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140<li><a href="#async_hooks_terminology">Terminology</a></li> 141<li><a href="#async_hooks_overview">Overview</a></li> 142<li><a href="#async_hooks_async_hooks_createhook_callbacks"><code>async_hooks.createHook(callbacks)</code></a> 143<ul> 144<li><a href="#async_hooks_error_handling">Error handling</a></li> 145<li><a href="#async_hooks_printing_in_asynchooks_callbacks">Printing in AsyncHooks callbacks</a></li> 146</ul> 147</li> 148<li><a href="#async_hooks_class_asynchook">Class: <code>AsyncHook</code></a> 149<ul> 150<li><a href="#async_hooks_asynchook_enable"><code>asyncHook.enable()</code></a></li> 151<li><a href="#async_hooks_asynchook_disable"><code>asyncHook.disable()</code></a></li> 152<li><a href="#async_hooks_hook_callbacks">Hook callbacks</a> 153<ul> 154<li><a href="#async_hooks_init_asyncid_type_triggerasyncid_resource"><code>init(asyncId, type, triggerAsyncId, resource)</code></a> 155<ul> 156<li><a href="#async_hooks_type"><code>type</code></a></li> 157<li><a href="#async_hooks_triggerasyncid"><code>triggerAsyncId</code></a></li> 158<li><a href="#async_hooks_resource"><code>resource</code></a></li> 159<li><a href="#async_hooks_asynchronous_context_example">Asynchronous context example</a></li> 160</ul> 161</li> 162<li><a href="#async_hooks_before_asyncid"><code>before(asyncId)</code></a></li> 163<li><a href="#async_hooks_after_asyncid"><code>after(asyncId)</code></a></li> 164<li><a href="#async_hooks_destroy_asyncid"><code>destroy(asyncId)</code></a></li> 165<li><a href="#async_hooks_promiseresolve_asyncid"><code>promiseResolve(asyncId)</code></a></li> 166</ul> 167</li> 168<li><a href="#async_hooks_async_hooks_executionasyncresource"><code>async_hooks.executionAsyncResource()</code></a></li> 169<li><a href="#async_hooks_async_hooks_executionasyncid"><code>async_hooks.executionAsyncId()</code></a></li> 170<li><a href="#async_hooks_async_hooks_triggerasyncid"><code>async_hooks.triggerAsyncId()</code></a></li> 171</ul> 172</li> 173<li><a href="#async_hooks_promise_execution_tracking">Promise execution tracking</a></li> 174<li><a href="#async_hooks_javascript_embedder_api">JavaScript embedder API</a> 175<ul> 176<li><a href="#async_hooks_class_asyncresource">Class: <code>AsyncResource</code></a> 177<ul> 178<li><a href="#async_hooks_new_asyncresource_type_options"><code>new AsyncResource(type[, options])</code></a></li> 179<li><a href="#async_hooks_static_method_asyncresource_bind_fn_type">Static method: <code>AsyncResource.bind(fn[, type])</code></a></li> 180<li><a href="#async_hooks_asyncresource_bind_fn"><code>asyncResource.bind(fn)</code></a></li> 181<li><a href="#async_hooks_asyncresource_runinasyncscope_fn_thisarg_args"><code>asyncResource.runInAsyncScope(fn[, thisArg, ...args])</code></a></li> 182<li><a href="#async_hooks_asyncresource_emitdestroy"><code>asyncResource.emitDestroy()</code></a></li> 183<li><a href="#async_hooks_asyncresource_asyncid"><code>asyncResource.asyncId()</code></a></li> 184<li><a href="#async_hooks_asyncresource_triggerasyncid"><code>asyncResource.triggerAsyncId()</code></a></li> 185</ul> 186</li> 187<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> 188<li><a href="#async_hooks_integrating_asyncresource_with_eventemitter">Integrating <code>AsyncResource</code> with <code>EventEmitter</code></a></li> 189</ul> 190</li> 191<li><a href="#async_hooks_class_asynclocalstorage">Class: <code>AsyncLocalStorage</code></a> 192<ul> 193<li><a href="#async_hooks_new_asynclocalstorage"><code>new AsyncLocalStorage()</code></a></li> 194<li><a href="#async_hooks_asynclocalstorage_disable"><code>asyncLocalStorage.disable()</code></a></li> 195<li><a href="#async_hooks_asynclocalstorage_getstore"><code>asyncLocalStorage.getStore()</code></a></li> 196<li><a href="#async_hooks_asynclocalstorage_enterwith_store"><code>asyncLocalStorage.enterWith(store)</code></a></li> 197<li><a href="#async_hooks_asynclocalstorage_run_store_callback_args"><code>asyncLocalStorage.run(store, callback[, ...args])</code></a></li> 198<li><a href="#async_hooks_asynclocalstorage_exit_callback_args"><code>asyncLocalStorage.exit(callback[, ...args])</code></a></li> 199<li><a href="#async_hooks_usage_with_async_await">Usage with <code>async/await</code></a></li> 200<li><a href="#async_hooks_troubleshooting">Troubleshooting</a></li> 201</ul> 202</li> 203</ul> 204</li> 205</ul></details> 206 207 <div id="apicontent"> 208 <h2>Async hooks<span><a class="mark" href="#async_hooks_async_hooks" id="async_hooks_async_hooks">#</a></span></h2> 209 210<p></p><div class="api_stability api_stability_1"><a href="documentation.html#documentation_stability_index">Stability: 1</a> - Experimental</div><p></p> 211<p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v14.19.1/lib/async_hooks.js">lib/async_hooks.js</a></p> 212<p>The <code>async_hooks</code> module provides an API to track asynchronous resources. It 213can be accessed using:</p> 214<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> 215<section><h3>Terminology<span><a class="mark" href="#async_hooks_terminology" id="async_hooks_terminology">#</a></span></h3> 216<p>An asynchronous resource represents an object with an associated callback. 217This callback may be called multiple times, for example, the <code>'connection'</code> 218event in <code>net.createServer()</code>, or just a single time like in <code>fs.open()</code>. 219A resource can also be closed before the callback is called. <code>AsyncHook</code> does 220not explicitly distinguish between these different cases but will represent them 221as the abstract concept that is a resource.</p> 222<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> 223interface, and each thread will use a new set of async IDs.</p> 224</section><section><h3>Overview<span><a class="mark" href="#async_hooks_overview" id="async_hooks_overview">#</a></span></h3> 225<p>Following is a simple overview of the public API.</p> 226<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>); 227 228<span class="hljs-comment">// Return the ID of the current execution context.</span> 229<span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 230 231<span class="hljs-comment">// Return the ID of the handle responsible for triggering the callback of the</span> 232<span class="hljs-comment">// current execution scope to call.</span> 233<span class="hljs-keyword">const</span> tid = async_hooks.<span class="hljs-title function_">triggerAsyncId</span>(); 234 235<span class="hljs-comment">// Create a new AsyncHook instance. All of these callbacks are optional.</span> 236<span class="hljs-keyword">const</span> asyncHook = 237 async_hooks.<span class="hljs-title function_">createHook</span>({ init, before, after, destroy, promiseResolve }); 238 239<span class="hljs-comment">// Allow callbacks of this AsyncHook instance to call. This is not an implicit</span> 240<span class="hljs-comment">// action after running the constructor, and must be explicitly run to begin</span> 241<span class="hljs-comment">// executing callbacks.</span> 242asyncHook.<span class="hljs-title function_">enable</span>(); 243 244<span class="hljs-comment">// Disable listening for new asynchronous events.</span> 245asyncHook.<span class="hljs-title function_">disable</span>(); 246 247<span class="hljs-comment">//</span> 248<span class="hljs-comment">// The following are the callbacks that can be passed to createHook().</span> 249<span class="hljs-comment">//</span> 250 251<span class="hljs-comment">// init is called during object construction. The resource may not have</span> 252<span class="hljs-comment">// completed construction when this callback runs, therefore all fields of the</span> 253<span class="hljs-comment">// resource referenced by "asyncId" may not have been populated.</span> 254<span class="hljs-keyword">function</span> <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { } 255 256<span class="hljs-comment">// Before is called just before the resource's callback is called. It can be</span> 257<span class="hljs-comment">// called 0-N times for handles (such as TCPWrap), and will be called exactly 1</span> 258<span class="hljs-comment">// time for requests (such as FSReqCallback).</span> 259<span class="hljs-keyword">function</span> <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { } 260 261<span class="hljs-comment">// After is called just after the resource's callback has finished.</span> 262<span class="hljs-keyword">function</span> <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { } 263 264<span class="hljs-comment">// Destroy is called when the resource is destroyed.