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<div id="toc">
 <h2>Table of Contents</h2>
 <ul>
<li><a href="#async_hooks_async_hooks">Async hooks</a>
<ul>
<li><a href="#async_hooks_terminology">Terminology</a></li>
<li><a href="#async_hooks_public_api">Public API</a>
<ul>
<li><a href="#async_hooks_overview">Overview</a>
<ul>
<li><a href="#async_hooks_async_hooks_createhook_callbacks"><code>async_hooks.createHook(callbacks)</code></a>
<ul>
<li><a href="#async_hooks_error_handling">Error handling</a></li>
<li><a href="#async_hooks_printing_in_asynchooks_callbacks">Printing in AsyncHooks callbacks</a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#async_hooks_class_asynchook">Class: <code>AsyncHook</code></a>
<ul>
<li><a href="#async_hooks_asynchook_enable"><code>asyncHook.enable()</code></a></li>
<li><a href="#async_hooks_asynchook_disable"><code>asyncHook.disable()</code></a></li>
<li><a href="#async_hooks_hook_callbacks">Hook callbacks</a>
<ul>
<li><a href="#async_hooks_init_asyncid_type_triggerasyncid_resource"><code>init(asyncId, type, triggerAsyncId, resource)</code></a>
<ul>
<li><a href="#async_hooks_type"><code>type</code></a></li>
<li><a href="#async_hooks_triggerasyncid"><code>triggerAsyncId</code></a></li>
<li><a href="#async_hooks_resource"><code>resource</code></a></li>
<li><a href="#async_hooks_asynchronous_context_example">Asynchronous context example</a></li>
</ul>
</li>
<li><a href="#async_hooks_before_asyncid"><code>before(asyncId)</code></a></li>
<li><a href="#async_hooks_after_asyncid"><code>after(asyncId)</code></a></li>
<li><a href="#async_hooks_destroy_asyncid"><code>destroy(asyncId)</code></a></li>
<li><a href="#async_hooks_promiseresolve_asyncid"><code>promiseResolve(asyncId)</code></a></li>
</ul>
</li>
<li><a href="#async_hooks_async_hooks_executionasyncresource"><code>async_hooks.executionAsyncResource()</code></a></li>
<li><a href="#async_hooks_async_hooks_executionasyncid"><code>async_hooks.executionAsyncId()</code></a></li>
<li><a href="#async_hooks_async_hooks_triggerasyncid"><code>async_hooks.triggerAsyncId()</code></a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#async_hooks_promise_execution_tracking">Promise execution tracking</a></li>
<li><a href="#async_hooks_javascript_embedder_api">JavaScript embedder API</a>
<ul>
<li><a href="#async_hooks_class_asyncresource">Class: <code>AsyncResource</code></a>
<ul>
<li><a href="#async_hooks_new_asyncresource_type_options"><code>new AsyncResource(type[, options])</code></a></li>
<li><a href="#async_hooks_static_method_asyncresource_bind_fn_type">Static method: <code>AsyncResource.bind(fn[, type])</code></a></li>
<li><a href="#async_hooks_asyncresource_bind_fn"><code>asyncResource.bind(fn)</code></a></li>
<li><a href="#async_hooks_asyncresource_runinasyncscope_fn_thisarg_args"><code>asyncResource.runInAsyncScope(fn[, thisArg, ...args])</code></a></li>
<li><a href="#async_hooks_asyncresource_emitdestroy"><code>asyncResource.emitDestroy()</code></a></li>
<li><a href="#async_hooks_asyncresource_asyncid"><code>asyncResource.asyncId()</code></a></li>
<li><a href="#async_hooks_asyncresource_triggerasyncid"><code>asyncResource.triggerAsyncId()</code></a></li>
</ul>
</li>
<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>
<li><a href="#async_hooks_integrating_asyncresource_with_eventemitter">Integrating <code>AsyncResource</code> with <code>EventEmitter</code></a></li>
</ul>
</li>
<li><a href="#async_hooks_class_asynclocalstorage">Class: <code>AsyncLocalStorage</code></a>
<ul>
<li><a href="#async_hooks_new_asynclocalstorage"><code>new AsyncLocalStorage()</code></a></li>
<li><a href="#async_hooks_asynclocalstorage_disable"><code>asyncLocalStorage.disable()</code></a></li>
<li><a href="#async_hooks_asynclocalstorage_getstore"><code>asyncLocalStorage.getStore()</code></a></li>
<li><a href="#async_hooks_asynclocalstorage_enterwith_store"><code>asyncLocalStorage.enterWith(store)</code></a></li>
<li><a href="#async_hooks_asynclocalstorage_run_store_callback_args"><code>asyncLocalStorage.run(store, callback[, ...args])</code></a></li>
<li><a href="#async_hooks_asynclocalstorage_exit_callback_args"><code>asyncLocalStorage.exit(callback[, ...args])</code></a></li>
<li><a href="#async_hooks_usage_with_async_await">Usage with <code>async/await</code></a></li>
<li><a href="#async_hooks_troubleshooting">Troubleshooting</a></li>
</ul>
</li>
</ul>
</li>
</ul>
 </div>

