---
title: util · Cloudflare Workers docs
description: The promisify and callbackify APIs in Node.js provide a means of bridging between a Promise-based programming model and a callback-based model.
lastUpdated: 2025-02-19T14:52:46.000Z
source_url:
  html: https://da735954.preview.developers.cloudflare.com/workers/runtime-apis/nodejs/util/index.md
  md: https://da735954.preview.developers.cloudflare.com/workers/runtime-apis/nodejs/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:

```js
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:

```js
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.

```js
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_of_HTTP/MIME_types). For example:

```js
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).