</span> 265<span class="hljs-keyword">function</span> <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { } 266 267<span class="hljs-comment">// promiseResolve is called only for promise resources, when the</span> 268<span class="hljs-comment">// `resolve` function passed to the `Promise` constructor is invoked</span> 269<span class="hljs-comment">// (either directly or through other means of resolving a promise).</span> 270<span class="hljs-keyword">function</span> <span class="hljs-title function_">promiseResolve</span>(<span class="hljs-params">asyncId</span>) { }</code></pre> 271</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> 272<div class="api_metadata"> 273<span>Added in: v8.1.0</span> 274</div> 275<ul> 276<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 277<ul> 278<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> 279<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> 280<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> 281<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> 282<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> 283</ul> 284</li> 285<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> 286</ul> 287<p>Registers functions to be called for different lifetime events of each async 288operation.</p> 289<p>The callbacks <code>init()</code>/<code>before()</code>/<code>after()</code>/<code>destroy()</code> are called for the 290respective asynchronous event during a resource's lifetime.</p> 291<p>All callbacks are optional. For example, if only resource cleanup needs to 292be tracked, then only the <code>destroy</code> callback needs to be passed. The 293specifics of all functions that can be passed to <code>callbacks</code> is in the 294<a href="#async_hooks_hook_callbacks">Hook Callbacks</a> section.</p> 295<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>); 296 297<span class="hljs-keyword">const</span> asyncHook = async_hooks.<span class="hljs-title function_">createHook</span>({ 298 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { }, 299 <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { } 300});</code></pre> 301<p>The callbacks will be inherited via the prototype chain:</p> 302<pre><code class="language-js"><span class="hljs-keyword">class</span> <span class="hljs-title class_">MyAsyncCallbacks</span> { 303 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { } 304 <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) {} 305} 306 307<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> { 308 <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { } 309 <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { } 310} 311 312<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> 313<p>Because promises are asynchronous resources whose lifecycle is tracked 314via the async hooks mechanism, the <code>init()</code>, <code>before()</code>, <code>after()</code>, and 315<code>destroy()</code> callbacks <em>must not</em> be async functions that return promises.</p> 316<h4>Error handling<span><a class="mark" href="#async_hooks_error_handling" id="async_hooks_error_handling">#</a></span></h4> 317<p>If any <code>AsyncHook</code> callbacks throw, the application will print the stack trace 318and exit. The exit path does follow that of an uncaught exception, but 319all <code>'uncaughtException'</code> listeners are removed, thus forcing the process to 320exit. The <code>'exit'</code> callbacks will still be called unless the application is run 321with <code>--abort-on-uncaught-exception</code>, in which case a stack trace will be 322printed and the application exits, leaving a core file.</p> 323<p>The reason for this error handling behavior is that these callbacks are running 324at potentially volatile points in an object's lifetime, for example during 325class construction and destruction. Because of this, it is deemed necessary to 326bring down the process quickly in order to prevent an unintentional abort in the 327future. This is subject to change in the future if a comprehensive analysis is 328performed to ensure an exception can follow the normal control flow without 329unintentional side effects.</p> 330<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> 331<p>Because printing to the console is an asynchronous operation, <code>console.log()</code> 332will cause the AsyncHooks callbacks to be called. Using <code>console.log()</code> or 333similar asynchronous operations inside an AsyncHooks callback function will thus 334cause an infinite recursion. An easy solution to this when debugging is to use a 335synchronous logging operation such as <code>fs.writeFileSync(file, msg, flag)</code>. 336This will print to the file and will not invoke AsyncHooks recursively because 337it is synchronous.</p> 338<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>); 339<span class="hljs-keyword">const</span> util = <span class="hljs-built_in">require</span>(<span class="hljs-string">'util'</span>); 340 341<span class="hljs-keyword">function</span> <span class="hljs-title function_">debug</span>(<span class="hljs-params">...args</span>) { 342 <span class="hljs-comment">// Use a function like this one when debugging inside an AsyncHooks callback</span> 343 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> }); 344}</code></pre> 345<p>If an asynchronous operation is needed for logging, it is possible to keep 346track of what caused the asynchronous operation using the information 347provided by AsyncHooks itself. The logging should then be skipped when 348it was the logging itself that caused AsyncHooks callback to call. By 349doing this the otherwise infinite recursion is broken.</p> 350</section><section><h3>Class: <code>AsyncHook</code><span><a class="mark" href="#async_hooks_class_asynchook" id="async_hooks_class_asynchook">#</a></span></h3> 351<p>The class <code>AsyncHook</code> exposes an interface for tracking lifetime events 352of asynchronous operations.</p> 353<h4><code>asyncHook.enable()</code><span><a class="mark" href="#async_hooks_asynchook_enable" id="async_hooks_asynchook_enable">#</a></span></h4> 354<ul> 355<li>Returns: <a href="async_hooks.html#async_hooks_async_hooks_createhook_callbacks" class="type"><AsyncHook></a> A reference to <code>asyncHook</code>.</li> 356</ul> 357<p>Enable the callbacks for a given <code>AsyncHook</code> instance. If no callbacks are 358provided, enabling is a no-op.</p> 359<p>The <code>AsyncHook</code> instance is disabled by default. If the <code>AsyncHook</code> instance 360should be enabled immediately after creation, the following pattern can be used.</p> 361<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>); 362 363<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> 364<h4><code>asyncHook.disable()</code><span><a class="mark" href="#async_hooks_asynchook_disable" id="async_hooks_asynchook_disable">#</a></span></h4> 365<ul> 366<li>Returns: <a href="async_hooks.html#async_hooks_async_hooks_createhook_callbacks" class="type"><AsyncHook></a> A reference to <code>asyncHook</code>.</li> 367</ul> 368<p>Disable the callbacks for a given <code>AsyncHook</code> instance from the global pool of 369<code>AsyncHook</code> callbacks to be executed. Once a hook has been disabled it will not 370be called again until enabled.</p> 371<p>For API consistency <code>disable()</code> also returns the <code>AsyncHook</code> instance.</p> 372<h4>Hook callbacks<span><a class="mark" href="#async_hooks_hook_callbacks" id="async_hooks_hook_callbacks">#</a></span></h4> 373<p>Key events in the lifetime of asynchronous events have been categorized into 374four areas: instantiation, before/after the callback is called, and when the 375instance is destroyed.</p> 376<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> 377<ul> 378<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> 379<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> 380<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 381execution context this async resource was created.</li> 382<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 383operation, needs to be released during <em>destroy</em>.</li> 384</ul> 385<p>Called when a class is constructed that has the <em>possibility</em> to emit an 386asynchronous event. This <em>does not</em> mean the instance must call 387<code>before</code>/<code>after</code> before <code>destroy</code> is called, only that the possibility 388exists.</p> 389<p>This behavior can be observed by doing something like opening a resource then 390closing it before the resource can be used. The following snippet demonstrates 391this.