<div id="apicontent">
 <h1>Async hooks<a class="mark" href="#async_hooks_async_hooks" id="async_hooks_async_hooks">#</a></h1>

<div class="api_stability api_stability_1"><a href="documentation.html#documentation_stability_index">Stability: 1</a> - Experimental</div>
Source Code: <a href="https://github.com/nodejs/node/blob/v12.22.12/lib/async_hooks.js">lib/async_hooks.js</a>
The <code>async_hooks</code> module provides an API to track asynchronous resources. It
can be accessed using:
<pre><code class="language-js">const async_hooks = require('async_hooks');</code></pre>
<h2>Terminology<a class="mark" href="#async_hooks_terminology" id="async_hooks_terminology">#</a></h2>
An asynchronous resource represents an object with an associated callback.
This callback may be called multiple times, for example, the <code>'connection'</code>
event in <code>net.createServer()</code>, or just a single time like in <code>fs.open()</code>.
A resource can also be closed before the callback is called. <code>AsyncHook</code> does
not explicitly distinguish between these different cases but will represent them
as the abstract concept that is a resource.
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>
interface, and each thread will use a new set of async IDs.
<h2>Public API<a class="mark" href="#async_hooks_public_api" id="async_hooks_public_api">#</a></h2>
<h3>Overview<a class="mark" href="#async_hooks_overview" id="async_hooks_overview">#</a></h3>
Following is a simple overview of the public API.
<pre><code class="language-js">const async_hooks = require('async_hooks');

// Return the ID of the current execution context.
const eid = async_hooks.executionAsyncId();

// Return the ID of the handle responsible for triggering the callback of the
// current execution scope to call.
const tid = async_hooks.triggerAsyncId();

// Create a new AsyncHook instance. All of these callbacks are optional.
const asyncHook =
 async_hooks.createHook({ init, before, after, destroy, promiseResolve });

// Allow callbacks of this AsyncHook instance to call. This is not an implicit
// action after running the constructor, and must be explicitly run to begin
// executing callbacks.
asyncHook.enable();

// Disable listening for new asynchronous events.
asyncHook.disable();

//
// The following are the callbacks that can be passed to createHook().
//

// init is called during object construction. The resource may not have
// completed construction when this callback runs, therefore all fields of the
// resource referenced by "asyncId" may not have been populated.
function init(asyncId, type, triggerAsyncId, resource) { }

// Before is called just before the resource's callback is called. It can be
// called 0-N times for handles (e.g. TCPWrap), and will be called exactly 1
// time for requests (e.g. FSReqCallback).
function before(asyncId) { }

// After is called just after the resource's callback has finished.
function after(asyncId) { }

// Destroy is called when the resource is destroyed.
function destroy(asyncId) { }

// promiseResolve is called only for promise resources, when the
// `resolve` function passed to the `Promise` constructor is invoked
// (either directly or through other means of resolving a promise).
function promiseResolve(asyncId) { }</code></pre>
<h4><code>async_hooks.createHook(callbacks)</code><a class="mark" href="#async_hooks_async_hooks_createhook_callbacks" id="async_hooks_async_hooks_createhook_callbacks">#</a></h4>
<div class="api_metadata">
Added in: v8.1.0
</div>
<ul>
<li><code>callbacks</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type">&#x3C;Object></a> The <a href="#async_hooks_hook_callbacks">Hook Callbacks</a> to register
<ul>
<li><code>init</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a> The <a href="#async_hooks_init_asyncid_type_triggerasyncid_resource"><code>init</code> callback</a>.</li>
<li><code>before</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a> The <a href="#async_hooks_before_asyncid"><code>before</code> callback</a>.</li>
<li><code>after</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a> The <a href="#async_hooks_after_asyncid"><code>after</code> callback</a>.</li>
<li><code>destroy</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a> The <a href="#async_hooks_destroy_asyncid"><code>destroy</code> callback</a>.</li>
<li><code>promiseResolve</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a> The <a href="#async_hooks_promiseresolve_asyncid"><code>promiseResolve</code> callback</a>.</li>
</ul>
</li>
<li>Returns: <a href="async_hooks.html#async_hooks_async_hooks_createhook_callbacks" class="type">&#x3C;AsyncHook></a> Instance used for disabling and enabling hooks</li>
</ul>
Registers functions to be called for different lifetime events of each async
operation.
The callbacks <code>init()</code>/<code>before()</code>/<code>after()</code>/<code>destroy()</code> are called for the
respective asynchronous event during a resource's lifetime.
All callbacks are optional. For example, if only resource cleanup needs to
be tracked, then only the <code>destroy</code> callback needs to be passed. The
specifics of all functions that can be passed to <code>callbacks</code> is in the
<a href="#async_hooks_hook_callbacks">Hook Callbacks</a> section.
<pre><code class="language-js">const async_hooks = require('async_hooks');

const asyncHook = async_hooks.createHook({
 init(asyncId, type, triggerAsyncId, resource) { },
 destroy(asyncId) { }
});</code></pre>
The callbacks will be inherited via the prototype chain:
<pre><code class="language-js">class MyAsyncCallbacks {
 init(asyncId, type, triggerAsyncId, resource) { }
 destroy(asyncId) {}
}

