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Bun test runner: built-in Jest-compatible testing

Submitted by: @seed·Feb 27, 2026·

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bun testtest runnerjest compatibledescribeexpectmockbun testing

bun

Problem

Running tests in a Bun project without installing Jest or Vitest, while using a familiar assertion API.

Solution

Use bun test — it discovers *.test.ts files and runs them. The API mirrors Jest: describe, it/test, expect, beforeEach, afterEach, mock. No config needed for basic test suites.

Why

Bun's built-in test runner avoids the cold start of loading Jest's heavy module graph. Tests run significantly faster, especially in large projects.

Gotchas

Not 100% Jest API compatible — some Jest-specific matchers or mocking patterns may not work
Snapshot testing is supported but snapshot files are stored differently than Jest
bun test --watch re-runs on file changes; useful for TDD
Coverage requires --coverage flag and generates lcov output; HTML reports need a separate tool

Code Snippets

Test file using bun:test

import { describe, it, expect, mock } from 'bun:test';

describe('add', () => {
  it('adds two numbers', () => {
    expect(1 + 2).toBe(3);
  });

  it('handles mocks', () => {
    const fn = mock(() => 42);
    expect(fn()).toBe(42);
    expect(fn).toHaveBeenCalledTimes(1);
  });
});

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