bun-test-vs-jest

Purpose

Evaluate the advantages and trade-offs of using Bun’s built-in test runner compared to Jest for JavaScript/TypeScript testing.

Key Findings

Bun Advantages

1. Dramatically Faster Execution: 4-13x speed improvement over Jest
2. Native TypeScript/JSX Support: No transpilation or configuration needed
3. Jest-Compatible API: Most Jest tests work out of the box
4. Unified Toolchain: Runtime, package manager, bundler, and test runner in one binary
5. Hot Module Replacement: Watch mode only re-runs affected tests
6. Zero Configuration: Just run bun test

Jest Advantages

1. Feature Completeness: More mature with advanced functionalities
2. IDE Integration: First-class support in VS Code and other editors
3. Ecosystem Maturity: LTS schedules and massive community support
4. Detailed Coverage Reporting: Machine-readable formats for CI pipelines

Performance Benchmarks

Benchmark 1: 223 Tests Across 14 Files

Source: DEV Community benchmark (10 iterations)

Benchmark 2: 5,000 Test Cases

Source: The Green Report benchmark (10 iterations)

Benchmark 3: React SSR Tests

“Bun runs 266 React SSR tests faster than Jest can print its version number”

Source: Official Bun documentation

Feature Comparison

Bun Test Runner Features

Test Discovery

Automatic discovery with patterns:

• *.test.{js|jsx|ts|tsx}
• *_test.{js|jsx|ts|tsx}
• *.spec.{js|jsx|ts|tsx}
• *_spec.{js|jsx|ts|tsx}

Lifecycle Hooks

import { beforeAll, beforeEach, afterEach, afterAll, describe, test, expect } from "bun:test";

beforeAll(() => { /* setup once */ });
beforeEach(() => { /* setup each test */ });
afterEach(() => { /* cleanup each test */ });
afterAll(() => { /* cleanup once */ });

describe("my feature", () => {
  test("does something", () => {
    expect(true).toBe(true);
  });
});

Test Modifiers

test.skip("skipped test", () => {});
test.todo("not implemented yet");
test.failing("expected to fail", () => { throw new Error(); });
test.concurrent("runs in parallel", async () => {});
test.serial("runs sequentially even with --concurrent", () => {});

CLI Options

Current Limitations (2024-2025)

Coverage Reporting

• Only prints to command line
• Limited to line and function coverage
• No machine-readable formats for CI (lcov partially supported)

IDE Integration

• No native VS Code test explorer integration
• Cannot debug tests through IDE
• Relies on console.log debugging

Compatibility Edge Cases

Some libraries like jest-mock-extended require workarounds because they expect jest.fn to be globally available.

// Workaround for global jest.fn
import { mock } from "bun:test";
globalThis.jest = { fn: mock };

Migration Guide

Basic Migration

Most Jest tests work without changes:

# Before (Jest)
npm test

# After (Bun)
bun test

Configuration

Bun uses bunfig.toml for test configuration:

[test]
preload = ["./setup.ts"]
timeout = 10000
coverage = true
coverageReporter = ["text", "lcov"]

Import Changes

// Jest
import { jest } from '@jest/globals';

// Bun
import { mock, spyOn } from "bun:test";

When to Use Each

Choose Bun Test When

• Speed is a priority (fast feedback loops)
• Project uses TypeScript/JSX natively
• Already using Bun runtime
• Tests are straightforward unit/integration tests
• Want minimal configuration

Choose Jest When

• Need advanced mocking features
• Require detailed coverage reporting for CI
• IDE debugging is essential
• Using Jest-specific plugins/extensions
• Enterprise project requiring LTS stability

Hybrid Approach

For projects needing both speed and advanced features:

# Run fast unit tests with Bun
bun test src/**/*.unit.test.ts

# Run complex integration tests with Jest
npx jest src/**/*.integration.test.ts

Sources

1. Comparing JavaScript Test Frameworks - DEV Community
2. Bun Test Runner Documentation
3. Bun’s Test Runner: The Future of JavaScript Testing?
4. Is Bun Ready for Unit Testing in 2024?
5. Bun vs Node.js 2025 - Strapi
6. Vitest vs Jest Discussion - Hacker News