove the `TaskInfo` associated with the Worker, and mark it as free</span> 905 <span class="hljs-comment">// again.</span> 906 worker[kTaskInfo].<span class="hljs-title function_">done</span>(<span class="hljs-literal">null</span>, result); 907 worker[kTaskInfo] = <span class="hljs-literal">null</span>; 908 <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-title function_">push</span>(worker); 909 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(kWorkerFreedEvent); 910 }); 911 worker.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">err</span>) =></span> { 912 <span class="hljs-comment">// In case of an uncaught exception: Call the callback that was passed to</span> 913 <span class="hljs-comment">// `runTask` with the error.</span> 914 <span class="hljs-keyword">if</span> (worker[kTaskInfo]) 915 worker[kTaskInfo].<span class="hljs-title function_">done</span>(err, <span class="hljs-literal">null</span>); 916 <span class="hljs-keyword">else</span> 917 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'error'</span>, err); 918 <span class="hljs-comment">// Remove the worker from the list and start a new Worker to replace the</span> 919 <span class="hljs-comment">// current one.</span> 920 <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>.<span class="hljs-title function_">splice</span>(<span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>.<span class="hljs-title function_">indexOf</span>(worker), <span class="hljs-number">1</span>); 921 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">addNewWorker</span>(); 922 }); 923 <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>.<span class="hljs-title function_">push</span>(worker); 924 <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-title function_">push</span>(worker); 925 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(kWorkerFreedEvent); 926 } 927 928 <span class="hljs-title function_">runTask</span>(<span class="hljs-params">task, callback</span>) { 929 <span class="hljs-keyword">if</span> (<span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-property">length</span> === <span class="hljs-number">0</span>) { 930 <span class="hljs-comment">// No free threads, wait until a worker thread becomes free.</span> 931 <span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span>.<span class="hljs-title function_">push</span>({ task, callback }); 932 <span class="hljs-keyword">return</span>; 933 } 934 935 <span class="hljs-keyword">const</span> worker = <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-title function_">pop</span>(); 936 worker[kTaskInfo] = <span class="hljs-keyword">new</span> <span class="hljs-title class_">WorkerPoolTaskInfo</span>(callback); 937 worker.<span class="hljs-title function_">postMessage</span>(task); 938 } 939 940 <span class="hljs-title function_">close</span>(<span class="hljs-params"></span>) { 941 <span class="hljs-keyword">for</span> (<span class="hljs-keyword">const</span> worker <span class="hljs-keyword">of</span> <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>) worker.<span class="hljs-title function_">terminate</span>(); 942 } 943} 944 945<span class="hljs-variable language_">module</span>.<span class="hljs-property">exports</span> = <span class="hljs-title class_">WorkerPool</span>;</code></pre> 946<p>Without the explicit tracking added by the <code>WorkerPoolTaskInfo</code> objects, 947it would appear that the callbacks are associated with the individual <code>Worker</code> 948objects. However, the creation of the <code>Worker</code>s is not associated with the 949creation of the tasks and does not provide information about when tasks 950were scheduled.</p> 951<p>This pool could be used as follows:</p> 952<pre><code class="language-js"><span class="hljs-keyword">const</span> <span class="hljs-title class_">WorkerPool</span> = <span class="hljs-built_in">require</span>(<span class="hljs-string">'./worker_pool.js'</span>); 953<span class="hljs-keyword">const</span> os = <span class="hljs-built_in">require</span>(<span class="hljs-string">'os'</span>); 954 955<span class="hljs-keyword">const</span> pool = <span class="hljs-keyword">new</span> <span class="hljs-title class_">WorkerPool</span>(os.<span class="hljs-title function_">cpus</span>().<span class="hljs-property">length</span>); 956 957<span class="hljs-keyword">let</span> finished = <span class="hljs-number">0</span>; 958<span class="hljs-keyword">for</span> (<span class="hljs-keyword">let</span> i = <span class="hljs-number">0</span>; i < <span class="hljs-number">10</span>; i++) { 959 pool.