</p> 392<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>(); }); 393<span class="hljs-comment">// OR</span> 394<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> 395<p>Every new resource is assigned an ID that is unique within the scope of the 396current Node.js instance.</p> 397<h6><code>type</code><span><a class="mark" href="#async_hooks_type" id="async_hooks_type">#</a></span></h6> 398<p>The <code>type</code> is a string identifying the type of resource that caused 399<code>init</code> to be called. Generally, it will correspond to the name of the 400resource's constructor.</p> 401<pre><code class="language-text">FSEVENTWRAP, FSREQCALLBACK, GETADDRINFOREQWRAP, GETNAMEINFOREQWRAP, HTTPINCOMINGMESSAGE, 402HTTPCLIENTREQUEST, JSSTREAM, PIPECONNECTWRAP, PIPEWRAP, PROCESSWRAP, QUERYWRAP, 403SHUTDOWNWRAP, SIGNALWRAP, STATWATCHER, TCPCONNECTWRAP, TCPSERVERWRAP, TCPWRAP, 404TTYWRAP, UDPSENDWRAP, UDPWRAP, WRITEWRAP, ZLIB, SSLCONNECTION, PBKDF2REQUEST, 405RANDOMBYTESREQUEST, TLSWRAP, Microtask, Timeout, Immediate, TickObject</code></pre> 406<p>There is also the <code>PROMISE</code> resource type, which is used to track <code>Promise</code> 407instances and asynchronous work scheduled by them.</p> 408<p>Users are able to define their own <code>type</code> when using the public embedder API.</p> 409<p>It is possible to have type name collisions. Embedders are encouraged to use 410unique prefixes, such as the npm package name, to prevent collisions when 411listening to the hooks.</p> 412<h6><code>triggerAsyncId</code><span><a class="mark" href="#async_hooks_triggerasyncid" id="async_hooks_triggerasyncid">#</a></span></h6> 413<p><code>triggerAsyncId</code> is the <code>asyncId</code> of the resource that caused (or "triggered") 414the new resource to initialize and that caused <code>init</code> to call. This is different 415from <code>async_hooks.executionAsyncId()</code> that only shows <em>when</em> a resource was 416created, while <code>triggerAsyncId</code> shows <em>why</em> a resource was created.</p> 417<p>The following is a simple demonstration of <code>triggerAsyncId</code>:</p> 418<pre><code class="language-js"><span class="hljs-keyword">const</span> { fd } = process.<span class="hljs-property">stdout</span>; 419 420async_hooks.<span class="hljs-title function_">createHook</span>({ 421 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId</span>) { 422 <span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 423 fs.<span class="hljs-title function_">writeSync</span>( 424 fd, 425 <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>); 426 } 427}).<span class="hljs-title function_">enable</span>(); 428 429net.<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> 430<p>Output when hitting the server with <code>nc localhost 8080</code>:</p> 431<pre><code class="language-console">TCPSERVERWRAP(5): trigger: 1 execution: 1 432TCPWRAP(7): trigger: 5 execution: 0</code></pre> 433<p>The <code>TCPSERVERWRAP</code> is the server which receives the connections.</p> 434<p>The <code>TCPWRAP</code> is the new connection from the client. When a new 435connection is made, the <code>TCPWrap</code> instance is immediately constructed. This 436happens outside of any JavaScript stack. (An <code>executionAsyncId()</code> of <code>0</code> means 437that it is being executed from C++ with no JavaScript stack above it.) With only 438that information, it would be impossible to link resources together in 439terms of what caused them to be created, so <code>triggerAsyncId</code> is given the task 440of propagating what resource is responsible for the new resource's existence.</p> 441<h6><code>resource</code><span><a class="mark" href="#async_hooks_resource" id="async_hooks_resource">#</a></span></h6> 442<p><code>resource</code> is an object that represents the actual async resource that has 443been initialized. This can contain useful information that can vary based on 444the value of <code>type</code>. For instance, for the <code>GETADDRINFOREQWRAP</code> resource type, 445<code>resource</code> provides the host name used when looking up the IP address for the 446host in <code>net.Server.listen()</code>. The API for accessing this information is 447not supported, but using the Embedder API, users can provide 448and document their own resource objects. For example, such a resource object 449could contain the SQL query being executed.</p> 450<p>In some cases the resource object is reused for performance reasons, it is 451thus not safe to use it as a key in a <code>WeakMap</code> or add properties to it.</p> 452<h6>Asynchronous context example<span><a class="mark" href="#async_hooks_asynchronous_context_example" id="async_hooks_asynchronous_context_example">#</a></span></h6> 453<p>The following is an example with additional information about the calls to 454<code>init</code> between the <code>before</code> and <code>after</code> calls, specifically what the 455callback to <code>listen()</code> will look like. The output formatting is slightly more 456elaborate to make calling context easier to see.</p> 457<pre><code class="language-js"><span class="hljs-keyword">const</span> { fd } = process.<span class="hljs-property">stdout</span>; 458 459<span class="hljs-keyword">let</span> indent = <span class="hljs-number">0</span>; 460async_hooks.<span class="hljs-title function_">createHook</span>({ 461 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId</span>) { 462 <span class="hljs-keyword">const</span> eid = async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 463 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 464 fs.<span class="hljs-title function_">writeSync</span>( 465 fd, 466 <span class="hljs-string">`<span class="hljs-subst">${indentStr}</span><span class="hljs-subst">${type}</span>(<span class="hljs-subst">${asyncId}</span>):`</span> + 467 <span class="hljs-string">` trigger: <span class="hljs-subst">${triggerAsyncId}</span> execution: <span class="hljs-subst">${eid}</span>\n`</span>); 468 }, 469 <span class="hljs-title function_">before</span>(<span class="hljs-params">asyncId</span>) { 470 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 471 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>); 472 indent += <span class="hljs-number">2</span>; 473 }, 474 <span class="hljs-title function_">after</span>(<span class="hljs-params">asyncId</span>) { 475 indent -= <span class="hljs-number">2</span>; 476 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 477 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>); 478 }, 479 <span class="hljs-title function_">destroy</span>(<span class="hljs-params">asyncId</span>) { 480 <span class="hljs-keyword">const</span> indentStr = <span class="hljs-string">' '</span>.<span class="hljs-title function_">repeat</span>(indent); 481 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>); 482 }, 483}).<span class="hljs-title function_">enable</span>(); 484 485net.<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> { 486 <span class="hljs-comment">// Let's wait 10ms before logging the server started.</span> 487 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { 488 <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>()); 489 }, <span class="hljs-number">10</span>); 490});</code></pre> 491<p>Output from only starting the server:</p> 492<pre><code class="language-console">TCPSERVERWRAP(5): trigger: 1 execution: 1 493TickObject(6): trigger: 5 execution: 1 494before: 6 495 Timeout(7): trigger: 6 execution: 6 496after: 6 497destroy: 6 498before: 7 499<span class="hljs-meta">></span><span class="language-bash">>> 7</span> 500 TickObject(8): trigger: 7 execution: 7 501after: 7 502before: 8 503after: 8</code></pre> 504<p>As illustrated in the example, <code>executionAsyncId()</code> and <code>execution</code> each specify 505the value of the current execution context; which is delineated by calls to 506<code>before</code> and <code>after</code>.</p> 507<p>Only using <code>execution</code> to graph resource allocation results in the following:</p> 508<pre><code class="language-console"> root(1) 509 ^ 510 | 511TickObject(6) 512 ^ 513 | 514 Timeout(7)</code></pre> 515<p>The <code>TCPSERVERWRAP</code> is not part of this graph, even though it was the reason for 516<code>console.log()</code> being called. This is because binding to a port without a host 517name is a <em>synchronous</em> operation, but to maintain a completely asynchronous 518API the user's callback is placed in a <code>process.nextTick()</code>. Which is why 519<code>TickObject</code> is present in the output and is a 'parent' for <code>.listen()</code> 520callback.