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 right into the runtime, with the same API, no configuration required, 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, so even a single test file can take a second or two before any test code actually runs.

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

It’s largely compatible

Bun’s test runner implements the full 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 any changes.

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

Switching

Remove Jest:

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

Remove any jest.config.ts or jest.config.js, along with 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 particularly well is filtering. In a pre-commit hook you rarely want to run every test, just the ones related to what you’re actually 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 with no extra packages required. With Jest you needed --coverage plus @jest/coverage-provider or a separately configured coverage provider. Bun just handles it natively.

Watch mode

bun test --watch

Re-runs affected tests on file change, and the feedback loop feels noticeably tighter than Jest’s watch mode because you’re not paying the startup cost on every re-run.

What to keep in mind

Bun’s test runner is stable but younger than Jest, so there are a few things worth knowing before you commit to a migration.

• Snapshot testing works, and the format is compatible enough with Jest’s that most existing snapshots migrate without issues.
• jsdom is not bundled. Tests that need a browser-like DOM environment require --dom or a separate setup, which matters for component testing but never comes up for backend or utility code.
• Module mocking works for most cases, though complex Jest factory patterns occasionally need minor adjustments.

For a TypeScript backend, CLI tool, or utility library, none of these caveats really apply and you can switch without hesitation. 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 feels 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, and even then a migration is usually shorter than you’d expect. For anything new, just start with bun test.