vitest-vs-bun

Overview

Vitest and Bun’s test runner represent two modern approaches to JavaScript testing: Vitest prioritizes ecosystem integration, correctness, and feature completeness, while Bun focuses on raw speed through native implementation. The performance story is more nuanced than “Bun is faster”—async vs sync workloads tell very different stories.

Performance Benchmarks: The Complete Picture

Synchronous Tests

Bun dominates synchronous test execution through its native implementation.

Asynchronous Tests (Critical Finding)

Vitest is 15× faster for async tests—a massive advantage for real-world applications using async/await, promises, API mocks, and database interactions.

Multi-Framework Benchmark (223 tests, 14 files)

Source: DEV Community benchmark

Performance Summary

• Sync-heavy suites: Bun wins decisively
• Async-heavy suites: Vitest wins decisively
• Mixed workloads: Results vary by async percentage
• Real-world apps: Most modern apps are async-heavy (APIs, databases, timers)

Feature Comparison Matrix

Critical Differences

1. Test Isolation (Correctness Issue)

Vitest: Runs each test suite in isolated global state

// Test suite A and B don't interfere
// Global pollution in one suite can't affect another

Bun: Tests share global state

// Side effects in test suite A can affect suite B
// Must manually reset globals between suites
// Can lead to flaky tests

Impact: Vitest provides stronger correctness guarantees by default.

2. Async Performance Architecture

Vitest: Optimized async execution through V8 integration and event loop management

Bun: Currently has async performance bottlenecks (15× slower)

This is the most significant practical difference for modern apps.

3. Vite Integration (Ecosystem)

Vitest: Shares Vite configuration automatically

// vitest.config.ts inherits from vite.config.ts
import { defineConfig } from 'vitest/config';

export default defineConfig({
  // Vite plugins, aliases, transforms automatically apply
  test: {
    // Vitest-specific config only
  }
});

Bun: Separate configuration in bunfig.toml

[test]
preload = ["./setup.ts"]
timeout = 10000

Impact: Vitest ensures tests use the same transformations as your app. Critical for monorepos and complex build setups.

4. Browser Mode (Frontend Testing)

Vitest: Runs tests in real browsers (Chrome, Firefox, Safari)

npx vitest --browser.name=chrome

• Real DOM APIs
• Actual browser behavior
• Component testing with @testing-library

Bun: Uses HappyDOM simulation only

• Not a real browser
• Can miss browser-specific bugs

Vitest Advantages

1. Feature Completeness

• Browser mode for component testing
• Benchmarking capabilities
• Type testing with expect-type
• UI mode with web dashboard
• Workspace/monorepo support
• Test sharding for CI

2. Correctness & Reliability

• Default test isolation prevents flaky tests
• Comprehensive fake timer implementation
• Better async handling

3. Ecosystem Integration

• Unified Vite configuration
• Works seamlessly with Vite plugins
• Better for frontend projects using Vite

4. Developer Experience

• VS Code extension with test explorer
• UI mode for visual test management
• Better error messages and stack traces
• More mature documentation

5. CI/CD Optimizations

• Test sharding across machines
• Better coverage reporting (v8, istanbul)
• More flexible reporters

Bun Test Advantages

1. Synchronous Performance

• 11× faster for sync-heavy test suites
• Excellent for pure computational tests
• Native implementation speed

2. Simplicity

• Zero dependencies (part of Bun binary)
• No separate installation needed
• Minimal configuration

3. Runtime Consistency

• Same runtime for tests and production (if using Bun)
• No transpilation overhead

4. Built-in Coverage

• Works without plugins (though limited)

Recommendation from Vitest Creator

Evan You (creator of Vite and Vitest) stated:

“Bun’s corresponding features are not fully equivalent replacements. Vite & Vitest gives you:

• Better non-React framework support
• Better production bundle”

Recommendation: Use Vitest even when running on Bun runtime.

When to Choose Each

Choose Vitest When

✅ Most projects should use Vitest

• Writing async-heavy code (APIs, databases, timers)
• Building frontend apps with Vite
• Need test isolation for reliability
• Component testing with browser mode
• Monorepo/workspace projects
• CI/CD with test sharding
• Value ecosystem maturity

Choose Bun Test When

• Sync-heavy test suites (pure computation, data transformation)
• Simple projects with basic test needs
• Already using Bun runtime and want consistency
• Don’t need advanced features
• Benchmarking shows acceptable async performance for your use case

Hybrid Approach: Best of Both Worlds

# Use Vitest for better features and async performance
npm install -D vitest

# But run it with Bun runtime for faster startup
bun --bun vitest

This gives you:

• Vitest’s feature completeness
• Test isolation
• Better async performance
• Bun’s fast startup and native TypeScript

Migration Examples

From Bun Test to Vitest

// Before (Bun)
import { test, expect } from "bun:test";

test("addition", () => {
  expect(1 + 1).toBe(2);
});

// After (Vitest)
import { test, expect } from "vitest";

test("addition", () => {
  expect(1 + 1).toBe(2);
});

Most Bun tests work with just import changes.

Configuration Migration

[test]
preload = ["./setup.ts"]
timeout = 10000

import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    setupFiles: ['./setup.ts'],
    testTimeout: 10000,
  }
});

Performance Optimization Tips

For Vitest

export default defineConfig({
  test: {
    // Use threads for CPU-heavy tests
    pool: 'threads',

    // Or forks for better isolation
    pool: 'forks',

    // Limit parallel tests
    maxConcurrency: 5,

    // Use v8 coverage (faster)
    coverage: {
      provider: 'v8'
    }
  }
});

For Bun Test

# Run tests in parallel
bun test --concurrent

# Limit concurrency
bun test --max-concurrency=10

# Skip slow async tests in dev
bun test --bail=1

Real-World Considerations

Modern App Characteristics

Most JavaScript applications are async-heavy:

• REST API calls (fetch, axios)
• Database queries (Prisma, Drizzle, TypeORM)
• Authentication flows
• File system operations
• Timer-based logic (debounce, throttle)

Implication: Vitest’s 15× async advantage matters more than Bun’s 11× sync advantage for most codebases.

Test Suite Composition Analysis

Analyze your test suite:

# Count async tests
grep -r "async" test/ | wc -l

# Count sync tests
find test/ -name "*.test.*" | wc -l

If async tests dominate → Vitest wins If sync tests dominate → Consider Bun

Feature Roadmap Outlook

Vitest (Mature)

• Active development
• Large community
• Stable API
• Continuous improvements

Bun Test (Evolving)

• Async performance improvements needed
• Missing critical features (fake timers, browser mode)
• May catch up over time
• Currently not feature-complete

Conclusion

TL;DR: Vitest is the more complete, production-ready choice for most projects in 2025, especially those with async code. Bun Test excels at synchronous benchmarks but lacks features and async optimization.

Final Recommendation

Use Vitest, optionally with Bun runtime:

bun --bun vitest

This combination provides:

• ✅ Vitest’s features and correctness
• ✅ Better async performance
• ✅ Bun’s fast startup
• ✅ Best of both worlds

Sources

1. Vitest Official Documentation
2. Vitest Configuration Guide
3. Vitest Features
4. Comparing JavaScript Test Frameworks (DEV Community)
5. Best Node.js Test Framework with Benchmarks
6. Bun vs Vitest GitHub Benchmark
7. Evan You on Vitest vs Bun
8. Vitest vs Jest Discussion (Hacker News)