</p> 521<p>The graph only shows <em>when</em> a resource was created, not <em>why</em>, so to track 522the <em>why</em> use <code>triggerAsyncId</code>. Which can be represented with the following 523graph:</p> 524<pre><code class="language-console"> bootstrap(1) 525 | 526 Ë… 527TCPSERVERWRAP(5) 528 | 529 Ë… 530 TickObject(6) 531 | 532 Ë… 533 Timeout(7)</code></pre> 534<h5><code>before(asyncId)</code><span><a class="mark" href="#async_hooks_before_asyncid" id="async_hooks_before_asyncid">#</a></span></h5> 535<ul> 536<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 537</ul> 538<p>When an asynchronous operation is initiated (such as a TCP server receiving a 539new connection) or completes (such as writing data to disk) a callback is 540called to notify the user. The <code>before</code> callback is called just before said 541callback is executed. <code>asyncId</code> is the unique identifier assigned to the 542resource about to execute the callback.</p> 543<p>The <code>before</code> callback will be called 0 to N times. The <code>before</code> callback 544will typically be called 0 times if the asynchronous operation was cancelled 545or, for example, if no connections are received by a TCP server. Persistent 546asynchronous resources like a TCP server will typically call the <code>before</code> 547callback multiple times, while other operations like <code>fs.open()</code> will call 548it only once.</p> 549<h5><code>after(asyncId)</code><span><a class="mark" href="#async_hooks_after_asyncid" id="async_hooks_after_asyncid">#</a></span></h5> 550<ul> 551<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 552</ul> 553<p>Called immediately after the callback specified in <code>before</code> is completed.</p> 554<p>If an uncaught exception occurs during execution of the callback, then <code>after</code> 555will run <em>after</em> the <code>'uncaughtException'</code> event is emitted or a <code>domain</code>'s 556handler runs.</p> 557<h5><code>destroy(asyncId)</code><span><a class="mark" href="#async_hooks_destroy_asyncid" id="async_hooks_destroy_asyncid">#</a></span></h5> 558<ul> 559<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 560</ul> 561<p>Called after the resource corresponding to <code>asyncId</code> is destroyed. It is also 562called asynchronously from the embedder API <code>emitDestroy()</code>.</p> 563<p>Some resources depend on garbage collection for cleanup, so if a reference is 564made to the <code>resource</code> object passed to <code>init</code> it is possible that <code>destroy</code> 565will never be called, causing a memory leak in the application. If the resource 566does not depend on garbage collection, then this will not be an issue.</p> 567<h5><code>promiseResolve(asyncId)</code><span><a class="mark" href="#async_hooks_promiseresolve_asyncid" id="async_hooks_promiseresolve_asyncid">#</a></span></h5> 568<div class="api_metadata"> 569<span>Added in: v8.6.0</span> 570</div> 571<ul> 572<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a></li> 573</ul> 574<p>Called when the <code>resolve</code> function passed to the <code>Promise</code> constructor is 575invoked (either directly or through other means of resolving a promise).</p> 576<p><code>resolve()</code> does not do any observable synchronous work.</p> 577<p>The <code>Promise</code> is not necessarily fulfilled or rejected at this point if the 578<code>Promise</code> was resolved by assuming the state of another <code>Promise</code>.</p> 579<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> 580<p>calls the following callbacks:</p> 581<pre><code class="language-text">init for PROMISE with id 5, trigger id: 1 582 promise resolve 5 # corresponds to resolve(true) 583init for PROMISE with id 6, trigger id: 5 # the Promise returned by then() 584 before 6 # the then() callback is entered 585 promise resolve 6 # the then() callback resolves the promise by returning 586 after 6</code></pre> 587<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> 588<div class="api_metadata"> 589<span>Added in: v13.9.0</span> 590</div> 591<ul> 592<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. 593Useful to store data within the resource.</li> 594</ul> 595<p>Resource objects returned by <code>executionAsyncResource()</code> are most often internal 596Node.js handle objects with undocumented APIs. Using any functions or properties 597on the object is likely to crash your application and should be avoided.</p> 598<p>Using <code>executionAsyncResource()</code> in the top-level execution context will 599return an empty object as there is no handle or request object to use, 600but having an object representing the top-level can be helpful.</p> 601<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>); 602<span class="hljs-keyword">const</span> { executionAsyncId, executionAsyncResource } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'async_hooks'</span>); 603 604<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> 605<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> { 606 <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> 607});</code></pre> 608<p>This can be used to implement continuation local storage without the 609use of a tracking <code>Map</code> to store the metadata:</p> 610<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>); 611<span class="hljs-keyword">const</span> { 612 executionAsyncId, 613 executionAsyncResource, 614 createHook 615} = <span class="hljs-built_in">require</span>(<span class="hljs-string">'async_hooks'</span>); 616<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> 617 618<span class="hljs-title function_">createHook</span>({ 619 <span class="hljs-title function_">init</span>(<span class="hljs-params">asyncId, type, triggerAsyncId, resource</span>) { 620 <span class="hljs-keyword">const</span> cr = <span class="hljs-title function_">executionAsyncResource</span>(); 621 <span class="hljs-keyword">if</span> (cr) { 622 resource[sym] = cr[sym]; 623 } 624 } 625}).<span class="hljs-title function_">enable</span>(); 626 627<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> { 628 <span class="hljs-title function_">executionAsyncResource</span>()[sym] = { <span class="hljs-attr">state</span>: req.<span class="hljs-property">url</span> }; 629 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-keyword">function</span>(<span class="hljs-params"></span>) { 630 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])); 631 }, <span class="hljs-number">100</span>); 632}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">3000</span>);</code></pre> 633<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> 634<div class="api_metadata"> 635<details class="changelog"><summary>History</summary> 636<table> 637<tbody><tr><th>Version</th><th>Changes</th></tr> 638<tr><td>v8.2.0</td> 639<td><p>Renamed from <code>currentId</code>.</p></td></tr> 640<tr><td>v8.1.0</td> 641<td><p><span>Added in: v8.1.0</span></p></td></tr> 642</tbody></table> 643</details> 644</div> 645<ul> 646<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 647track when something calls.</li> 648</ul> 649<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>); 650 651<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> 652fs.<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> { 653 <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> 654});</code></pre> 655<p>The ID returned from <code>executionAsyncId()</code> is related to execution timing, not 656causality (which is covered by <code>triggerAsyncId()</code>):</p> 657<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> { 658 <span class="hljs-comment">// Returns the ID of the server, not of the new connection, because the</span> 659 <span class="hljs-comment">// callback runs in the execution scope of the server's MakeCallback().</span> 660 async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 661 662}).<span class="hljs-title function_">listen</span>(port, <span class="hljs-function">() =></span> { 663 <span class="hljs-comment">// Returns the ID of a TickObject (process.nextTick()) because all</span> 664 <span class="hljs-comment">// callbacks passed to .listen() are wrapped in a nextTick().</span> 665 async_hooks.<span class="hljs-title function_">executionAsyncId</span>(); 666});</code></pre> 667<p>Promise contexts may not get precise <code>executionAsyncIds</code> by default. 668See the section on <a href="#async_hooks_promise_execution_tracking">promise execution tracking</a>.