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How do you mock modules, timers, and network calls in Node.js tests?

Q: How do you mock modules, timers, and network calls in Node.js tests?

**TL;DR.** Use the runner's mock API ( mock.fn , mock.method , mock.timers ) or **Sinon** to replace dependencies and control time, and **nock** or undici's **MockAgent** to intercept HTTP. Mocking **isolates** the unit under test and makes async/time-dependent code **deterministic**. unit under test HTTP → MockAgent time → fake timers <text

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Isolating units with mocks, fake timers, and HTTP intercept.

nodejstestingmockingtimers

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Fake timers + mocked fetch

import { test, mock } from 'node:test';
import { MockAgent, setGlobalDispatcher } from 'undici';

test('retries after delay', () => {
  mock.timers.enable({ apis: ['setTimeout'] });
  const agent = new MockAgent();
  setGlobalDispatcher(agent);
  agent.get('https://api').intercept({ path: '/x' }).reply(200, { ok: true });

  const p = fetchWithRetry('https://api/x');
  mock.timers.tick(1000);     // advance time instantly
  return p;
});

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