---
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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