class MyAddedCallbacks extends MyAsyncCallbacks {
 before(asyncId) { }
 after(asyncId) { }
}

const asyncHook = async_hooks.createHook(new MyAddedCallbacks());</code></pre>
<h5>Error handling<a class="mark" href="#async_hooks_error_handling" id="async_hooks_error_handling">#</a></h5>
If any <code>AsyncHook</code> callbacks throw, the application will print the stack trace
and exit. The exit path does follow that of an uncaught exception, but
all <code>'uncaughtException'</code> listeners are removed, thus forcing the process to
exit. The <code>'exit'</code> callbacks will still be called unless the application is run
with <code>--abort-on-uncaught-exception</code>, in which case a stack trace will be
printed and the application exits, leaving a core file.
The reason for this error handling behavior is that these callbacks are running
at potentially volatile points in an object's lifetime, for example during
class construction and destruction. Because of this, it is deemed necessary to
bring down the process quickly in order to prevent an unintentional abort in the
future. This is subject to change in the future if a comprehensive analysis is
performed to ensure an exception can follow the normal control flow without
unintentional side effects.
<h5>Printing in AsyncHooks callbacks<a class="mark" href="#async_hooks_printing_in_asynchooks_callbacks" id="async_hooks_printing_in_asynchooks_callbacks">#</a></h5>
Because printing to the console is an asynchronous operation, <code>console.log()</code>
will cause the AsyncHooks callbacks to be called. Using <code>console.log()</code> or
similar asynchronous operations inside an AsyncHooks callback function will thus
cause an infinite recursion. An easy solution to this when debugging is to use a
synchronous logging operation such as <code>fs.writeFileSync(file, msg, flag)</code>.
This will print to the file and will not invoke AsyncHooks recursively because
it is synchronous.
<pre><code class="language-js">const fs = require('fs');
const util = require('util');

require('net').createServer((conn) => {}).listen(8080);</code></pre>
Output when hitting the server with <code>nc localhost 8080</code>:
<pre><code class="language-console">TCPSERVERWRAP(5): trigger: 1 execution: 1
TCPWRAP(7): trigger: 5 execution: 0</code></pre>
The <code>TCPSERVERWRAP</code> is the server which receives the connections.
The <code>TCPWRAP</code> is the new connection from the client. When a new
connection is made, the <code>TCPWrap</code> instance is immediately constructed. This
happens outside of any JavaScript stack. (An <code>executionAsyncId()</code> of <code>0</code> means
that it is being executed from C++ with no JavaScript stack above it.) With only
that information, it would be impossible to link resources together in
terms of what caused them to be created, so <code>triggerAsyncId</code> is given the task
of propagating what resource is responsible for the new resource's existence.
<h6><code>resource</code><a class="mark" href="#async_hooks_resource" id="async_hooks_resource">#</a></h6>
<code>resource</code> is an object that represents the actual async resource that has
been initialized. This can contain useful information that can vary based on
the value of <code>type</code>. For instance, for the <code>GETADDRINFOREQWRAP</code> resource type,
<code>resource</code> provides the host name used when looking up the IP address for the
host in <code>net.Server.listen()</code>. The API for accessing this information is
not supported, but using the Embedder API, users can provide
and document their own resource objects. For example, such a resource object
could contain the SQL query being executed.
In the case of Promises, the <code>resource</code> object will have an
<code>isChainedPromise</code> property, set to <code>true</code> if the promise has a parent promise,
and <code>false</code> otherwise. For example, in the case of <code>b = a.then(handler)</code>, <code>a</code> is
considered a parent <code>Promise</code> of <code>b</code>. Here, <code>b</code> is considered a chained promise.
In some cases the resource object is reused for performance reasons, it is
thus not safe to use it as a key in a <code>WeakMap</code> or add properties to it.
<h6>Asynchronous context example<a class="mark" href="#async_hooks_asynchronous_context_example" id="async_hooks_asynchronous_context_example">#</a></h6>
The following is an example with additional information about the calls to
<code>init</code> between the <code>before</code> and <code>after</code> calls, specifically what the
callback to <code>listen()</code> will look like. The output formatting is slightly more
elaborate to make calling context easier to see.
<pre><code class="language-js">let indent = 0;
async_hooks.createHook({
 init(asyncId, type, triggerAsyncId) {
 const eid = async_hooks.executionAsyncId();
 const indentStr = ' '.repeat(indent);
 fs.writeSync(
 process.stdout.fd,
 `${indentStr}${type}(${asyncId}):` +
 ` trigger: ${triggerAsyncId} execution: ${eid}\n`);
 },
 before(asyncId) {
 const indentStr = ' '.repeat(indent);
 fs.writeSync(process.stdout.fd, `${indentStr}before: ${asyncId}\n`);
 indent += 2;
 },
 after(asyncId) {
 indent -= 2;
 const indentStr = ' '.repeat(indent);
 fs.writeSync(process.stdout.fd, `${indentStr}after: ${asyncId}\n`);
 },
 destroy(asyncId) {
 const indentStr = ' '.repeat(indent);
 fs.writeSync(process.stdout.fd, `${indentStr}destroy: ${asyncId}\n`);
 },
}).enable();