<span class="hljs-title function_">runTask</span>({ <span class="hljs-attr">a</span>: <span class="hljs-number">42</span>, <span class="hljs-attr">b</span>: <span class="hljs-number">100</span> }, <span class="hljs-function">(<span class="hljs-params">err, result</span>) =></span> { 960 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(i, err, result); 961 <span class="hljs-keyword">if</span> (++finished === <span class="hljs-number">10</span>) 962 pool.<span class="hljs-title function_">close</span>(); 963 }); 964}</code></pre> 965<h4>Integrating <code>AsyncResource</code> with <code>EventEmitter</code><span><a class="mark" href="#async_hooks_integrating_asyncresource_with_eventemitter" id="async_hooks_integrating_asyncresource_with_eventemitter">#</a></span></h4> 966<p>Event listeners triggered by an <a href="events.html#events_class_eventemitter"><code>EventEmitter</code></a> may be run in a different 967execution context than the one that was active when <code>eventEmitter.on()</code> was 968called.</p> 969<p>The following example shows how to use the <code>AsyncResource</code> class to properly 970associate an event listener with the correct execution context. The same 971approach can be applied to a <a href="stream.html#stream_stream"><code>Stream</code></a> or a similar event-driven class.</p> 972<pre><code class="language-js"><span class="hljs-keyword">const</span> { createServer } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'http'</span>); 973<span class="hljs-keyword">const</span> { <span class="hljs-title class_">AsyncResource</span>, executionAsyncId } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'async_hooks'</span>); 974 975<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> { 976 req.<span class="hljs-title function_">on</span>(<span class="hljs-string">'close'</span>, <span class="hljs-title class_">AsyncResource</span>.<span class="hljs-title function_">bind</span>(<span class="hljs-function">() =></span> { 977 <span class="hljs-comment">// Execution context is bound to the current outer scope.</span> 978 })); 979 req.<span class="hljs-title function_">on</span>(<span class="hljs-string">'close'</span>, <span class="hljs-function">() =></span> { 980 <span class="hljs-comment">// Execution context is bound to the scope that caused 'close' to emit.</span> 981 }); 982 res.<span class="hljs-title function_">end</span>(); 983}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">3000</span>);</code></pre> 984</section><section><h3>Class: <code>AsyncLocalStorage</code><span><a class="mark" href="#async_hooks_class_asynclocalstorage" id="async_hooks_class_asynclocalstorage">#</a></span></h3> 985<div class="api_metadata"> 986<span>Added in: v13.10.0</span> 987</div> 988<p>This class is used to create asynchronous state within callbacks and promise 989chains. It allows storing data throughout the lifetime of a web request 990or any other asynchronous duration. It is similar to thread-local storage 991in other languages.</p> 992<p>While you can create your own implementation on top of the <code>async_hooks</code> module, 993<code>AsyncLocalStorage</code> should be preferred as it is a performant and memory safe 994implementation that involves significant optimizations that are non-obvious to 995implement.</p> 996<p>The following example uses <code>AsyncLocalStorage</code> to build a simple logger 997that assigns IDs to incoming HTTP requests and includes them in messages 998logged within each request.</p> 999<pre><code class="language-js"><span class="hljs-keyword">const</span> http = <span class="hljs-built_in">require</span>(<span class="hljs-string">'http'</span>); 1000<span class="hljs-keyword">const</span> { <span class="hljs-title class_">AsyncLocalStorage</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'async_hooks'</span>); 1001 1002<span class="hljs-keyword">const</span> asyncLocalStorage = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncLocalStorage</span>(); 1003 1004<span class="hljs-keyword">function</span> <span class="hljs-title function_">logWithId</span>(<span class="hljs-params">msg</span>) { 1005 <span class="hljs-keyword">const</span> id = asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); 1006 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`<span class="hljs-subst">${id !