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What is undici and why is the global fetch built on it in modern Node.js?

Q: What is undici and why is the global fetch built on it in modern Node.js?

**TL;DR.** **undici** is a fast, from-scratch **HTTP/1.1 client** written for Node, and the **global fetch ** is implemented on top of it. It provides **connection pooling** ( Agent / Pool ), lower overhead than the legacy http client, and high-throughput APIs ( request , stream , pipeline ). fetch() undici pooled keep-alive <path class='d-edge-accent' d='M304 77 H352' marker-end='

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Understand the problem

Node's from-scratch HTTP/1.1 client behind global fetch.

nodejshttpundicifetch

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Read the code

Tune fetch's pool + mock it in tests

import { Agent, setGlobalDispatcher, MockAgent } from 'undici';

// Bigger keep-alive pool for a chatty service:
setGlobalDispatcher(new Agent({ connections: 128, keepAliveTimeout: 10_000 }));
const res = await fetch('https://api.example.com/data');

// In tests, intercept without real network:
const mock = new MockAgent();
mock.get('https://api.example.com').intercept({ path: '/data' }).reply(200, { ok: true });

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