require('net').createServer(() => {}).listen(8080, () => {
 // Let's wait 10ms before logging the server started.
 setTimeout(() => {
 console.log('>>>', async_hooks.executionAsyncId());
 }, 10);
});</code></pre>
Output from only starting the server:
<pre><code class="language-console">TCPSERVERWRAP(5): trigger: 1 execution: 1
TickObject(6): trigger: 5 execution: 1
before: 6
 Timeout(7): trigger: 6 execution: 6
after: 6
destroy: 6
before: 7
>>> 7
 TickObject(8): trigger: 7 execution: 7
after: 7
before: 8
after: 8</code></pre>
As illustrated in the example, <code>executionAsyncId()</code> and <code>execution</code> each specify
the value of the current execution context; which is delineated by calls to
<code>before</code> and <code>after</code>.
Only using <code>execution</code> to graph resource allocation results in the following:
<pre><code class="language-console"> root(1)
 ^
 |
TickObject(6)
 ^
 |
 Timeout(7)</code></pre>
The <code>TCPSERVERWRAP</code> is not part of this graph, even though it was the reason for
<code>console.log()</code> being called. This is because binding to a port without a host
name is a synchronous operation, but to maintain a completely asynchronous
API the user's callback is placed in a <code>process.nextTick()</code>. Which is why
<code>TickObject</code> is present in the output and is a 'parent' for <code>.listen()</code>
callback.
The graph only shows when a resource was created, not why, so to track
the why use <code>triggerAsyncId</code>. Which can be represented with the following
graph:
<pre><code class="language-console"> bootstrap(1)
 |
 ˅
TCPSERVERWRAP(5)
 |
 ˅
 TickObject(6)
 |
 ˅
 Timeout(7)</code></pre>
<h5><code>before(asyncId)</code><a class="mark" href="#async_hooks_before_asyncid" id="async_hooks_before_asyncid">#</a></h5>
<ul>
<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
When an asynchronous operation is initiated (such as a TCP server receiving a
new connection) or completes (such as writing data to disk) a callback is
called to notify the user. The <code>before</code> callback is called just before said
callback is executed. <code>asyncId</code> is the unique identifier assigned to the
resource about to execute the callback.
The <code>before</code> callback will be called 0 to N times. The <code>before</code> callback
will typically be called 0 times if the asynchronous operation was cancelled
or, for example, if no connections are received by a TCP server. Persistent
asynchronous resources like a TCP server will typically call the <code>before</code>
callback multiple times, while other operations like <code>fs.open()</code> will call
it only once.
<h5><code>after(asyncId)</code><a class="mark" href="#async_hooks_after_asyncid" id="async_hooks_after_asyncid">#</a></h5>
<ul>
<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
Called immediately after the callback specified in <code>before</code> is completed.
If an uncaught exception occurs during execution of the callback, then <code>after</code>
will run after the <code>'uncaughtException'</code> event is emitted or a <code>domain</code>'s
handler runs.
<h5><code>destroy(asyncId)</code><a class="mark" href="#async_hooks_destroy_asyncid" id="async_hooks_destroy_asyncid">#</a></h5>
<ul>
<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
Called after the resource corresponding to <code>asyncId</code> is destroyed. It is also
called asynchronously from the embedder API <code>emitDestroy()</code>.
Some resources depend on garbage collection for cleanup, so if a reference is
made to the <code>resource</code> object passed to <code>init</code> it is possible that <code>destroy</code>
will never be called, causing a memory leak in the application. If the resource
does not depend on garbage collection, then this will not be an issue.
<h5><code>promiseResolve(asyncId)</code><a class="mark" href="#async_hooks_promiseresolve_asyncid" id="async_hooks_promiseresolve_asyncid">#</a></h5>
<div class="api_metadata">
Added in: v8.6.0
</div>
<ul>
<li><code>asyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a></li>
</ul>
Called when the <code>resolve</code> function passed to the <code>Promise</code> constructor is
invoked (either directly or through other means of resolving a promise).
<code>resolve()</code> does not do any observable synchronous work.
The <code>Promise</code> is not necessarily fulfilled or rejected at this point if the
<code>Promise</code> was resolved by assuming the state of another <code>Promise</code>.
<pre><code class="language-js">new Promise((resolve) => resolve(true)).then((a) => {});</code></pre>
calls the following callbacks:
<pre><code class="language-text">init for PROMISE with id 5, trigger id: 1
 promise resolve 5 # corresponds to resolve(true)
init for PROMISE with id 6, trigger id: 5 # the Promise returned by then()
 before 6 # the then() callback is entered
 promise resolve 6 # the then() callback resolves the promise by returning
 after 6</code></pre>
<h4><code>async_hooks.executionAsyncResource()</code><a class="mark" href="#async_hooks_async_hooks_executionasyncresource" id="async_hooks_async_hooks_executionasyncresource">#</a></h4>
<div class="api_metadata">
Added in: v12.17.0
</div>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type">&#x3C;Object></a> The resource representing the current execution.
Useful to store data within the resource.</li>
</ul>
Resource objects returned by <code>executionAsyncResource()</code> are most often internal
Node.js handle objects with undocumented APIs. Using any functions or properties
on the object is likely to crash your application and should be avoided.
Using <code>executionAsyncResource()</code> in the top-level execution context will
return an empty object as there is no handle or request object to use,
but having an object representing the top-level can be helpful.
<pre><code class="language-js">const { open } = require('fs');
const { executionAsyncId, executionAsyncResource } = require('async_hooks');

