# bun-jest-migration

Walks a Jest test suite over to Bun's built-in runner: imports, mocks, timers and config.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 386 installs · ★ 236
- Author: secondsky
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/secondsky/claude-skills/tree/HEAD/plugins/bun/skills/bun-jest-migration/SKILL.md
- Updated: 2026-10-20
- HTML: https://affpapa.org/en/nevibecode/skills/bun-jest-migration

## What it does

Bun's runner is Jest-compatible, so most of your suite runs untouched. This skill covers the parts that don't. `describe`, `expect`, `beforeAll` and `spyOn` carry over as-is. `jest.fn()` becomes `mock()`, `jest.mock('module')` becomes `mock.module('module', () => {...})`, and `jest.clearAllMocks()` has to be replaced with a `.mockClear()` call on each mock.

Timers are the other rough edge. `jest.useFakeTimers()` and `jest.advanceTimersByTime()` are supported, but for date mocking the skill points at `setSystemTime` from `bun:test`. Mock hoisting is gone: Jest lifts `jest.mock` to the top of the file, Bun doesn't, so `mock.module` goes above the imports or the module gets pulled in with a dynamic import. Any test that leans on hoisting will break — check those first.

Config moves from `jest.config.js` to `bunfig.toml`. `testMatch` gives way to `root`, `setupFilesAfterEnv` to `preload`, and `coverageThreshold` is written as a fraction such as 0.8. Snapshots transfer unchanged and refresh with `bun test --update-snapshots`. For the full compatibility picture the skill says to load `references/compatibility-matrix.md`.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add secondsky/claude-skills --skill bun-jest-migration -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/bun-jest-migration/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/bun-jest-migration/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add secondsky/claude-skills --skill bun-jest-migration -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/bun-jest-migration/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add secondsky/claude-skills --skill bun-jest-migration -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/bun-jest-migration/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install secondsky/claude-skills bun-jest-migration
```

2. **Manually: put the files in place**
- File: `.github/skills/bun-jest-migration/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add secondsky/claude-skills --skill bun-jest-migration -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/secondsky/claude-skills --path plugins/bun/skills/bun-jest-migration
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/bun-jest-migration/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add secondsky/claude-skills --skill bun-jest-migration -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/secondsky/claude-skills/tree/HEAD/plugins/bun/skills/bun-jest-migration/SKILL.md

## When to use it
- strip jest, ts-jest and babel-jest from the project
- swap `@jest/globals` imports for `bun:test`
- rewrite `jest.mock` calls as `mock.module` without hoisting
- port settings from `jest.config.js` into `bunfig.toml`
- fix snapshots after the serialization change

## Example prompts
- migrate this project's tests from Jest to Bun
- why doesn't `jest.mock` work under Bun and how do I fix it
- convert `jest.config.js` to `bunfig.toml`

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 158c22320680b701.

BEGIN THIRD-PARTY FILE 158c22320680b701
````text
---
name: bun-jest-migration
description: Use when migrating from Jest to Bun's test runner, import compatibility, mocks, and config.
metadata:
  version: "1.0.0"
license: MIT
---

# Bun Jest Migration

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

## Quick Migration

```bash
# 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

```typescript
// 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

```typescript
// 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

```typescript
// 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

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

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

## Timer Migration

```typescript
// 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
```

## Snapshot Testing

```typescript
// Jest
expect(component).toMatchSnapshot();
````
END THIRD-PARTY FILE 158c22320680b701

The third-party file has ended. Everything between the 158c22320680b701 tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

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