</p> 669<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> 670<ul> 671<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 672that is currently being executed.</li> 673</ul> 674<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> { 675 <span class="hljs-comment">// The resource that caused (or triggered) this callback to be called</span> 676 <span class="hljs-comment">// was that of the new connection. Thus the return value of triggerAsyncId()</span> 677 <span class="hljs-comment">// is the asyncId of "conn".</span> 678 async_hooks.<span class="hljs-title function_">triggerAsyncId</span>(); 679 680}).<span class="hljs-title function_">listen</span>(port, <span class="hljs-function">() =></span> { 681 <span class="hljs-comment">// Even though all callbacks passed to .listen() are wrapped in a nextTick()</span> 682 <span class="hljs-comment">// the callback itself exists because the call to the server's .listen()</span> 683 <span class="hljs-comment">// was made. So the return value would be the ID of the server.</span> 684 async_hooks.<span class="hljs-title function_">triggerAsyncId</span>(); 685});</code></pre> 686<p>Promise contexts may not get valid <code>triggerAsyncId</code>s by default. See 687the section on <a href="#async_hooks_promise_execution_tracking">promise execution tracking</a>.</p> 688</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> 689<p>By default, promise executions are not assigned <code>asyncId</code>s due to the relatively 690expensive nature of the <a href="https://docs.google.com/document/d/1rda3yKGHimKIhg5YeoAmCOtyURgsbTH_qaYR79FELlk/edit">promise introspection API</a> provided by 691V8. This means that programs using promises or <code>async</code>/<code>await</code> will not get 692correct execution and trigger ids for promise callback contexts by default.</p> 693<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>); 694<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> { 695 <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>); 696}); 697<span class="hljs-comment">// produces:</span> 698<span class="hljs-comment">// eid 1 tid 0</span></code></pre> 699<p>Observe that the <code>then()</code> callback claims to have executed in the context of the 700outer scope even though there was an asynchronous hop involved. Also, 701the <code>triggerAsyncId</code> value is <code>0</code>, which means that we are missing context about 702the resource that caused (triggered) the <code>then()</code> callback to be executed.</p> 703<p>Installing async hooks via <code>async_hooks.createHook</code> enables promise execution 704tracking:</p> 705<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>); 706ah.<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> 707<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> { 708 <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>); 709}); 710<span class="hljs-comment">// produces:</span> 711<span class="hljs-comment">// eid 7 tid 6</span></code></pre> 712<p>In this example, adding any actual hook function enabled the tracking of 713promises. There are two promises in the example above; the promise created by 714<code>Promise.resolve()</code> and the promise returned by the call to <code>then()</code>. In the 715example above, the first promise got the <code>asyncId</code> <code>6</code> and the latter got 716<code>asyncId</code> <code>7</code>. During the execution of the <code>then()</code> callback, we are executing 717in the context of promise with <code>asyncId</code> <code>7</code>. This promise was triggered by 718async resource <code>6</code>.</p> 719<p>Another subtlety with promises is that <code>before</code> and <code>after</code> callbacks are run 720only on chained promises. That means promises not created by <code>then()</code>/<code>catch()</code> 721will not have the <code>before</code> and <code>after</code> callbacks fired on them. For more details 722see the details of the V8 <a href="https://docs.google.com/document/d/1rda3yKGHimKIhg5YeoAmCOtyURgsbTH_qaYR79FELlk/edit">PromiseHooks</a> API.</p> 723</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> 724<p>Library developers that handle their own asynchronous resources performing tasks 725like I/O, connection pooling, or managing callback queues may use the 726<code>AsyncResource</code> JavaScript API so that all the appropriate callbacks are called.</p> 727<h4>Class: <code>AsyncResource</code><span><a class="mark" href="#async_hooks_class_asyncresource" id="async_hooks_class_asyncresource">#</a></span></h4> 728<p>The class <code>AsyncResource</code> is designed to be extended by the embedder's async 729resources. Using this, users can easily trigger the lifetime events of their 730own resources.</p> 731<p>The <code>init</code> hook will trigger when an <code>AsyncResource</code> is instantiated.</p> 732<p>The following is an overview of the <code>AsyncResource</code> API.</p> 733<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>); 734 735<span class="hljs-comment">// AsyncResource() is meant to be extended. Instantiating a</span> 736<span class="hljs-comment">// new AsyncResource() also triggers init. If triggerAsyncId is omitted then</span> 737<span class="hljs-comment">// async_hook.executionAsyncId() is used.</span> 738<span class="hljs-keyword">const</span> asyncResource = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncResource</span>( 739 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> } 740); 741 742<span class="hljs-comment">// Run a function in the execution context of the resource. This will</span> 743<span class="hljs-comment">// * establish the context of the resource</span> 744<span class="hljs-comment">// * trigger the AsyncHooks before callbacks</span> 745<span class="hljs-comment">// * call the provided function `fn` with the supplied arguments</span> 746<span class="hljs-comment">// * trigger the AsyncHooks after callbacks</span> 747<span class="hljs-comment">// * restore the original execution context</span> 748asyncResource.<span class="hljs-title function_">runInAsyncScope</span>(fn, thisArg, ...args); 749 750<span class="hljs-comment">// Call AsyncHooks destroy callbacks.</span> 751asyncResource.<span class="hljs-title function_">emitDestroy</span>(); 752 753<span class="hljs-comment">// Return the unique ID assigned to the AsyncResource instance.</span> 754asyncResource.<span class="hljs-title function_">asyncId</span>(); 755 756<span class="hljs-comment">// Return the trigger ID for the AsyncResource instance.</span> 757asyncResource.<span class="hljs-title function_">triggerAsyncId</span>();</code></pre> 758<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> 759<ul> 760<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> 761<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> 762<ul> 763<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 764async event. <strong>Default:</strong> <code>executionAsyncId()</code>.</li> 765<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> 766when the object is garbage collected. This usually does not need to be set 767(even if <code>emitDestroy</code> is called manually), unless the resource's <code>asyncId</code> 768is retrieved and the sensitive API's <code>emitDestroy</code> is called with it. 769When set to <code>false</code>, the <code>emitDestroy</code> call on garbage collection 770will only take place if there is at least one active <code>destroy</code> hook. 771<strong>Default:</strong> <code>false</code>.</li> 772</ul> 773</li> 774</ul> 775<p>Example usage:</p> 776<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> { 777 <span class="hljs-title function_">constructor</span>(<span class="hljs-params">db</span>) { 778 <span class="hljs-variable language_">super</span>(<span class="hljs-string">'DBQuery'</span>); 779 <span class="hljs-variable language_">this</span>.<span class="hljs-property">db</span> = db; 780 } 781 782 <span class="hljs-title function_">getInfo</span>(<span class="hljs-params">query, callback</span>) { 783 <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> { 784 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">runInAsyncScope</span>(callback, <span class="hljs-literal">null</span>, err, data); 785 }); 786 } 787 788 <span class="hljs-title function_">close</span>(<span class="hljs-params"></span>) { 789 <span class="hljs-variable language_">this</span>.<span class="hljs-property">db</span> = <span class="hljs-literal">null</span>; 790 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emitDestroy</span>(); 791 } 792}</code></pre> 793<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> 794<div class="api_metadata"> 795<span>Added in: v14.8.0</span> 796</div> 797<ul> 798<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> 799<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 800<code>AsyncResource</code>.