bun-jest-migration

Bun Jest Migration

Bun's test runner is Jest-compatible. Most Jest tests run without changes.

Quick Migration

# 1. Remove Jest dependencies
bun remove jest ts-jest @types/jest babel-jest

# 2. Update test script
# package.json: "test": "bun test"

# 3. Run tests
bun test

Import Changes

// Before (Jest)
import { describe, it, expect, jest } from '@jest/globals';

// After (Bun) - No import needed, or explicit:
import { describe, test, expect, mock, spyOn } from "bun:test";

API Compatibility

Fully Compatible

Jest	Bun	Notes
describe()	describe()	Identical
it() / test()	test()	Use test()
expect()	expect()	Same matchers
beforeAll/Each	beforeAll/Each	Identical
afterAll/Each	afterAll/Each	Identical
jest.fn()	mock()	Use mock()
jest.spyOn()	spyOn()	Identical

Requires Changes

Jest	Bun Equivalent
jest.mock('module')	mock.module('module', () => {...})
jest.useFakeTimers()	import { setSystemTime } from "bun:test"
jest.setTimeout()	Third argument to test()
jest.clearAllMocks()	Call .mockClear() on each mock

Mock Migration

Mock Functions

// Jest
const fn = jest.fn().mockReturnValue('value');

// Bun
const fn = mock(() => 'value');
// Or for compatibility:
import { jest } from "bun:test";
const fn = jest.fn(() => 'value');

Module Mocking

// Jest (top-level hoisting)
jest.mock('./utils', () => ({
  helper: jest.fn(() => 'mocked')
}));

// Bun (inline, no hoisting)
import { mock } from "bun:test";
mock.module('./utils', () => ({
  helper: mock(() => 'mocked')
}));

Spy Migration

// Jest
jest.spyOn(console, 'log').mockImplementation(() => {});

// Bun (identical)
spyOn(console, 'log').mockImplementation(() => {});

Timer Migration

// Jest
jest.useFakeTimers();
jest.setSystemTime(new Date('2024-01-01'));
jest.advanceTimersByTime(1000);

// Bun - supports Jest-compatible timer APIs
import { setSystemTime } from "bun:test";
import { jest } from "bun:test";

jest.useFakeTimers();
jest.setSystemTime(new Date('2024-01-01'));
jest.advanceTimersByTime(1000);  // Now supported