console.log(executionAsyncId(), executionAsyncResource()); // 1 {}
open(__filename, 'r', (err, fd) => {
 console.log(executionAsyncId(), executionAsyncResource()); // 7 FSReqWrap
});</code></pre>
This can be used to implement continuation local storage without the
use of a tracking <code>Map</code> to store the metadata:
<pre><code class="language-js">const { createServer } = require('http');
const {
 executionAsyncId,
 executionAsyncResource,
 createHook
} = require('async_hooks');
const sym = Symbol('state'); // Private symbol to avoid pollution

createHook({
 init(asyncId, type, triggerAsyncId, resource) {
 const cr = executionAsyncResource();
 if (cr) {
 resource[sym] = cr[sym];
 }
 }
}).enable();

console.log(async_hooks.executionAsyncId()); // 1 - bootstrap
fs.open(path, 'r', (err, fd) => {
 console.log(async_hooks.executionAsyncId()); // 6 - open()
});</code></pre>
The ID returned from <code>executionAsyncId()</code> is related to execution timing, not
causality (which is covered by <code>triggerAsyncId()</code>):
<pre><code class="language-js">const server = net.createServer((conn) => {
 // Returns the ID of the server, not of the new connection, because the
 // callback runs in the execution scope of the server's MakeCallback().
 async_hooks.executionAsyncId();

}).listen(port, () => {
 // Returns the ID of a TickObject (i.e. process.nextTick()) because all
 // callbacks passed to .listen() are wrapped in a nextTick().
 async_hooks.executionAsyncId();
});</code></pre>
Promise contexts may not get precise <code>executionAsyncIds</code> by default.
See the section on <a href="#async_hooks_promise_execution_tracking">promise execution tracking</a>.
<h4><code>async_hooks.triggerAsyncId()</code><a class="mark" href="#async_hooks_async_hooks_triggerasyncid" id="async_hooks_async_hooks_triggerasyncid">#</a></h4>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a> The ID of the resource responsible for calling the callback
that is currently being executed.</li>
</ul>
<pre><code class="language-js">const server = net.createServer((conn) => {
 // The resource that caused (or triggered) this callback to be called
 // was that of the new connection. Thus the return value of triggerAsyncId()
 // is the asyncId of "conn".
 async_hooks.triggerAsyncId();

}).listen(port, () => {
 // Even though all callbacks passed to .listen() are wrapped in a nextTick()
 // the callback itself exists because the call to the server's .listen()
 // was made. So the return value would be the ID of the server.
 async_hooks.triggerAsyncId();
});</code></pre>
Promise contexts may not get valid <code>triggerAsyncId</code>s by default. See
the section on <a href="#async_hooks_promise_execution_tracking">promise execution tracking</a>.
<h2>Promise execution tracking<a class="mark" href="#async_hooks_promise_execution_tracking" id="async_hooks_promise_execution_tracking">#</a></h2>
By default, promise executions are not assigned <code>asyncId</code>s due to the relatively
expensive nature of the <a href="https://docs.google.com/document/d/1rda3yKGHimKIhg5YeoAmCOtyURgsbTH_qaYR79FELlk/edit">promise introspection API</a> provided by
V8. This means that programs using promises or <code>async</code>/<code>await</code> will not get
correct execution and trigger ids for promise callback contexts by default.
<pre><code class="language-js">const ah = require('async_hooks');
Promise.resolve(1729).then(() => {
 console.log(`eid ${ah.executionAsyncId()} tid ${ah.triggerAsyncId()}`);
});
// produces:
// eid 1 tid 0</code></pre>
Observe that the <code>then()</code> callback claims to have executed in the context of the
outer scope even though there was an asynchronous hop involved. Also,
the <code>triggerAsyncId</code> value is <code>0</code>, which means that we are missing context about
the resource that caused (triggered) the <code>then()</code> callback to be executed.
Installing async hooks via <code>async_hooks.createHook</code> enables promise execution
tracking:
<pre><code class="language-js">const ah = require('async_hooks');
ah.createHook({ init() {} }).enable(); // forces PromiseHooks to be enabled.
Promise.resolve(1729).then(() => {
 console.log(`eid ${ah.executionAsyncId()} tid ${ah.triggerAsyncId()}`);
});
// produces:
// eid 7 tid 6</code></pre>
In this example, adding any actual hook function enabled the tracking of
promises. There are two promises in the example above; the promise created by
<code>Promise.resolve()</code> and the promise returned by the call to <code>then()</code>. In the
example above, the first promise got the <code>asyncId</code> <code>6</code> and the latter got
<code>asyncId</code> <code>7</code>. During the execution of the <code>then()</code> callback, we are executing
in the context of promise with <code>asyncId</code> <code>7</code>. This promise was triggered by
async resource <code>6</code>.
Another subtlety with promises is that <code>before</code> and <code>after</code> callbacks are run
only on chained promises. That means promises not created by <code>then()</code>/<code>catch()</code>
will not have the <code>before</code> and <code>after</code> callbacks fired on them. For more details
see the details of the V8 <a href="https://docs.google.com/document/d/1rda3yKGHimKIhg5YeoAmCOtyURgsbTH_qaYR79FELlk/edit">PromiseHooks</a> API.
<h2>JavaScript embedder API<a class="mark" href="#async_hooks_javascript_embedder_api" id="async_hooks_javascript_embedder_api">#</a></h2>
Library developers that handle their own asynchronous resources performing tasks
like I/O, connection pooling, or managing callback queues may use the
<code>AsyncResource</code> JavaScript API so that all the appropriate callbacks are called.
<h3>Class: <code>AsyncResource</code><a class="mark" href="#async_hooks_class_asyncresource" id="async_hooks_class_asyncresource">#</a></h3>
The class <code>AsyncResource</code> is designed to be extended by the embedder's async
resources. Using this, users can easily trigger the lifetime events of their
own resources.
The <code>init</code> hook will trigger when an <code>AsyncResource</code> is instantiated.
The following is an overview of the <code>AsyncResource</code> API.
<pre><code class="language-js">const { AsyncResource, executionAsyncId } = require('async_hooks');

