undici

An HTTP/1.1 client, written from scratch for Node.js.

Undici means eleven in Italian. 1.1 -> 11 -> Eleven -> Undici. It is also a Stranger Things reference.

How to get involved

Have a question about using Undici? Open a Q&A Discussion or join our official OpenJS Slack channel.

Looking to contribute? Start by reading the contributing guide

Install

npm i undici

Benchmarks

The benchmark is a simple getting data example using a 50 TCP connections with a pipelining depth of 10 running on Node 22.11.0.

┌────────────────────────┬─────────┬────────────────────┬────────────┬─────────────────────────┐
│  Tests                 │ Samples │ Result             │ Tolerance  │ Difference with slowest │
├────────────────────────┼─────────┼────────────────────┼────────────┼─────────────────────────┤
│  'axios'               │ 15      │ '5708.26 req/sec'  │ '± 2.91 %' │ '-'                     │
│  'http - no keepalive' │ 10      │ '5809.80 req/sec'  │ '± 2.30 %' │ '+ 1.78 %'              │
│  'request'             │ 30      │ '5828.80 req/sec'  │ '± 2.91 %' │ '+ 2.11 %'              │
│  'undici - fetch'      │ 40      │ '5903.78 req/sec'  │ '± 2.87 %' │ '+ 3.43 %'              │
│  'node-fetch'          │ 10      │ '5945.40 req/sec'  │ '± 2.13 %' │ '+ 4.15 %'              │
│  'got'                 │ 35      │ '6511.45 req/sec'  │ '± 2.84 %' │ '+ 14.07 %'             │
│  'http - keepalive'    │ 65      │ '9193.24 req/sec'  │ '± 2.92 %' │ '+ 61.05 %'             │
│  'superagent'          │ 35      │ '9339.43 req/sec'  │ '± 2.95 %' │ '+ 63.61 %'             │
│  'undici - pipeline'   │ 50      │ '13364.62 req/sec' │ '± 2.93 %' │ '+ 134.13 %'            │
│  'undici - stream'     │ 95      │ '18245.36 req/sec' │ '± 2.99 %' │ '+ 219.63 %'            │
│  'undici - request'    │ 50      │ '18340.17 req/sec' │ '± 2.84 %' │ '+ 221.29 %'            │
│  'undici - dispatch'   │ 40      │ '22234.42 req/sec' │ '± 2.94 %' │ '+ 289.51 %'            │
└────────────────────────┴─────────┴────────────────────┴────────────┴─────────────────────────┘

Undici vs. Fetch

Overview

Node.js includes a built-in fetch() implementation powered by undici starting from Node.js v18. However, there are important differences between using the built-in fetch and installing undici as a separate module.

Built-in Fetch (Node.js v18+)

Node.js's built-in fetch is powered by a bundled version of undici:

1// Available globally in Node.js v18+
2const response = await fetch('https://api.example.com/data');
3const data = await response.json();
4
5// Check the bundled undici version
6console.log(process.versions.undici); // e.g., "5.28.4"

Pros:

• No additional dependencies required
• Works across different JavaScript runtimes
• Automatic compression handling (gzip, deflate, br)
• Built-in caching support (in development)

Cons:

• Limited to the undici version bundled with your Node.js version
• Less control over connection pooling and advanced features
• Error handling follows Web API standards (errors wrapped in TypeError)
• Performance overhead due to Web Streams implementation

Undici Module

Installing undici as a separate module gives you access to the latest features and APIs:

1npm install undici

1import { request, fetch, Agent, setGlobalDispatcher } from 'undici';
2
3// Use undici.request for maximum performance
4const { statusCode, headers, body } = await request('https://api.example.com/data');
5const data = await body.json();
6
7// Or use undici.fetch with custom configuration
8const agent = new Agent({ keepAliveTimeout: 10000 });
9setGlobalDispatcher(agent);
10const response = await fetch('https://api.example.com/data');

Pros:

• Latest undici features and bug fixes
• Access to advanced APIs (request, stream, pipeline)
• Fine-grained control over connection pooling
• Better error handling with clearer error messages
• Superior performance, especially with undici.request
• HTTP/1.1 pipelining support
• Custom interceptors and middleware
• Advanced features like ProxyAgent, MockAgent

Cons:

• Additional dependency to manage
• Larger bundle size

When to Use Each

Use Built-in Fetch When:

• You want zero dependencies
• Building isomorphic code that runs in browsers and Node.js
• Publishing to npm and want to maximize compatibility with JS runtimes
• Simple HTTP requests without advanced configuration
• You're publishing to npm and you want to maximize compatiblity
• You don't depend on features from a specific version of undici

Use Undici Module When:

