---
title: AsyncLocalStorage
description: Use the Node.js AsyncLocalStorage API in Cloudflare Workers to maintain context across asynchronous operations.
image: https://developers.cloudflare.com/dev-products-preview.png
---

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# AsyncLocalStorage

## Background

Note

To enable built-in Node.js APIs and polyfills, add the nodejs\_compat compatibility flag to your [Wrangler configuration file](https://developers.cloudflare.com/workers/wrangler/configuration/). This also enables nodejs\_compat\_v2 as long as your compatibility date is 2024-09-23 or later. [Learn more about the Node.js compatibility flag and v2](https://developers.cloudflare.com/workers/configuration/compatibility-flags/#nodejs-compatibility-flag).

Cloudflare Workers provides an implementation of a subset of the Node.js [AsyncLocalStorage ↗](https://nodejs.org/dist/latest-v18.x/docs/api/async%5Fcontext.html#class-asynclocalstorage) API for creating in-memory stores that remain coherent through asynchronous operations.

## Constructor

JavaScript

```
import { AsyncLocalStorage } from "node:async_hooks";
const asyncLocalStorage = new AsyncLocalStorage();
```

* `new AsyncLocalStorage()` : AsyncLocalStorage  
  * Returns a new `AsyncLocalStorage` instance.

## Methods

* `getStore()` : any

  * Returns the current store. If called outside of an asynchronous context initialized by calling `asyncLocalStorage.run()`, it returns `undefined`.
* `run(storeany, callbackfunction, ...argsarguments)` : any

  * Runs a function synchronously within a context and returns its return value. The store is not accessible outside of the callback function. The store is accessible to any asynchronous operations created within the callback. The optional `args` are passed to the callback function. If the callback function throws an error, the error is thrown by `run()` also.
* `exit(callbackfunction, ...argsarguments)` : any

  * Runs a function synchronously outside of a context and returns its return value. This method is equivalent to calling `run()` with the `store` value set to `undefined`.

## Static Methods

* `AsyncLocalStorage.bind(fn)` : function

  * Captures the asynchronous context that is current when `bind()` is called and returns a function that enters that context before calling the passed in function.
* `AsyncLocalStorage.snapshot()` : function

  * Captures the asynchronous context that is current when `snapshot()` is called and returns a function that enters that context before calling a given function.

## Examples

### Fetch Listener

JavaScript

```
import { AsyncLocalStorage } from 'node:async_hooks';
const asyncLocalStorage = new AsyncLocalStorage();let idSeq = 0;
export default {  async fetch(req) {    return asyncLocalStorage.run(idSeq++, () => {      // Simulate some async activity...      await scheduler.wait(1000);      return new Response(asyncLocalStorage.getStore());    });  }};
```

### Multiple stores

The API supports multiple `AsyncLocalStorage` instances to be used concurrently.

JavaScript

```
import { AsyncLocalStorage } from 'node:async_hooks';
const als1 = new AsyncLocalStorage();const als2 = new AsyncLocalStorage();
export default {  async fetch(req) {    return als1.run(123, () => {      return als2.run(321, () => {        // Simulate some async activity...        await scheduler.wait(1000);        return new Response(`${als1.getStore()}-${als2.getStore()}`);      });    });  }};
```

### Unhandled Rejections

When a `Promise` rejects and the rejection is unhandled, the async context propagates to the `'unhandledrejection'` event handler:

JavaScript

```
import { AsyncLocalStorage } from "node:async_hooks";
const asyncLocalStorage = new AsyncLocalStorage();let idSeq = 0;
addEventListener("unhandledrejection", (event) => {  console.log(asyncLocalStorage.getStore(), "unhandled rejection!");});
export default {  async fetch(req) {    return asyncLocalStorage.run(idSeq++, () => {      // Cause an unhandled rejection!      throw new Error("boom");    });  },};
```

### `AsyncLocalStorage.bind()` and `AsyncLocalStorage.snapshot()`

JavaScript

```
import { AsyncLocalStorage } from "node:async_hooks";
const als = new AsyncLocalStorage();
function foo() {  console.log(als.getStore());}function bar() {  console.log(als.getStore());}
const oneFoo = als.run(123, () => AsyncLocalStorage.bind(foo));oneFoo(); // prints 123
const snapshot = als.run("abc", () => AsyncLocalStorage.snapshot());snapshot(foo); // prints 'abc'snapshot(bar); // prints 'abc'
```

JavaScript

```
import { AsyncLocalStorage } from "node:async_hooks";
const als = new AsyncLocalStorage();
class MyResource {  #runInAsyncScope = AsyncLocalStorage.snapshot();
  doSomething() {    this.#runInAsyncScope(() => {      return als.getStore();    });  }}
const myResource = als.run(123, () => new MyResource());console.log(myResource.doSomething()); // prints 123
```

## `AsyncResource`

The [AsyncResource ↗](https://nodejs.org/dist/latest-v18.x/docs/api/async%5Fcontext.html#class-asyncresource) class is a component of Node.js' async context tracking API that allows users to create their own async contexts. Objects that extend from `AsyncResource` are capable of propagating the async context in much the same way as promises.

Note that `AsyncLocalStorage.snapshot()` and `AsyncLocalStorage.bind()` provide a better approach. `AsyncResource` is provided solely for backwards compatibility with Node.js.

### Constructor

JavaScript

```
import { AsyncResource, AsyncLocalStorage } from "node:async_hooks";
const als = new AsyncLocalStorage();
class MyResource extends AsyncResource {  constructor() {    // The type string is required by Node.js but unused in Workers.    super("MyResource");  }
  doSomething() {    this.runInAsyncScope(() => {      return als.getStore();    });  }}
const myResource = als.run(123, () => new MyResource());console.log(myResource.doSomething()); // prints 123
```

* `new AsyncResource(typestring, optionsAsyncResourceOptions)` : AsyncResource

  * Returns a new `AsyncResource`. Importantly, while the constructor arguments are required in Node.js' implementation of `AsyncResource`, they are not used in Workers.
* `AsyncResource.bind(fnfunction, typestring, thisArgany)`  
  * Binds the given function to the current async context.

### Methods

* `asyncResource.bind(fnfunction, thisArgany)`  
  * Binds the given function to the async context associated with this `AsyncResource`.
* `asyncResource.runInAsyncScope(fnfunction, thisArgany, ...argsarguments)`  
  * Call the provided function with the given arguments in the async context associated with this `AsyncResource`.

## Caveats

* The `AsyncLocalStorage` implementation provided by Workers intentionally omits support for the [asyncLocalStorage.enterWith() ↗](https://nodejs.org/dist/latest-v18.x/docs/api/async%5Fcontext.html#asynclocalstorageenterwithstore) and [asyncLocalStorage.disable() ↗](https://nodejs.org/dist/latest-v18.x/docs/api/async%5Fcontext.html#asynclocalstoragedisable) methods.
* Workers does not implement the full [async\_hooks ↗](https://nodejs.org/dist/latest-v18.x/docs/api/async%5Fhooks.html) API upon which Node.js' implementation of `AsyncLocalStorage` is built.
* Workers does not implement the ability to create an `AsyncResource` with an explicitly identified trigger context as allowed by Node.js. This means that a new `AsyncResource` will always be bound to the async context in which it was created.
* Thenables (non-Promise objects that expose a `then()` method) are not fully supported when using `AsyncLocalStorage`. When working with thenables, instead use [AsyncLocalStorage.snapshot() ↗](https://nodejs.org/api/async%5Fcontext.html#static-method-asynclocalstoragesnapshot) to capture a snapshot of the current context.

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