---
title: util
description: Use the Node.js util module in Workers for promisify, callbackify, types, and MIMEType support.
image: https://developers.cloudflare.com/dev-products-preview.png
---

> Documentation Index  
> Fetch the complete documentation index at: https://developers.cloudflare.com/workers/llms.txt  
> Use this file to discover all available pages before exploring further.

[Skip to content](#%5Ftop) 

# util

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).

## promisify/callbackify

The `promisify` and `callbackify` APIs in Node.js provide a means of bridging between a Promise-based programming model and a callback-based model.

The `promisify` method allows taking a Node.js-style callback function and converting it into a Promise-returning async function:

JavaScript

```

import { promisify } from "node:util";


function foo(args, callback) {

  try {

    callback(null, 1);

  } catch (err) {

    // Errors are emitted to the callback via the first argument.

    callback(err);

  }

}


const promisifiedFoo = promisify(foo);

await promisifiedFoo(args);


```

Similarly to `promisify`, `callbackify` converts a Promise-returning async function into a Node.js-style callback function:

JavaScript

```

import { callbackify } from 'node:util';


async function foo(args) {

  throw new Error('boom');

}


const callbackifiedFoo = callbackify(foo);


callbackifiedFoo(args, (err, value) => {

  if (err) throw err;

});


```

`callbackify` and `promisify` make it easy to handle all of the challenges that come with bridging between callbacks and promises.

Refer to the [Node.js documentation for callbackify ↗](https://nodejs.org/dist/latest-v19.x/docs/api/util.html#utilcallbackifyoriginal) and [Node.js documentation for promisify ↗](https://nodejs.org/dist/latest-v19.x/docs/api/util.html#utilpromisifyoriginal) for more information.

## util.types

The `util.types` API provides a reliable and efficient way of checking that values are instances of various built-in types.

JavaScript

```

import { types } from "node:util";


types.isAnyArrayBuffer(new ArrayBuffer()); // Returns true

types.isAnyArrayBuffer(new SharedArrayBuffer()); // Returns true

types.isArrayBufferView(new Int8Array()); // true

types.isArrayBufferView(Buffer.from("hello world")); // true

types.isArrayBufferView(new DataView(new ArrayBuffer(16))); // true

types.isArrayBufferView(new ArrayBuffer()); // false

function foo() {

  types.isArgumentsObject(arguments); // Returns true

}

types.isAsyncFunction(function foo() {}); // Returns false

types.isAsyncFunction(async function foo() {}); // Returns true

// .. and so on


```

Warning

The Workers implementation currently does not provide implementations of the `util.types.isExternal()`, `util.types.isProxy()`, `util.types.isKeyObject()`, or `util.type.isWebAssemblyCompiledModule()` APIs.

For more about `util.types`, refer to the [Node.js documentation for util.types ↗](https://nodejs.org/dist/latest-v19.x/docs/api/util.html#utiltypes).

## util.MIMEType

`util.MIMEType` provides convenience methods that allow you to more easily work with and manipulate [MIME types ↗](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics%5Fof%5FHTTP/MIME%5Ftypes). For example:

JavaScript

```

import { MIMEType } from "node:util";


const myMIME = new MIMEType("text/javascript;key=value");


console.log(myMIME.type);

// Prints: text


console.log(myMIME.essence);

// Prints: text/javascript


console.log(myMIME.subtype);

// Prints: javascript


console.log(String(myMIME));

// Prints: application/javascript;key=value


```

For more about `util.MIMEType`, refer to the [Node.js documentation for util.MIMEType ↗](https://nodejs.org/api/util.html#class-utilmimetype).

```json
{"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"item":{"@id":"/directory/","name":"Directory"}},{"@type":"ListItem","position":2,"item":{"@id":"/workers/","name":"Workers"}},{"@type":"ListItem","position":3,"item":{"@id":"/workers/runtime-apis/","name":"Runtime APIs"}},{"@type":"ListItem","position":4,"item":{"@id":"/workers/runtime-apis/nodejs/","name":"Node.js compatibility"}},{"@type":"ListItem","position":5,"item":{"@id":"/workers/runtime-apis/nodejs/util/","name":"util"}}]}
```