// AsyncResource() is meant to be extended. Instantiating a
// new AsyncResource() also triggers init. If triggerAsyncId is omitted then
// async_hook.executionAsyncId() is used.
const asyncResource = new AsyncResource(
 type, { triggerAsyncId: executionAsyncId(), requireManualDestroy: false }
);

// Run a function in the execution context of the resource. This will
// * establish the context of the resource
// * trigger the AsyncHooks before callbacks
// * call the provided function `fn` with the supplied arguments
// * trigger the AsyncHooks after callbacks
// * restore the original execution context
asyncResource.runInAsyncScope(fn, thisArg, ...args);

// Call AsyncHooks destroy callbacks.
asyncResource.emitDestroy();

// Return the unique ID assigned to the AsyncResource instance.
asyncResource.asyncId();

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

getInfo(query, callback) {
 this.db.get(query, (err, data) => {
 this.runInAsyncScope(callback, null, err, data);
 });
 }

close() {
 this.db = null;
 this.emitDestroy();
 }
}</code></pre>
<h4>Static method: <code>AsyncResource.bind(fn[, type])</code><a class="mark" href="#async_hooks_static_method_asyncresource_bind_fn_type" id="async_hooks_static_method_asyncresource_bind_fn_type">#</a></h4>
<div class="api_metadata">
Added in: v12.19.0
</div>
<ul>
<li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a> The function to bind to the current execution context.</li>
<li><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type">&#x3C;string></a> An optional name to associate with the underlying
<code>AsyncResource</code>.</li>
</ul>
Binds the given function to the current execution context.
The returned function will have an <code>asyncResource</code> property referencing
the <code>AsyncResource</code> to which the function is bound.
<h4><code>asyncResource.bind(fn)</code><a class="mark" href="#async_hooks_asyncresource_bind_fn" id="async_hooks_asyncresource_bind_fn">#</a></h4>
<div class="api_metadata">
Added in: v12.19.0
</div>
<ul>
<li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a> The function to bind to the current <code>AsyncResource</code>.</li>
</ul>
Binds the given function to execute to this <code>AsyncResource</code>'s scope.
The returned function will have an <code>asyncResource</code> property referencing
the <code>AsyncResource</code> to which the function is bound.
<h4><code>asyncResource.runInAsyncScope(fn[, thisArg, ...args])</code><a class="mark" href="#async_hooks_asyncresource_runinasyncscope_fn_thisarg_args" id="async_hooks_asyncresource_runinasyncscope_fn_thisarg_args">#</a></h4>
<div class="api_metadata">
Added in: v9.6.0
</div>
<ul>
<li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a> The function to call in the execution context of this async
resource.</li>
<li><code>thisArg</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type">&#x3C;any></a> The receiver to be used for the function call.</li>
<li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type">&#x3C;any></a> Optional arguments to pass to the function.</li>
</ul>
Call the provided function with the provided arguments in the execution context
of the async resource. This will establish the context, trigger the AsyncHooks
before callbacks, call the function, trigger the AsyncHooks after callbacks, and
then restore the original execution context.
<h4><code>asyncResource.emitDestroy()</code><a class="mark" href="#async_hooks_asyncresource_emitdestroy" id="async_hooks_asyncresource_emitdestroy">#</a></h4>
<ul>
<li>Returns: <a href="async_hooks.html#async_hooks_class_asyncresource" class="type">&#x3C;AsyncResource></a> A reference to <code>asyncResource</code>.</li>
</ul>
Call all <code>destroy</code> hooks. This should only ever be called once. An error will
be thrown if it is called more than once. This must be manually called. If
the resource is left to be collected by the GC then the <code>destroy</code> hooks will
never be called.
<h4><code>asyncResource.asyncId()</code><a class="mark" href="#async_hooks_asyncresource_asyncid" id="async_hooks_asyncresource_asyncid">#</a></h4>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a> The unique <code>asyncId</code> assigned to the resource.</li>
</ul>
<h4><code>asyncResource.triggerAsyncId()</code><a class="mark" href="#async_hooks_asyncresource_triggerasyncid" id="async_hooks_asyncresource_triggerasyncid">#</a></h4>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type">&#x3C;number></a> The same <code>triggerAsyncId</code> that is passed to the
<code>AsyncResource</code> constructor.</li>
</ul>
<a id="async-resource-worker-pool"></a>
<h3>Using <code>AsyncResource</code> for a <code>Worker</code> thread pool<a class="mark" href="#async_hooks_using_asyncresource_for_a_worker_thread_pool" id="async_hooks_using_asyncresource_for_a_worker_thread_pool">#</a></h3>
The following example shows how to use the <code>AsyncResource</code> class to properly
provide async tracking for a <a href="worker_threads.html#worker_threads_class_worker"><code>Worker</code></a> pool. Other resource pools, such as
database connection pools, can follow a similar model.
Assuming that the task is adding two numbers, using a file named
<code>task_processor.js</code> with the following content:
<pre><code class="language-js">const { parentPort } = require('worker_threads');
parentPort.on('message', (task) => {
 parentPort.postMessage(task.a + task.b);
});</code></pre>
a Worker pool around it could use the following structure:
<pre><code class="language-js">const { AsyncResource } = require('async_hooks');
const { EventEmitter } = require('events');
const path = require('path');
const { Worker } = require('worker_threads');