</li> 801</ul> 802<p>Binds the given function to the current execution context.</p> 803<p>The returned function will have an <code>asyncResource</code> property referencing 804the <code>AsyncResource</code> to which the function is bound.</p> 805<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> 806<div class="api_metadata"> 807<span>Added in: v14.8.0</span> 808</div> 809<ul> 810<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> 811</ul> 812<p>Binds the given function to execute to this <code>AsyncResource</code>'s scope.</p> 813<p>The returned function will have an <code>asyncResource</code> property referencing 814the <code>AsyncResource</code> to which the function is bound.</p> 815<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> 816<div class="api_metadata"> 817<span>Added in: v9.6.0</span> 818</div> 819<ul> 820<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 821resource.</li> 822<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> 823<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> 824</ul> 825<p>Call the provided function with the provided arguments in the execution context 826of the async resource. This will establish the context, trigger the AsyncHooks 827before callbacks, call the function, trigger the AsyncHooks after callbacks, and 828then restore the original execution context.</p> 829<h5><code>asyncResource.emitDestroy()</code><span><a class="mark" href="#async_hooks_asyncresource_emitdestroy" id="async_hooks_asyncresource_emitdestroy">#</a></span></h5> 830<ul> 831<li>Returns: <a href="async_hooks.html#async_hooks_class_asyncresource" class="type"><AsyncResource></a> A reference to <code>asyncResource</code>.</li> 832</ul> 833<p>Call all <code>destroy</code> hooks. This should only ever be called once. An error will 834be thrown if it is called more than once. This <strong>must</strong> be manually called. If 835the resource is left to be collected by the GC then the <code>destroy</code> hooks will 836never be called.</p> 837<h5><code>asyncResource.asyncId()</code><span><a class="mark" href="#async_hooks_asyncresource_asyncid" id="async_hooks_asyncresource_asyncid">#</a></span></h5> 838<ul> 839<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> 840</ul> 841<h5><code>asyncResource.triggerAsyncId()</code><span><a class="mark" href="#async_hooks_asyncresource_triggerasyncid" id="async_hooks_asyncresource_triggerasyncid">#</a></span></h5> 842<ul> 843<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 844<code>AsyncResource</code> constructor.</li> 845</ul> 846<p><a id="async-resource-worker-pool"></a></p> 847<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> 848<p>The following example shows how to use the <code>AsyncResource</code> class to properly 849provide async tracking for a <a href="worker_threads.html#worker_threads_class_worker"><code>Worker</code></a> pool. Other resource pools, such as 850database connection pools, can follow a similar model.</p> 851<p>Assuming that the task is adding two numbers, using a file named 852<code>task_processor.js</code> with the following content:</p> 853<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>); 854parentPort.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">task</span>) =></span> { 855 parentPort.<span class="hljs-title function_">postMessage</span>(task.<span class="hljs-property">a</span> + task.<span class="hljs-property">b</span>); 856});</code></pre> 857<p>a Worker pool around it could use the following structure:</p> 858<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>); 859<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>); 860<span class="hljs-keyword">const</span> path = <span class="hljs-built_in">require</span>(<span class="hljs-string">'path'</span>); 861<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>); 862 863<span class="hljs-keyword">const</span> kTaskInfo = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'kTaskInfo'</span>); 864<span class="hljs-keyword">const</span> kWorkerFreedEvent = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'kWorkerFreedEvent'</span>); 865 866<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> { 867 <span class="hljs-title function_">constructor</span>(<span class="hljs-params">callback</span>) { 868 <span class="hljs-variable language_">super</span>(<span class="hljs-string">'WorkerPoolTaskInfo'</span>); 869 <span class="hljs-variable language_">this</span>.<span class="hljs-property">callback</span> = callback; 870 } 871 872 <span class="hljs-title function_">done</span>(<span class="hljs-params">err, result</span>) { 873 <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); 874 <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> 875 } 876} 877 878<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> { 879 <span class="hljs-title function_">constructor</span>(<span class="hljs-params">numThreads</span>) { 880 <span class="hljs-variable language_">super</span>(); 881 <span class="hljs-variable language_">this</span>.<span class="hljs-property">numThreads</span> = numThreads; 882 <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span> = []; 883 <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span> = []; 884 <span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span> = []; 885 886 <span class="hljs-keyword">for</span> (<span class="hljs-keyword">let</span> i = <span class="hljs-number">0</span>; i < numThreads; i++) 887 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">addNewWorker</span>(); 888 889 <span class="hljs-comment">// Any time the kWorkerFreedEvent is emitted, dispatch</span> 890 <span class="hljs-comment">// the next task pending in the queue, if any.</span> 891 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">on</span>(kWorkerFreedEvent, <span class="hljs-function">() =></span> { 892 <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>) { 893 <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>(); 894 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">runTask</span>(task, callback); 895 } 896 }); 897 } 898 899 <span class="hljs-title function_">addNewWorker</span>(<span class="hljs-params"></span>) { 900 <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>)); 901 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> { 902 <span class="hljs-comment">// In case of success: Call the callback that was passed to `runTask`,</span> 903 <span class="hljs-comment">// remove the `TaskInfo` associated with the Worker, and mark it as free</span> 904 <span class="hljs-comment">// again.</span> 905 worker[kTaskInfo].<span class="hljs-title function_">done</span>(<span class="hljs-literal">null</span>, result); 906 worker[kTaskInfo] = <span class="hljs-literal">null</span>; 907 <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-title function_">push</span>(worker); 908 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(kWorkerFreedEvent); 909 }); 910 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> { 911 <span class="hljs-comment">// In case of an uncaught exception: Call the callback that was passed to</span> 912 <span class="hljs-comment">// `runTask` with the error.</span> 913 <span class="hljs-keyword">if</span> (worker[kTaskInfo]) 914 worker[kTaskInfo].<span class="hljs-title function_">done</span>(err, <span class="hljs-literal">null</span>); 915 <span class="hljs-keyword">else</span> 916 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'error'</span>, err); 917 <span class="hljs-comment">// Remove the worker from the list and start a new Worker to replace the</span> 918 <span class="hljs-comment">// current one.</span> 919 <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>); 920 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">addNewWorker</span>(); 921 }); 922 <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>.<span class="hljs-title function_">push</span>(worker); 923 <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-title function_">push</span>(worker); 924 <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(kWorkerFreedEvent); 925 } 926 927 <span class="hljs-title function_">runTask</span>(<span class="hljs-params">task, callback</span>) { 928 <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>) { 929 <span class="hljs-comment">// No free threads, wait until a worker thread becomes free.</span> 930 <span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span>.