== <span class="hljs-literal">undefined</span> ? id : <span class="hljs-string">'-'</span>}</span>:`</span>, msg); 1007} 1008 1009<span class="hljs-keyword">let</span> idSeq = <span class="hljs-number">0</span>; 1010http.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> { 1011 asyncLocalStorage.<span class="hljs-title function_">run</span>(idSeq++, <span class="hljs-function">() =></span> { 1012 <span class="hljs-title function_">logWithId</span>(<span class="hljs-string">'start'</span>); 1013 <span class="hljs-comment">// Imagine any chain of async operations here</span> 1014 <span class="hljs-title function_">setImmediate</span>(<span class="hljs-function">() =></span> { 1015 <span class="hljs-title function_">logWithId</span>(<span class="hljs-string">'finish'</span>); 1016 res.<span class="hljs-title function_">end</span>(); 1017 }); 1018 }); 1019}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">8080</span>); 1020 1021http.<span class="hljs-title function_">get</span>(<span class="hljs-string">'http://localhost:8080'</span>); 1022http.<span class="hljs-title function_">get</span>(<span class="hljs-string">'http://localhost:8080'</span>); 1023<span class="hljs-comment">// Prints:</span> 1024<span class="hljs-comment">// 0: start</span> 1025<span class="hljs-comment">// 1: start</span> 1026<span class="hljs-comment">// 0: finish</span> 1027<span class="hljs-comment">// 1: finish</span></code></pre> 1028<p>When having multiple instances of <code>AsyncLocalStorage</code>, they are independent 1029from each other. It is safe to instantiate this class multiple times.</p> 1030<h4><code>new AsyncLocalStorage()</code><span><a class="mark" href="#async_hooks_new_asynclocalstorage" id="async_hooks_new_asynclocalstorage">#</a></span></h4> 1031<div class="api_metadata"> 1032<span>Added in: v13.10.0</span> 1033</div> 1034<p>Creates a new instance of <code>AsyncLocalStorage</code>. Store is only provided within a 1035<code>run()</code> call or after an <code>enterWith()</code> call.</p> 1036<h4><code>asyncLocalStorage.disable()</code><span><a class="mark" href="#async_hooks_asynclocalstorage_disable" id="async_hooks_asynclocalstorage_disable">#</a></span></h4> 1037<div class="api_metadata"> 1038<span>Added in: v13.10.0</span> 1039</div> 1040<p>Disables the instance of <code>AsyncLocalStorage</code>. All subsequent calls 1041to <code>asyncLocalStorage.getStore()</code> will return <code>undefined</code> until 1042<code>asyncLocalStorage.run()</code> or <code>asyncLocalStorage.enterWith()</code> is called again.</p> 1043<p>When calling <code>asyncLocalStorage.disable()</code>, all current contexts linked to the 1044instance will be exited.</p> 1045<p>Calling <code>asyncLocalStorage.disable()</code> is required before the 1046<code>asyncLocalStorage</code> can be garbage collected. This does not apply to stores 1047provided by the <code>asyncLocalStorage</code>, as those objects are garbage collected 1048along with the corresponding async resources.</p> 1049<p>Use this method when the <code>asyncLocalStorage</code> is not in use anymore 1050in the current process.</p> 1051<h4><code>asyncLocalStorage.getStore()</code><span><a class="mark" href="#async_hooks_asynclocalstorage_getstore" id="async_hooks_asynclocalstorage_getstore">#</a></span></h4> 1052<div class="api_metadata"> 1053<span>Added in: v13.10.0</span> 1054</div> 1055<ul> 1056<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 1057</ul> 1058<p>Returns the current store. 1059If called outside of an asynchronous context initialized by 1060calling <code>asyncLocalStorage.run()</code> or <code>asyncLocalStorage.enterWith()</code>, it 1061returns <code>undefined</code>.</p> 1062<h4><code>asyncLocalStorage.enterWith(store)</code><span><a class="mark" href="#async_hooks_asynclocalstorage_enterwith_store" id="async_hooks_asynclocalstorage_enterwith_store">#</a></span></h4> 1063<div class="api_metadata"> 1064<span>Added in: v13.11.0</span> 1065</div> 1066<ul> 1067<li><code>store</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 1068</ul> 1069<p>Transitions into the context for the remainder of the current 1070synchronous execution and then persists the store through any following 1071asynchronous calls.</p> 1072<p>Example:</p> 1073<pre><code class="language-js"><span class="hljs-keyword">const</span> store = { <span class="hljs-attr">id</span>: <span class="hljs-number">1</span> }; 1074<span class="hljs-comment">// Replaces previous store with the given store object</span> 1075asyncLocalStorage.