Migration from Jest¶

rjest is designed as a drop-in replacement for Jest. Most projects can migrate with minimal changes.

Compatibility¶

Fully Supported¶

All standard matchers (toBe, toEqual, toContain, etc.)
Mock functions (jest.fn(), jest.spyOn())
Snapshot testing
Async testing (Promises, async/await)
describe, test, it blocks
beforeAll, afterAll, beforeEach, afterEach hooks
--testNamePattern / -t filtering
JSON output (--json)
TypeScript and JSX out of the box

Partially Supported¶

Feature	Status	Notes
`jest.mock()`	Basic	Module mocking works, factory functions supported
Fake timers	Basic	`useFakeTimers()`, `advanceTimersByTime()`
Coverage	Not yet	Code coverage not implemented
Watch mode	Basic	File watching works, interactive mode limited

Not Yet Supported¶

Code coverage (--coverage)
Interactive watch mode commands
Custom reporters
jest.requireActual()
Module reset between tests
testEnvironment options (jsdom, etc.)

Migration Steps¶

Step 1: Install rjest¶

# Build from source
git clone https://github.com/user/rjest.git
cd rjest
cargo build --release

# Add to PATH
export PATH="$PATH:$(pwd)/target/release"

Step 2: Verify Configuration¶

rjest reads your existing jest.config.js:

jest.config.js

module.exports = {
  // These options are supported
  testMatch: ['**/*.test.ts', '**/*.test.tsx'],
  moduleFileExtensions: ['ts', 'tsx', 'js', 'jsx', 'json'],
  testPathIgnorePatterns: ['/node_modules/'],

  // These can be removed (built-in to rjest)
  // transform: { ... },  // Not needed - native SWC
  // preset: 'ts-jest',   // Not needed
};

Step 3: Remove Transform Configuration¶

rjest has built-in TypeScript/JSX support via SWC. Remove these:

jest.config.js

module.exports = {
  // REMOVE these - not needed with rjest
  // transform: {
  //   '^.+\\.tsx?$': 'ts-jest',
  // },
  // preset: 'ts-jest',
  // globals: {
  //   'ts-jest': { ... }
  // },
};

Step 4: Run Tests¶

# Run all tests
jest

# If you see issues, try with verbose output
jest --verbose

Common Migration Issues¶

Issue: "Cannot use import statement outside a module"¶

This error means rjest isn't transforming your TypeScript files.

Solution: Ensure the daemon is running with the latest code:

jest --daemon-stop
jest

Issue: Tests Pass in Jest but Fail in rjest¶

Check if you're using unsupported features:

// These may not work yet
jest.requireActual('./module');
jest.isolateModules(() => { ... });

Issue: Mock Not Working¶

Ensure you're using supported mock patterns:

// Supported
const mockFn = jest.fn();
const mockFn = jest.fn().mockReturnValue(42);
const mockFn = jest.fn().mockResolvedValue('data');

// Supported
jest.spyOn(object, 'method');

// Basic support
jest.mock('./module');

Issue: Snapshot Mismatch¶

Snapshot format is compatible, but if you see mismatches:

# Update snapshots
jest -u

Running Both Jest and rjest¶

During migration, you can run both:

package.json

{
  "scripts": {
    "test": "jest",
    "test:fast": "rjest"
  }
}

Performance Comparison¶

After migration, you should see significant speedups:

Scenario	Jest	rjest	Speedup
Cold start	~14s	~9s	1.5x
Warm run	~14s	~100ms	95x

Getting Help¶

If you encounter issues:

Check Troubleshooting
Run with RUST_LOG=debug jest for verbose output
File an issue on GitHub