<span class="hljs-title function_">push</span>({ task, callback }); 931 <span class="hljs-keyword">return</span>; 932 } 933 934 <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>(); 935 worker[kTaskInfo] = <span class="hljs-keyword">new</span> <span class="hljs-title class_">WorkerPoolTaskInfo</span>(callback); 936 worker.<span class="hljs-title function_">postMessage</span>(task); 937 } 938 939 <span class="hljs-title function_">close</span>(<span class="hljs-params"></span>) { 940 <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>(); 941 } 942} 943 944<span class="hljs-variable language_">module</span>.<span class="hljs-property">exports</span> = <span class="hljs-title class_">WorkerPool</span>;</code></pre> 945<p>Without the explicit tracking added by the <code>WorkerPoolTaskInfo</code> objects, 946it would appear that the callbacks are associated with the individual <code>Worker</code> 947objects. However, the creation of the <code>Worker</code>s is not associated with the 948creation of the tasks and does not provide information about when tasks 949were scheduled.</p> 950<p>This pool could be used as follows:</p> 951<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>); 952<span class="hljs-keyword">const</span> os = <span class="hljs-built_in">require</span>(<span class="hljs-string">'os'</span>); 953 954<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>); 955 956<span class="hljs-keyword">let</span> finished = <span class="hljs-number">0</span>; 957<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++) { 958 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> { 959 <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(i, err, result); 960 <span class="hljs-keyword">if</span> (++finished === <span class="hljs-number">10</span>) 961 pool.<span class="hljs-title function_">close</span>(); 962 }); 963}</code></pre> 964<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> 965<p>Event listeners triggered by an <a href="events.html#events_class_eventemitter"><code>EventEmitter</code></a> may be run in a different 966execution context than the one that was active when <code>eventEmitter.on()</code> was 967called.</p> 968<p>The following example shows how to use the <code>AsyncResource</code> class to properly 969associate an event listener with the correct execution context. The same 970approach can be applied to a <a href="stream.html#stream_stream"><code>Stream</code></a> or a similar event-driven class.</p> 971<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>); 972<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>); 973 974<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> { 975 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> { 976 <span class="hljs-comment">// Execution context is bound to the current outer scope.</span> 977 })); 978 req.<span class="hljs-title function_">on</span>(<span class="hljs-string">'close'</span>, <span class="hljs-function">() =></span> { 979 <span class="hljs-comment">// Execution context is bound to the scope that caused 'close' to emit.</span> 980 }); 981 res.<span class="hljs-title function_">end</span>(); 982}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">3000</span>);</code></pre> 983</section><section><h3>Class: <code>AsyncLocalStorage</code><span><a class="mark" href="#async_hooks_class_asynclocalstorage" id="async_hooks_class_asynclocalstorage">#</a></span></h3> 984<div class="api_metadata"> 985<span>Added in: v13.10.0</span> 986</div> 987<p>This class is used to create asynchronous state within callbacks and promise 988chains. It allows storing data throughout the lifetime of a web request 989or any other asynchronous duration. It is similar to thread-local storage 990in other languages.</p> 991<p>While you can create your own implementation on top of the <code>async_hooks</code> module, 992<code>AsyncLocalStorage</code> should be preferred as it is a performant and memory safe 993implementation that involves significant optimizations that are non-obvious to 994implement.</p> 995<p>The following example uses <code>AsyncLocalStorage</code> to build a simple logger 996that assigns IDs to incoming HTTP requests and includes them in messages 997logged within each request.</p> 998<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>); 999<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>); 1000 1001<span class="hljs-keyword">const</span> asyncLocalStorage = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncLocalStorage</span>(); 1002 1003<span class="hljs-keyword">function</span> <span class="hljs-title function_">logWithId</span>(<span class="hljs-params">msg</span>) { 1004 <span class="hljs-keyword">const</span> id = asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); 1005 <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); 1006} 1007 1008<span class="hljs-keyword">let</span> idSeq = <span class="hljs-number">0</span>; 1009http.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> { 1010 asyncLocalStorage.<span class="hljs-title function_">run</span>(idSeq++, <span class="hljs-function">() =></span> { 1011 <span class="hljs-title function_">logWithId</span>(<span class="hljs-string">'start'</span>); 1012 <span class="hljs-comment">// Imagine any chain of async operations here</span> 1013 <span class="hljs-title function_">setImmediate</span>(<span class="hljs-function">() =></span> { 1014 <span class="hljs-title function_">logWithId</span>(<span class="hljs-string">'finish'</span>); 1015 res.<span class="hljs-title function_">end</span>(); 1016 }); 1017 }); 1018}).<span class="hljs-title function_">listen</span>(<span class="hljs-number">8080</span>); 1019 1020http.<span class="hljs-title function_">get</span>(<span class="hljs-string">'http://localhost:8080'</span>); 1021http.<span class="hljs-title function_">get</span>(<span class="hljs-string">'http://localhost:8080'</span>); 1022<span class="hljs-comment">// Prints:</span> 1023<span class="hljs-comment">// 0: start</span> 1024<span class="hljs-comment">// 1: start</span> 1025<span class="hljs-comment">// 0: finish</span> 1026<span class="hljs-comment">// 1: finish</span></code></pre> 1027<p>When having multiple instances of <code>AsyncLocalStorage</code>, they are independent 1028from each other. It is safe to instantiate this class multiple times.</p> 1029<h4><code>new AsyncLocalStorage()</code><span><a class="mark" href="#async_hooks_new_asynclocalstorage" id="async_hooks_new_asynclocalstorage">#</a></span></h4> 1030<div class="api_metadata"> 1031<span>Added in: v13.10.0</span> 1032</div> 1033<p>Creates a new instance of <code>AsyncLocalStorage</code>. Store is only provided within a 1034<code>run()</code> call or after an <code>enterWith()</code> call.</p> 1035<h4><code>asyncLocalStorage.disable()</code><span><a class="mark" href="#async_hooks_asynclocalstorage_disable" id="async_hooks_asynclocalstorage_disable">#</a></span></h4> 1036<div class="api_metadata"> 1037<span>Added in: v13.10.0</span> 1038</div> 1039<p>Disables the instance of <code>AsyncLocalStorage</code>. All subsequent calls 1040to <code>asyncLocalStorage.getStore()</code> will return <code>undefined</code> until 1041<code>asyncLocalStorage.run()</code> or <code>asyncLocalStorage.enterWith()</code> is called again.</p> 1042<p>When calling <code>asyncLocalStorage.disable()</code>, all current contexts linked to the 1043instance will be exited.</p> 1044<p>Calling <code>asyncLocalStorage.disable()</code> is required before the 1045<code>asyncLocalStorage</code> can be garbage collected. This does not apply to stores 1046provided by the <code>asyncLocalStorage</code>, as those objects are garbage collected 1047along with the corresponding async resources.</p> 1048<p>Use this method when the <code>asyncLocalStorage</code> is not in use anymore 1049in the current process.</p> 1050<h4><code>asyncLocalStorage.getStore()</code><span><a class="mark" href="#async_hooks_asynclocalstorage_getstore" id="async_hooks_asynclocalstorage_getstore">#</a></span></h4> 1051<div class="api_metadata"> 1052<span>Added in: v13.10.0</span> 1053</div> 1054<ul> 1055<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 1056</ul> 1057<p>Returns the current store. 1058If called outside of an asynchronous context initialized by 1059calling <code>asyncLocalStorage.run()</code> or <code>asyncLocalStorage.enterWith()</code>, it 1060returns <code>undefined</code>.