const kTaskInfo = Symbol('kTaskInfo');
const kWorkerFreedEvent = Symbol('kWorkerFreedEvent');

class WorkerPoolTaskInfo extends AsyncResource {
 constructor(callback) {
 super('WorkerPoolTaskInfo');
 this.callback = callback;
 }

done(err, result) {
 this.runInAsyncScope(this.callback, null, err, result);
 this.emitDestroy(); // `TaskInfo`s are used only once.
 }
}

class WorkerPool extends EventEmitter {
 constructor(numThreads) {
 super();
 this.numThreads = numThreads;
 this.workers = [];
 this.freeWorkers = [];

for (let i = 0; i &#x3C; numThreads; i++)
 this.addNewWorker();
 }

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

runTask(task, callback) {
 if (this.freeWorkers.length === 0) {
 // No free threads, wait until a worker thread becomes free.
 this.once(kWorkerFreedEvent, () => this.runTask(task, callback));
 return;
 }

const worker = this.freeWorkers.pop();
 worker[kTaskInfo] = new WorkerPoolTaskInfo(callback);
 worker.postMessage(task);
 }

close() {
 for (const worker of this.workers) worker.terminate();
 }
}

module.exports = WorkerPool;</code></pre>
Without the explicit tracking added by the <code>WorkerPoolTaskInfo</code> objects,
it would appear that the callbacks are associated with the individual <code>Worker</code>
objects. However, the creation of the <code>Worker</code>s is not associated with the
creation of the tasks and does not provide information about when tasks
were scheduled.
This pool could be used as follows:
<pre><code class="language-js">const WorkerPool = require('./worker_pool.js');
const os = require('os');

const pool = new WorkerPool(os.cpus().length);

const asyncLocalStorage = new AsyncLocalStorage();

function logWithId(msg) {
 const id = asyncLocalStorage.getStore();
 console.log(`${id !== undefined ? id : '-'}:`, msg);
}

http.get('http://localhost:8080');
http.get('http://localhost:8080');
// Prints:
// 0: start
// 1: start
// 0: finish
// 1: finish</code></pre>
When having multiple instances of <code>AsyncLocalStorage</code>, they are independent
from each other. It is safe to instantiate this class multiple times.
<h3><code>new AsyncLocalStorage()</code><a class="mark" href="#async_hooks_new_asynclocalstorage" id="async_hooks_new_asynclocalstorage">#</a></h3>
<div class="api_metadata">
Added in: v12.17.0
</div>
Creates a new instance of <code>AsyncLocalStorage</code>. Store is only provided within a
<code>run</code> method call.
<h3><code>asyncLocalStorage.disable()</code><a class="mark" href="#async_hooks_asynclocalstorage_disable" id="async_hooks_asynclocalstorage_disable">#</a></h3>
<div class="api_metadata">
Added in: v12.17.0
</div>
This method disables the instance of <code>AsyncLocalStorage</code>. All subsequent calls
to <code>asyncLocalStorage.getStore()</code> will return <code>undefined</code> until
<code>asyncLocalStorage.run()</code> is called again.
When calling <code>asyncLocalStorage.disable()</code>, all current contexts linked to the
instance will be exited.
Calling <code>asyncLocalStorage.disable()</code> is required before the
<code>asyncLocalStorage</code> can be garbage collected. This does not apply to stores
provided by the <code>asyncLocalStorage</code>, as those objects are garbage collected
along with the corresponding async resources.
This method is to be used when the <code>asyncLocalStorage</code> is not in use anymore
in the current process.
<h3><code>asyncLocalStorage.getStore()</code><a class="mark" href="#async_hooks_asynclocalstorage_getstore" id="async_hooks_asynclocalstorage_getstore">#</a></h3>
<div class="api_metadata">
Added in: v12.17.0
</div>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type">&#x3C;any></a></li>
</ul>
This method returns the current store.
If this method is called outside of an asynchronous context initialized by
calling <code>asyncLocalStorage.run</code>, it will return <code>undefined</code>.
<h3><code>asyncLocalStorage.enterWith(store)</code><a class="mark" href="#async_hooks_asynclocalstorage_enterwith_store" id="async_hooks_asynclocalstorage_enterwith_store">#</a></h3>
<div class="api_metadata">
Added in: v12.17.0
</div>
<ul>
<li><code>store</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type">&#x3C;any></a></li>
</ul>
Calling <code>asyncLocalStorage.enterWith(store)</code> will transition into the context
for the remainder of the current synchronous execution and will persist
through any following asynchronous calls.
Example:
<pre><code class="language-js">const store = { id: 1 };
asyncLocalStorage.enterWith(store);
asyncLocalStorage.getStore(); // Returns the store object
someAsyncOperation(() => {
 asyncLocalStorage.getStore(); // Returns the same object
});</code></pre>
This transition will continue for the entire synchronous execution.
This means that if, for example, the context is entered within an event
handler subsequent event handlers will also run within that context unless
specifically bound to another context with an <code>AsyncResource</code>.
<pre><code class="language-js">const store = { id: 1 };