• You need the latest undici features and performance improvements
• You require advanced connection pooling configuration
• You need APIs not available in the built-in fetch (ProxyAgent, MockAgent, etc.)
• Performance is critical (use undici.request for maximum speed)
• You want better error handling and debugging capabilities
• You need HTTP/1.1 pipelining or advanced interceptors
• You prefer decoupled protocol and API interfaces

Performance Comparison

Based on benchmarks, here's the typical performance hierarchy:

1. undici.request() - Fastest, most efficient
2. undici.fetch() - Good performance, standard compliance
3. Node.js http/https - Baseline performance

Migration Guide

If you're currently using built-in fetch and want to migrate to undici:

1// Before: Built-in fetch
2const response = await fetch('https://api.example.com/data');
3
4// After: Undici fetch (drop-in replacement)
5import { fetch } from 'undici';
6const response = await fetch('https://api.example.com/data');
7
8// Or: Undici request (better performance)
9import { request } from 'undici';
10const { statusCode, body } = await request('https://api.example.com/data');
11const data = await body.json();

Version Compatibility

You can check which version of undici is bundled with your Node.js version:

1console.log(process.versions.undici);

Installing undici as a module allows you to use a newer version than what's bundled with Node.js, giving you access to the latest features and performance improvements.

Quick Start

1import { request } from 'undici'
2
3const {
4  statusCode,
5  headers,
6  trailers,
7  body
8} = await request('http://localhost:3000/foo')
9
10console.log('response received', statusCode)
11console.log('headers', headers)
12
13for await (const data of body) { console.log('data', data) }
14
15console.log('trailers', trailers)

Global Installation

Undici provides an install() function to add all WHATWG fetch classes to globalThis, making them available globally:

1import { install } from 'undici'
2
3// Install all WHATWG fetch classes globally
4install()
5
6// Now you can use fetch classes globally without importing
7const response = await fetch('https://api.example.com/data')
8const data = await response.json()
9
10// All classes are available globally:
11const headers = new Headers([['content-type', 'application/json']])
12const request = new Request('https://example.com')
13const formData = new FormData()
14const ws = new WebSocket('wss://example.com')
15const eventSource = new EventSource('https://example.com/events')

The install() function adds the following classes to globalThis:

• fetch - The fetch function
• Headers - HTTP headers management
• Response - HTTP response representation
• Request - HTTP request representation
• FormData - Form data handling
• WebSocket - WebSocket client
• CloseEvent, ErrorEvent, MessageEvent - WebSocket events
• EventSource - Server-sent events client

This is useful for:

• Polyfilling environments that don't have fetch
• Ensuring consistent fetch behavior across different Node.js versions
• Making undici's implementations available globally for libraries that expect them

Body Mixins

The body mixins are the most common way to format the request/response body. Mixins include:

• .arrayBuffer()
• .blob()
• .bytes()
• .json()
• .text()

[!NOTE] The body returned from undici.request does not implement .formData().

Example usage:

1import { request } from 'undici'
2
3const {
4  statusCode,
5  headers,
6  trailers,
7  body
8} = await request('http://localhost:3000/foo')
9
10console.log('response received', statusCode)
11console.log('headers', headers)
12console.log('data', await body.json())
13console.log('trailers', trailers)

Note: Once a mixin has been called then the body cannot be reused, thus calling additional mixins on .body, e.g. .body.json(); .body.text() will result in an error TypeError: unusable being thrown and returned through the Promise rejection.

Should you need to access the body in plain-text after using a mixin, the best practice is to use the .text() mixin first and then manually parse the text to the desired format.

For more information about their behavior, please reference the body mixin from the Fetch Standard.

Common API Methods

This section documents our most commonly used API methods. Additional APIs are documented in their own files within the docs folder and are accessible via the navigation list on the left side of the docs site.

undici.request([url, options]): Promise

Arguments:

• url string | URL | UrlObject
• options RequestOptions
  • dispatcher Dispatcher - Default: getGlobalDispatcher
  • method String - Default: PUT if options.body, otherwise GET

Returns a promise with the result of the Dispatcher.request method.

Calls options.dispatcher.request(options).

See Dispatcher.request for more details, and request examples for examples.

undici.stream([url, options, ]factory): Promise

Arguments:

• url string | URL | UrlObject
• options StreamOptions
  • dispatcher Dispatcher - Default: getGlobalDispatcher
  • method String - Default: PUT if options.body, otherwise GET
• factory Dispatcher.stream.factory

Returns a promise with the result of the Dispatcher.stream method.

Calls options.dispatcher.stream(options, factory).

See Dispatcher.stream for more details.

undici.pipeline([url, options, ]handler): Duplex

Arguments:

• url string | URL | UrlObject
• options PipelineOptions
  • dispatcher Dispatcher - Default: getGlobalDispatcher
  • method String - Default: PUT if options.body, otherwise GET
• handler Dispatcher.pipeline.handler

Returns: stream.Duplex

Calls options.dispatch.pipeline(options, handler).

