Bun Test Migration: Performance Gains and Compatibility Gotchas

Last month, our CI pipeline was dragging. With over 1,200 unit tests running on Vitest, we were burning through nearly 12 minutes of build time on every PR. I started looking for ways to trim the fat, and while Bun runtime performance: Why it’s the shift you need had already convinced me to move our production workloads, I was skeptical about swapping our entire test runner.

The promise of Bun Test is simple: it’s a drop-in replacement for Jest that leverages Bun’s native speed. But in practice, "drop-in" usually means "drop-in, then spend three days fixing broken mocks."

Why bother with a Bun Test migration?

The primary driver here is execution overhead. Vitest is an incredible tool, but it's built on top of Vite. It’s optimized for the developer experience—hot module replacement, fast watch modes, and deep integration with the ecosystem. However, that abstraction layer costs CPU cycles.

When we ran our suite using bun test, the execution time dropped from roughly 12 minutes to about 4 minutes. That’s a 3x speedup on a cold start. If you’re tired of watching your terminal scroll while waiting for simple unit tests to finish, the raw speed of Bun is genuinely impressive.

The compatibility wall

Before you jump, understand that your vitest.config.ts won't just port over. Bun Test is API-compatible with Jest, but it doesn't support the full Vitest plugin ecosystem.

We hit three major hurdles immediately:

1. Mocking global objects: We relied heavily on vi.spyOn. Bun Test uses jest.spyOn. While the syntax is similar, the behavior of deep-mocking modules in ESM vs. CJS caused some non-deterministic failures in our auth.test.ts file.
2. Environment isolation: Vitest’s jsdom and happy-dom integrations are highly refined. Bun’s built-in DOM support is evolving, but if you’re doing heavy component testing that requires complex browser behavior, you might find it lacking compared to what you’re used to.
3. Plugin ecosystem: If your tests depend on specific Vite plugins to resolve aliases or transform custom file types, you’re stuck. Bun doesn't know what a Vite plugin is. We had to rewrite our path alias resolution using a manual bunfig.toml configuration.

Benchmarking the reality

In JavaScript testing, performance isn't just about the runner; it’s about the setup. Here is what our transition looked like:

• Vitest (v0.34.6): 712 seconds (Total suite time)
• Bun Test (v1.1.20): 248 seconds (Total suite time)

The speedup is massive, but it’s skewed. Bun shines when it doesn't have to compile your entire dependency tree through a heavy transformer. If your codebase is already highly modular and uses standard ESM, you’ll see the best results. If you have a legacy monolith with hundreds of circular dependencies, the speed gains might be eaten up by Bun’s stricter module resolution.

How to manage the switch

If you’re ready to try it, don't delete your vitest.config.ts yet. Start by running a small subset of your tests with Bun:

Bash
# Run a single test file to check for syntax errors
bun test ./src/components/Button.test.ts

# Run the whole suite with a reporter that mimics Jest
bun test --reporter=jest

I recommend keeping both runners for a few weeks. We used a conditional check in our CI script: if USE_BUN is set, we run the suite through Bun. This allowed us to fix individual test files one by one without breaking the main branch.

Is it worth it?

If you’re a senior engineer looking to optimize developer tooling, the unit testing performance gains from Bun are hard to ignore. It makes the feedback loop almost instantaneous. However, if your team relies heavily on Vitest’s specialized features like UI mode or advanced coverage reporting, the migration cost might outweigh the build-time savings.

Next time, I’d probably start by migrating the utility libraries first—those are usually stateless and easy to port. We tried to jump into the UI component tests too early, which led to a lot of frustration with the DOM environment.

I’m still not convinced that Bun is a total replacement for the entire ecosystem yet, but for high-speed unit testing, it’s clearly the new standard to beat.

FAQ

Does Bun Test support Snapshot testing? Yes, it does. It follows the Jest snapshot format, so you can often just copy your existing __snapshots__ folders over, though you might need to re-run them once to account for minor differences in string serialization.

Can I use TypeScript without extra configuration? Absolutely. Bun handles TypeScript natively. You don't need ts-jest or vite-node. It just works, which is one of the biggest ergonomic wins.

What happens if I use msw for API mocking? It works fine, but you’ll need to ensure you’re using the msw/node integration. Since Bun implements a lot of the Node.js API surface, it’s surprisingly compatible with most network-level mocking libraries.