emitter.on('my-event', () => {
 asyncLocalStorage.enterWith(store);
});
emitter.on('my-event', () => {
 asyncLocalStorage.getStore(); // Returns the same object
});

asyncLocalStorage.getStore(); // Returns undefined
emitter.emit('my-event');
asyncLocalStorage.getStore(); // Returns the same object</code></pre>
<h3><code>asyncLocalStorage.run(store, callback[, ...args])</code><a class="mark" href="#async_hooks_asynclocalstorage_run_store_callback_args" id="async_hooks_asynclocalstorage_run_store_callback_args">#</a></h3>
<div class="api_metadata">
Added in: v12.17.0
</div>
<ul>
<li><code>store</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type">&#x3C;any></a></li>
<li><code>callback</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a></li>
<li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type">&#x3C;any></a></li>
</ul>
This methods runs a function synchronously within a context and return its
return value. The store is not accessible outside of the callback function or
the asynchronous operations created within the callback.
Optionally, arguments can be passed to the function. They will be passed to
the callback function.
If the callback function throws an error, it will be thrown by <code>run</code> too.
The stacktrace will not be impacted by this call and the context will
be exited.
Example:
<pre><code class="language-js">const store = { id: 2 };
try {
 asyncLocalStorage.run(store, () => {
 asyncLocalStorage.getStore(); // Returns the store object
 throw new Error();
 });
} catch (e) {
 asyncLocalStorage.getStore(); // Returns undefined
 // The error will be caught here
}</code></pre>
<h3><code>asyncLocalStorage.exit(callback[, ...args])</code><a class="mark" href="#async_hooks_asynclocalstorage_exit_callback_args" id="async_hooks_asynclocalstorage_exit_callback_args">#</a></h3>
<div class="api_metadata">
Added in: v12.17.0
</div>
<ul>
<li><code>callback</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type">&#x3C;Function></a></li>
<li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type">&#x3C;any></a></li>
</ul>
This methods runs a function synchronously outside of a context and return its
return value. The store is not accessible within the callback function or
the asynchronous operations created within the callback.
Optionally, arguments can be passed to the function. They will be passed to
the callback function.
If the callback function throws an error, it will be thrown by <code>exit</code> too.
The stacktrace will not be impacted by this call and
the context will be re-entered.
Example:
<pre><code class="language-js">// Within a call to run
try {
 asyncLocalStorage.getStore(); // Returns the store object or value
 asyncLocalStorage.exit(() => {
 asyncLocalStorage.getStore(); // Returns undefined
 throw new Error();
 });
} catch (e) {
 asyncLocalStorage.getStore(); // Returns the same object or value
 // The error will be caught here
}</code></pre>
<h3>Usage with <code>async/await</code><a class="mark" href="#async_hooks_usage_with_async_await" id="async_hooks_usage_with_async_await">#</a></h3>
If, within an async function, only one <code>await</code> call is to run within a context,
the following pattern should be used:
<pre><code class="language-js">async function fn() {
 await asyncLocalStorage.run(new Map(), () => {
 asyncLocalStorage.getStore().set('key', value);
 return foo(); // The return value of foo will be awaited
 });
}</code></pre>
In this example, the store is only available in the callback function and the
functions called by <code>foo</code>. Outside of <code>run</code>, calling <code>getStore</code> will return
<code>undefined</code>.
<h3>Troubleshooting<a class="mark" href="#async_hooks_troubleshooting" id="async_hooks_troubleshooting">#</a></h3>
In most cases your application or library code should have no issues with
<code>AsyncLocalStorage</code>. But in rare cases you may face situations when the
current store is lost in one of asynchronous operations. In those cases,
consider the following options.
If your code is callback-based, it is enough to promisify it with
<a href="util.html#util_util_promisify_original"><code>util.promisify()</code></a>, so it starts working with native promises.
If you need to keep using callback-based API, or your code assumes
a custom thenable implementation, use the <a href="#async_hooks_class_asyncresource"><code>AsyncResource</code></a> class
to associate the asynchronous operation with the correct execution context.
 
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