See Dispatcher.pipeline for more details.

undici.connect([url, options]): Promise

Starts two-way communications with the requested resource using HTTP CONNECT.

Arguments:

• url string | URL | UrlObject
• options ConnectOptions
  • dispatcher Dispatcher - Default: getGlobalDispatcher
• callback (err: Error | null, data: ConnectData | null) => void (optional)

Returns a promise with the result of the Dispatcher.connect method.

Calls options.dispatch.connect(options).

See Dispatcher.connect for more details.

undici.fetch(input[, init]): Promise

Implements fetch.

• https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch
• https://fetch.spec.whatwg.org/#fetch-method

Basic usage example:

1import { fetch } from 'undici'
2
3
4const res = await fetch('https://example.com')
5const json = await res.json()
6console.log(json)

You can pass an optional dispatcher to fetch as:

1import { fetch, Agent } from 'undici'
2
3const res = await fetch('https://example.com', {
4  // Mocks are also supported
5  dispatcher: new Agent({
6    keepAliveTimeout: 10,
7    keepAliveMaxTimeout: 10
8  })
9})
10const json = await res.json()
11console.log(json)

request.body

A body can be of the following types:

• ArrayBuffer
• ArrayBufferView
• AsyncIterables
• Blob
• Iterables
• String
• URLSearchParams
• FormData

In this implementation of fetch, request.body now accepts Async Iterables. It is not present in the Fetch Standard.

1import { fetch } from 'undici'
2
3const data = {
4  async *[Symbol.asyncIterator]() {
5    yield 'hello'
6    yield 'world'
7  },
8}
9
10await fetch('https://example.com', { body: data, method: 'POST', duplex: 'half' })

FormData besides text data and buffers can also utilize streams via Blob objects:

1import { openAsBlob } from 'node:fs'
2
3const file = await openAsBlob('./big.csv')
4const body = new FormData()
5body.set('file', file, 'big.csv')
6
7await fetch('http://example.com', { method: 'POST', body })

request.duplex

• 'half'

In this implementation of fetch, request.duplex must be set if request.body is ReadableStream or Async Iterables, however, even though the value must be set to 'half', it is actually a full duplex. For more detail refer to the Fetch Standard.

response.body

Nodejs has two kinds of streams: web streams, which follow the API of the WHATWG web standard found in browsers, and an older Node-specific streams API. response.body returns a readable web stream. If you would prefer to work with a Node stream you can convert a web stream using .fromWeb().

1import { fetch } from 'undici'
2import { Readable } from 'node:stream'
3
4const response = await fetch('https://example.com')
5const readableWebStream = response.body
6const readableNodeStream = Readable.fromWeb(readableWebStream)

Specification Compliance

This section documents parts of the HTTP/1.1 and Fetch Standard that Undici does not support or does not fully implement.

CORS

Unlike browsers, Undici does not implement CORS (Cross-Origin Resource Sharing) checks by default. This means:

• No preflight requests are automatically sent for cross-origin requests
• No validation of Access-Control-Allow-Origin headers is performed
• Requests to any origin are allowed regardless of the source

This behavior is intentional for server-side environments where CORS restrictions are typically unnecessary. If your application requires CORS-like protections, you will need to implement these checks manually.

Garbage Collection

• https://fetch.spec.whatwg.org/#garbage-collection

The Fetch Standard allows users to skip consuming the response body by relying on garbage collection to release connection resources. Undici does not do the same. Therefore, it is important to always either consume or cancel the response body.

Garbage collection in Node is less aggressive and deterministic (due to the lack of clear idle periods that browsers have through the rendering refresh rate) which means that leaving the release of connection resources to the garbage collector can lead to excessive connection usage, reduced performance (due to less connection re-use), and even stalls or deadlocks when running out of connections.

1// Do
2const { body, headers } = await fetch(url);
3for await (const chunk of body) {
4  // force consumption of body
5}
6
7// Do not
8const { headers } = await fetch(url);

The same applies for request too:

1// Do
2const { body, headers } = await request(url);
3await res.body.dump(); // force consumption of body
4
5// Do not
6const { headers } = await request(url);

However, if you want to get only headers, it might be better to use HEAD request method. Usage of this method will obviate the need for consumption or cancelling of the response body. See MDN - HTTP - HTTP request methods - HEAD for more details.

