bun-jest-migration

// 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";

// 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');

// 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')
}));

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

// 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

// Jest
expect(component).toMatchSnapshot();
expect(data).toMatchInlineSnapshot(`"expected"`);
 
// Bun (identical)
expect(component).toMatchSnapshot();
expect(data).toMatchInlineSnapshot(`"expected"`);

bun test --update-snapshots

// jest.config.js (before)
module.exports = {
  testMatch: ['**/*.test.ts'],
  testTimeout: 10000,
  setupFilesAfterEnv: ['./jest.setup.ts'],
  collectCoverage: true,
  coverageThreshold: { global: { lines: 80 } }
};

# bunfig.toml (after)
[test]
root = "./"
preload = ["./jest.setup.ts"]
timeout = 10000
coverage = true
coverageThreshold = 0.8

// Jest mock hoisting doesn't exist in Bun
// Move mock.module before imports or use dynamic imports
 
// Solution 1: Use mock.module at top
mock.module('./api', () => ({ fetch: mock() }));
import { fetch } from './api';
 
// Solution 2: Dynamic import
const mockFetch = mock();
mock.module('./api', () => ({ fetch: mockFetch }));
const { fetch } = await import('./api');

// Bun timer support is limited
// Use setSystemTime for date mocking
import { setSystemTime } from "bun:test";
 
beforeEach(() => {
  setSystemTime(new Date('2024-01-01'));
});
 
afterEach(() => {
  setSystemTime(); // Reset to real time
});

// Jest
expect.extend({ toBeWithinRange(received, floor, ceiling) {...} });
 
// Bun (same API)
import { expect } from "bun:test";
expect.extend({
  toBeWithinRange(received, floor, ceiling) {
    const pass = received >= floor && received <= ceiling;
    return {
      pass,
      message: () => `expected ${received} to be within ${floor}-${ceiling}`
    };
  }
});