<span class="hljs-title function_">enterWith</span>(store); 1076asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the store object</span> 1077<span class="hljs-title function_">someAsyncOperation</span>(<span class="hljs-function">() =></span> { 1078 asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the same object</span> 1079});</code></pre> 1080<p>This transition will continue for the <em>entire</em> synchronous execution. 1081This means that if, for example, the context is entered within an event 1082handler subsequent event handlers will also run within that context unless 1083specifically bound to another context with an <code>AsyncResource</code>. That is why 1084<code>run()</code> should be preferred over <code>enterWith()</code> unless there are strong reasons 1085to use the latter method.</p> 1086<pre><code class="language-js"><span class="hljs-keyword">const</span> store = { <span class="hljs-attr">id</span>: <span class="hljs-number">1</span> }; 1087 1088emitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'my-event'</span>, <span class="hljs-function">() =></span> { 1089 asyncLocalStorage.<span class="hljs-title function_">enterWith</span>(store); 1090}); 1091emitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'my-event'</span>, <span class="hljs-function">() =></span> { 1092 asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the same object</span> 1093}); 1094 1095asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns undefined</span> 1096emitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'my-event'</span>); 1097asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the same object</span></code></pre> 1098<h4><code>asyncLocalStorage.run(store, callback[, ...args])</code><span><a class="mark" href="#async_hooks_asynclocalstorage_run_store_callback_args" id="async_hooks_asynclocalstorage_run_store_callback_args">#</a></span></h4> 1099<div class="api_metadata"> 1100<span>Added in: v13.10.0</span> 1101</div> 1102<ul> 1103<li><code>store</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 1104<li><code>callback</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li> 1105<li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 1106</ul> 1107<p>Runs a function synchronously within a context and returns its 1108return value. The store is not accessible outside of the callback function. 1109The store is accessible to any asynchronous operations created within the 1110callback.</p> 1111<p>The optional <code>args</code> are passed to the callback function.</p> 1112<p>If the callback function throws an error, the error is thrown by <code>run()</code> too. 1113The stacktrace is not impacted by this call and the context is exited.</p> 1114<p>Example:</p> 1115<pre><code class="language-js"><span class="hljs-keyword">const</span> store = { <span class="hljs-attr">id</span>: <span class="hljs-number">2</span> }; 1116<span class="hljs-keyword">try</span> { 1117 asyncLocalStorage.<span class="hljs-title function_">run</span>(store, <span class="hljs-function">() =></span> { 1118 asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the store object</span> 1119 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { 1120 asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the store object</span> 1121 }, <span class="hljs-number">200</span>); 1122 <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>(); 1123 }); 1124} <span class="hljs-keyword">catch</span> (e) { 1125 asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns undefined</span> 1126 <span class="hljs-comment">// The error will be caught here</span> 1127}</code></pre> 1128<h4><code>asyncLocalStorage.exit(callback[, ...args])</code><span><a class="mark" href="#async_hooks_asynclocalstorage_exit_callback_args" id="async_hooks_asynclocalstorage_exit_callback_args">#</a></span></h4> 1129<div class="api_metadata"> 1130<span>Added in: v13.10.0</span> 1131</div> 1132<ul> 1133<li><code>callback</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li> 1134<li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 1135</ul> 1136<p>Runs a function synchronously outside of a context and returns its 1137return value. The store is not accessible within the callback function or 1138the asynchronous operations created within the callback. Any <code>getStore()</code> 1139call done within the callback function will always return <code>undefined</code>.