</p> 1061<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> 1062<div class="api_metadata"> 1063<span>Added in: v13.11.0</span> 1064</div> 1065<ul> 1066<li><code>store</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 1067</ul> 1068<p>Transitions into the context for the remainder of the current 1069synchronous execution and then persists the store through any following 1070asynchronous calls.</p> 1071<p>Example:</p> 1072<pre><code class="language-js"><span class="hljs-keyword">const</span> store = { <span class="hljs-attr">id</span>: <span class="hljs-number">1</span> }; 1073<span class="hljs-comment">// Replaces previous store with the given store object</span> 1074asyncLocalStorage.<span class="hljs-title function_">enterWith</span>(store); 1075asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the store object</span> 1076<span class="hljs-title function_">someAsyncOperation</span>(<span class="hljs-function">() =></span> { 1077 asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the same object</span> 1078});</code></pre> 1079<p>This transition will continue for the <em>entire</em> synchronous execution. 1080This means that if, for example, the context is entered within an event 1081handler subsequent event handlers will also run within that context unless 1082specifically bound to another context with an <code>AsyncResource</code>. That is why 1083<code>run()</code> should be preferred over <code>enterWith()</code> unless there are strong reasons 1084to use the latter method.</p> 1085<pre><code class="language-js"><span class="hljs-keyword">const</span> store = { <span class="hljs-attr">id</span>: <span class="hljs-number">1</span> }; 1086 1087emitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'my-event'</span>, <span class="hljs-function">() =></span> { 1088 asyncLocalStorage.<span class="hljs-title function_">enterWith</span>(store); 1089}); 1090emitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'my-event'</span>, <span class="hljs-function">() =></span> { 1091 asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the same object</span> 1092}); 1093 1094asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns undefined</span> 1095emitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'my-event'</span>); 1096asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the same object</span></code></pre> 1097<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> 1098<div class="api_metadata"> 1099<span>Added in: v13.10.0</span> 1100</div> 1101<ul> 1102<li><code>store</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 1103<li><code>callback</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li> 1104<li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 1105</ul> 1106<p>Runs a function synchronously within a context and returns its 1107return value. The store is not accessible outside of the callback function. 1108The store is accessible to any asynchronous operations created within the 1109callback.</p> 1110<p>The optional <code>args</code> are passed to the callback function.</p> 1111<p>If the callback function throws an error, the error is thrown by <code>run()</code> too. 1112The stacktrace is not impacted by this call and the context is exited.</p> 1113<p>Example:</p> 1114<pre><code class="language-js"><span class="hljs-keyword">const</span> store = { <span class="hljs-attr">id</span>: <span class="hljs-number">2</span> }; 1115<span class="hljs-keyword">try</span> { 1116 asyncLocalStorage.<span class="hljs-title function_">run</span>(store, <span class="hljs-function">() =></span> { 1117 asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the store object</span> 1118 <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { 1119 asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the store object</span> 1120 }, <span class="hljs-number">200</span>); 1121 <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>(); 1122 }); 1123} <span class="hljs-keyword">catch</span> (e) { 1124 asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns undefined</span> 1125 <span class="hljs-comment">// The error will be caught here</span> 1126}</code></pre> 1127<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> 1128<div class="api_metadata"> 1129<span>Added in: v13.10.0</span> 1130</div> 1131<ul> 1132<li><code>callback</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li> 1133<li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> 1134</ul> 1135<p>Runs a function synchronously outside of a context and returns its 1136return value. The store is not accessible within the callback function or 1137the asynchronous operations created within the callback. Any <code>getStore()</code> 1138call done within the callback function will always return <code>undefined</code>.</p> 1139<p>The optional <code>args</code> are passed to the callback function.</p> 1140<p>If the callback function throws an error, the error is thrown by <code>exit()</code> too. 1141The stacktrace is not impacted by this call and the context is re-entered.</p> 1142<p>Example:</p> 1143<pre><code class="language-js"><span class="hljs-comment">// Within a call to run</span> 1144<span class="hljs-keyword">try</span> { 1145 asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the store object or value</span> 1146 asyncLocalStorage.<span class="hljs-title function_">exit</span>(<span class="hljs-function">() =></span> { 1147 asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns undefined</span> 1148 <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>(); 1149 }); 1150} <span class="hljs-keyword">catch</span> (e) { 1151 asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the same object or value</span> 1152 <span class="hljs-comment">// The error will be caught here</span> 1153}</code></pre> 1154<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> 1155<p>If, within an async function, only one <code>await</code> call is to run within a context, 1156the following pattern should be used:</p> 1157<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>) { 1158 <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> { 1159 asyncLocalStorage.<span class="hljs-title function_">getStore</span>().<span class="hljs-title function_">set</span>(<span class="hljs-string">'key'</span>, value); 1160 <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> 1161 }); 1162}</code></pre> 1163<p>In this example, the store is only available in the callback function and the 1164functions called by <code>foo</code>. Outside of <code>run</code>, calling <code>getStore</code> will return 1165<code>undefined</code>.</p> 1166<h4>Troubleshooting<span><a class="mark" href="#async_hooks_troubleshooting" id="async_hooks_troubleshooting">#</a></span></h4> 1167<p>In most cases your application or library code should have no issues with 1168<code>AsyncLocalStorage</code>. But in rare cases you may face situations when the 1169current store is lost in one of asynchronous operations. In those cases, 1170consider the following options.</p> 1171<p>If your code is callback-based, it is enough to promisify it with 1172<a href="util.html#util_util_promisify_original"><code>util.promisify()</code></a>, so it starts working with native promises.</p> 1173<p>If you need to keep using callback-based API, or your code assumes 1174a custom thenable implementation, use the <a href="#async_hooks_class_asyncresource"><code>AsyncResource</code></a> class 1175to associate the asynchronous operation with the correct execution context.</p></section> 1176 <!-- API END --> 1177 </div> 1178 </div> 1179 </div> 1180 <script> 1181 'use strict'; 1182 { 1183 const kCustomPreference = 'customDarkTheme'; 1184 const userSettings = sessionStorage.getItem(kCustomPreference); 1185 const themeToggleButton = document.getElementById('theme-toggle-btn'); 1186 if (userSettings === null && window.matchMedia) { 1187 const mq = window.matchMedia('(prefers-color-scheme: dark)'); 1188 if ('onchange' in mq) { 1189 function mqChangeListener(e) { 1190 document.body.classList.toggle('dark-mode', e.matches); 1191 } 1192 mq.addEventListener('change', mqChangeListener); 1193 if (themeToggleButton) { 1194 themeToggleButton.addEventListener('click', function() { 1195 mq.removeEventListener('change', mqChangeListener); 1196 }, { once: true }); 1197 } 1198 } 1199 if (mq.matches) { 1200 document.body.classList.add('dark-mode'); 1201 } 1202 } else if (userSettings === 'true') { 1203 document.body.classList.add('dark-mode'); 1204 } 1205 if (themeToggleButton) { 1206 themeToggleButton.hidden = false; 1207 themeToggleButton.addEventListener('click', function() { 1208 sessionStorage.setItem( 1209 kCustomPreference, 1210 document.body.classList.toggle('dark-mode') 1211 ); 1212 }); 1213 } 1214 } 1215 </script> 1216</body> 1217</html> 1218