Global Installation

Undici provides an install() function to add fetch-related and other web API classes to globalThis, making them available globally without requiring imports.

install(): void

Install undici's global web APIs on globalThis.

Example:

import { install } from 'undici'

// Install undici's global web APIs
install()

// Now you can use fetch classes globally without importing
const response = await fetch('https://api.example.com/data')
const data = await response.json()

// All classes are available globally:
const headers = new Headers([['content-type', 'application/json']])
const request = new Request('https://example.com')
const formData = new FormData()
const ws = new WebSocket('wss://example.com')
const eventSource = new EventSource('https://example.com/events')

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

If you send a FormData body, use matching implementations for fetch and FormData.

These two patterns are safe:

// Built-in globals from Node.js
const body = new FormData()
await fetch('https://example.com', {
  method: 'POST',
  body
})

After install(), fetch, Headers, Response, Request, and FormData all come from the installed undici package, so they work as a matching set. WebSocket, CloseEvent, ErrorEvent, MessageEvent, and EventSource also come from the installed undici package.

If you do not want to install globals, import both from undici instead:

import { fetch, FormData } from 'undici'

const body = new FormData()
await fetch('https://example.com', {
  method: 'POST',
  body
})

Avoid mixing a global FormData with undici.fetch(), or undici.FormData with the built-in global fetch(). Keeping them paired avoids surprising multipart behavior across Node.js and undici versions.

Global installation is useful for:

• Polyfilling environments that don't have native fetch support
• Ensuring consistent behavior across different Node.js versions
• Library compatibility when third-party libraries expect global fetch
• Migration scenarios where you want to replace built-in implementations
• Testing environments where you need predictable fetch behavior

import { install } from 'undici'

// Check if fetch is available and install if needed
if (typeof globalThis.fetch === 'undefined') {
  install()
  console.log('Undici fetch installed globally')
}

// Now fetch is guaranteed to be available
const response = await fetch('https://api.example.com')

import { install } from 'undici'

// In test setup, ensure consistent fetch behavior
install()

// Now all tests use undici's implementations
test('fetch API test', async () => {
  const response = await fetch('https://example.com')
  expect(response).toBeInstanceOf(Response)
})

• The install() function overwrites any existing global implementations
• Classes installed are undici's implementations, not Node.js built-ins
• This provides access to undici's latest fetch, WebSocket, and EventSource features and performance improvements
• The global installation persists for the lifetime of the process