</p> 1140<p>The optional <code>args</code> are passed to the callback function.</p> 1141<p>If the callback function throws an error, the error is thrown by <code>exit()</code> too. 1142The stacktrace is not impacted by this call and the context is re-entered.</p> 1143<p>Example:</p> 1144<pre><code class="language-js"><span class="hljs-comment">// Within a call to run</span> 1145<span class="hljs-keyword">try</span> { 1146 asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the store object or value</span> 1147 asyncLocalStorage.<span class="hljs-title function_">exit</span>(<span class="hljs-function">() =></span> { 1148 asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns undefined</span> 1149 <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>(); 1150 }); 1151} <span class="hljs-keyword">catch</span> (e) { 1152 asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the same object or value</span> 1153 <span class="hljs-comment">// The error will be caught here</span> 1154}</code></pre> 1155<h4>Usage with <code>async/await</code><span><a class="mark" href="#async_hooks_usage_with_async_await" id="async_hooks_usage_with_async_await">#</a></span></h4> 1156<p>If, within an async function, only one <code>await</code> call is to run within a context, 1157the following pattern should be used:</p> 1158<pre><code class="language-js"><span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">fn</span>(<span class="hljs-params"></span>) { 1159 <span class="hljs-keyword">await</span> asyncLocalStorage.<span class="hljs-title function_">run</span>(<span class="hljs-keyword">new</span> <span class="hljs-title class_">Map</span>(), <span class="hljs-function">() =></span> { 1160 asyncLocalStorage.<span class="hljs-title function_">getStore</span>().<span class="hljs-title function_">set</span>(<span class="hljs-string">'key'</span>, value); 1161 <span class="hljs-keyword">return</span> <span class="hljs-title function_">foo</span>(); <span class="hljs-comment">// The return value of foo will be awaited</span> 1162 }); 1163}</code></pre> 1164<p>In this example, the store is only available in the callback function and the 1165functions called by <code>foo</code>. Outside of <code>run</code>, calling <code>getStore</code> will return 1166<code>undefined</code>.</p> 1167<h4>Troubleshooting<span><a class="mark" href="#async_hooks_troubleshooting" id="async_hooks_troubleshooting">#</a></span></h4> 1168<p>In most cases your application or library code should have no issues with 1169<code>AsyncLocalStorage</code>. But in rare cases you may face situations when the 1170current store is lost in one of asynchronous operations. In those cases, 1171consider the following options.</p> 1172<p>If your code is callback-based, it is enough to promisify it with 1173<a href="util.html#util_util_promisify_original"><code>util.promisify()</code></a>, so it starts working with native promises.</p> 1174<p>If you need to keep using callback-based API, or your code assumes 1175a custom thenable implementation, use the <a href="#async_hooks_class_asyncresource"><code>AsyncResource</code></a> class 1176to associate the asynchronous operation with the correct execution context.</p></section> 1177 <!-- API END --> 1178 </div> 1179 </div> 1180 </div> 1181 <script> 1182 'use strict'; 1183 { 1184 const kCustomPreference = 'customDarkTheme'; 1185 const userSettings = sessionStorage.getItem(kCustomPreference); 1186 const themeToggleButton = document.getElementById('theme-toggle-btn'); 1187 if (userSettings === null && window.matchMedia) { 1188 const mq = window.matchMedia('(prefers-color-scheme: dark)'); 1189 if ('onchange' in mq) { 1190 function mqChangeListener(e) { 1191 document.body.classList.toggle('dark-mode', e.matches); 1192 } 1193 mq.addEventListener('change', mqChangeListener); 1194 if (themeToggleButton) { 1195 themeToggleButton.addEventListener('click', function() { 1196 mq.removeEventListener('change', mqChangeListener); 1197 }, { once: true }); 1198 } 1199 } 1200 if (mq.matches) { 1201 document.body.classList.add('dark-mode'); 1202 } 1203 } else if (userSettings === 'true') { 1204 document.body.classList.add('dark-mode'); 1205 } 1206 if (themeToggleButton) { 1207 themeToggleButton.hidden = false; 1208 themeToggleButton.addEventListener('click', function() { 1209 sessionStorage.setItem( 1210 kCustomPreference, 1211 document.body.classList.toggle('dark-mode') 1212 ); 1213 }); 1214 } 1215 } 1216 </script> 1217</body> 1218</html> 1219