Jest is the default test runner for JavaScript projects. It’s battle-tested, has a massive ecosystem, and works fine. It’s also slow in a way you stop noticing until you switch away from it.

Bun ships a test runner built into the runtime. Same API, no configuration, and startup times that make Jest feel like a different era.

The speed difference

Jest’s slowness comes from two places: startup cost and transform overhead. Every test run spins up Node, loads Jest’s module system, and transpiles your TypeScript through Babel or ts-jest or esbuild. Even a single test file takes a second or two before any test code runs.

Bun runs TypeScript natively. No transpile step, no separate transform config. A test file that takes 2.3 seconds to start in Jest starts in under 100ms in Bun. On a suite of 50 test files the cumulative difference is minutes, not seconds.

It’s largely compatible

Bun’s test runner implements the Jest API: describe, it, test, expect, beforeEach, afterEach, beforeAll, afterAll, mock, spyOn. If your tests don’t rely on Jest-specific internals, they’ll run under bun test without changes.

The main things that don’t carry over are Jest’s custom runner plugins and some snapshot serializer customizations. For most application test suites, those aren’t factors.

Switching

Remove Jest:

npm uninstall jest ts-jest @types/jest babel-jest

Remove any jest.config.ts or jest.config.js. Remove the transform and testEnvironment config if you had them.

Add Bun (if not already installed):

curl -fsSL https://bun.sh/install | bash

Update your package.json scripts:

{
  "scripts": {
    "test": "bun test",
    "test:watch": "bun test --watch"
  }
}

Run your tests:

bun test

That’s it for most projects.

Running only staged files

One thing bun test does well is filtering. In a pre-commit hook you don’t want to run every test, just the ones related to what you’re changing:

bun test --testPathPattern=src/utils

Or with lefthook, pass staged files directly:

pre-commit:
  commands:
    test-related:
      glob: "**/*.{ts,tsx}"
      run: bun test {staged_files}

This gives you a fast sanity check on commit without waiting for the full suite.

Coverage

bun test --coverage

Coverage output works out of the box, no extra packages required. Jest needed --coverage plus @jest/coverage-provider or a specific coverage provider configured. Bun handles it natively.

Watch mode

bun test --watch

Re-runs affected tests on file change. The feedback loop is noticeably tighter than Jest’s watch mode because the startup cost isn’t paid on every re-run.

What to keep in mind

Bun’s test runner is stable but younger than Jest. A few things to be aware of:

• Snapshot testing works, but if you have an existing Jest snapshot suite, the format is compatible enough that most snapshots migrate without issues.
• jsdom is not bundled. For tests that need a browser-like DOM environment, you need to configure --dom or use a separate setup. This matters for component testing; for backend or utility code it doesn’t come up.
• Module mocking works for most cases, but complex Jest factory patterns occasionally need minor adjustments.

For a TypeScript backend, CLI tool, or utility library, none of these caveats apply. Switch immediately. For a React component test suite, expect a short migration session rather than a zero-touch swap.

The bottom line

Jest made sense when the alternatives were worse. In 2026, bun test is faster, requires no configuration, handles TypeScript natively, and covers 95% of what Jest does. The main reason to stay on Jest is an existing suite with deep Jest-specific dependencies. For anything new, start with bun test.