1const headers = await fetch(url, { method: 'HEAD' })
2  .then(res => res.headers)

Forbidden and Safelisted Header Names

• https://fetch.spec.whatwg.org/#cors-safelisted-response-header-name
• https://fetch.spec.whatwg.org/#forbidden-header-name
• https://fetch.spec.whatwg.org/#forbidden-response-header-name
• https://github.com/wintercg/fetch/issues/6

The Fetch Standard requires implementations to exclude certain headers from requests and responses. In browser environments, some headers are forbidden so the user agent remains in full control over them. In Undici, these constraints are removed to give more control to the user.

undici.upgrade([url, options]): Promise

Upgrade to a different protocol. See MDN - HTTP - Protocol upgrade mechanism for more details.

Arguments:

• url string | URL | UrlObject
• options UpgradeOptions
  • dispatcher Dispatcher - Default: getGlobalDispatcher
• callback (error: Error | null, data: UpgradeData) => void (optional)

Returns a promise with the result of the Dispatcher.upgrade method.

Calls options.dispatcher.upgrade(options).

See Dispatcher.upgrade for more details.

undici.setGlobalDispatcher(dispatcher)

• dispatcher Dispatcher

Sets the global dispatcher used by Common API Methods. Global dispatcher is shared among compatible undici modules, including undici that is bundled internally with node.js.

undici.getGlobalDispatcher()

Gets the global dispatcher used by Common API Methods.

Returns: Dispatcher

undici.setGlobalOrigin(origin)

• origin string | URL | undefined

Sets the global origin used in fetch.

If undefined is passed, the global origin will be reset. This will cause Response.redirect, new Request(), and fetch to throw an error when a relative path is passed.

1setGlobalOrigin('http://localhost:3000')
2
3const response = await fetch('/api/ping')
4
5console.log(response.url) // http://localhost:3000/api/ping

undici.getGlobalOrigin()

Gets the global origin used in fetch.

Returns: URL

UrlObject

• port string | number (optional)
• path string (optional)
• pathname string (optional)
• hostname string (optional)
• origin string (optional)
• protocol string (optional)
• search string (optional)

Expect

Undici does not support the Expect request header field. The request body is always immediately sent and the 100 Continue response will be ignored.

Refs: https://tools.ietf.org/html/rfc7231#section-5.1.1

Pipelining

Undici will only use pipelining if configured with a pipelining factor greater than 1. Also it is important to pass blocking: false to the request options to properly pipeline requests.

Undici always assumes that connections are persistent and will immediately pipeline requests, without checking whether the connection is persistent. Hence, automatic fallback to HTTP/1.0 or HTTP/1.1 without pipelining is not supported.

Undici will immediately pipeline when retrying requests after a failed connection. However, Undici will not retry the first remaining requests in the prior pipeline and instead error the corresponding callback/promise/stream.

Undici will abort all running requests in the pipeline when any of them are aborted.

• Refs: https://tools.ietf.org/html/rfc2616#section-8.1.2.2
• Refs: https://tools.ietf.org/html/rfc7230#section-6.3.2

Manual Redirect

Since it is not possible to manually follow an HTTP redirect on the server-side, Undici returns the actual response instead of an opaqueredirect filtered one when invoked with a manual redirect. This aligns fetch() with the other implementations in Deno and Cloudflare Workers.

Refs: https://fetch.spec.whatwg.org/#atomic-http-redirect-handling

Workarounds

Network address family autoselection.

If you experience problem when connecting to a remote server that is resolved by your DNS servers to a IPv6 (AAAA record) first, there are chances that your local router or ISP might have problem connecting to IPv6 networks. In that case undici will throw an error with code UND_ERR_CONNECT_TIMEOUT.

If the target server resolves to both a IPv6 and IPv4 (A records) address and you are using a compatible Node version (18.3.0 and above), you can fix the problem by providing the autoSelectFamily option (support by both undici.request and undici.Agent) which will enable the family autoselection algorithm when establishing the connection.

Collaborators

• Daniele Belardi, https://www.npmjs.com/~dnlup
• Ethan Arrowood, https://www.npmjs.com/~ethan_arrowood
• Matteo Collina, https://www.npmjs.com/~matteo.collina
• Matthew Aitken, https://www.npmjs.com/~khaf
• Robert Nagy, https://www.npmjs.com/~ronag
• Szymon Marczak, https://www.npmjs.com/~szmarczak

Past Collaborators

• Tomas Della Vedova, https://www.npmjs.com/~delvedor

Releasers

• Ethan Arrowood, https://www.npmjs.com/~ethan_arrowood
• Matteo Collina, https://www.npmjs.com/~matteo.collina
• Robert Nagy, https://www.npmjs.com/~ronag
• Matthew Aitken, https://www.npmjs.com/~khaf

Long Term Support

Undici aligns with the Node.js LTS schedule. The following table shows the supported versions:

Version	Node.js	End of Life
5.x	v18.x	2024-04-30
6.x	v20.x v22.x	2026-04-30
7.x	v24.x	2027-04-30

License

MIT