Unit Testing in Bun.js

🧪 Unit Testing in Bun.js: Speedy & Straightforward

In modern JavaScript development, unit testing has become essential to ensure code reliability and maintainability. With Bun.js, unit testing is simpler, faster, and built right into the runtime. Forget bulky third-party test runners—Bun provides a zero-config, native solution that feels effortless and lightning-fast ⚡.

🚀 Why Use Bun.js for Unit Testing?

Bun.js eliminates the overhead of traditional test setups. No more installing Jest or Mocha. With Bun, just write your test files and run bun test. It supports modern syntax, parallel execution, and clear, minimal APIs—perfect for writing unit tests that scale well and run fast.

📂 Project Setup for Unit Testing

Let’s get started. Make sure you have Bun installed. If not, install it via:

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

Now create a simple folder structure:

/my-app
  /src
    math.js
  /tests
    math.test.js

🔢 Sample Unit Function

Here's a basic utility we want to test:

// src/math.js
export function add(a, b) {
  return a + b;
}

export function subtract(a, b) {
  return a - b;
}

🧪 Writing Your First Unit Tests

Now let’s write unit tests using Bun's native test runner:

// tests/math.test.js
import { test, expect } from "bun";
import { add, subtract } from "../src/math.js";

test("adds two numbers", () => {
  expect(add(2, 3)).toBe(5);
});

test("subtracts two numbers", () => {
  expect(subtract(10, 5)).toBe(5);
});

▶️ Running the Unit Tests

Just execute this in your terminal:

bun test

Bun automatically detects files ending with .test.js, .test.ts, .spec.js, or .spec.ts and executes them in parallel for blazing-fast feedback 🚀.

🔍 Understanding Assertions

Bun provides an expect() API similar to Jest, making assertions easy and intuitive:

expect(true).toBe(true);
expect([1, 2, 3]).toEqual([1, 2, 3]);
expect("bun").toContain("bu");
expect(() => JSON.parse("<invalid>")).toThrow();

🧪 Test Multiple Cases Using Loops

You can iterate through multiple inputs with ease:

const testCases = [
  { a: 1, b: 2, expected: 3 },
  { a: -1, b: -1, expected: -2 },
];

testCases.forEach(({ a, b, expected }) => {
  test(`add(${a}, ${b}) = ${expected}`, () => {
    expect(add(a, b)).toBe(expected);
  });
});

🧰 Tips for Effective Unit Testing

• Test one thing per test case
• Use descriptive test names
• Keep tests isolated and deterministic
• Avoid external API calls in unit tests
• Use utility functions or fixtures for repeatable logic

❌ Common Mistakes to Avoid

Many developers struggle with test accuracy. Here are some pitfalls to avoid:

• Over-mocking everything — keep it real for unit tests
• Not testing edge cases or invalid inputs
• Making assumptions about execution order

🧠 Bonus: Organize Tests by Modules

Structure tests based on features or modules for better maintainability:

/src
  /auth
    login.js
/tests
  /auth
    login.test.js

This helps your test suite scale with your application.

✅ Final Thoughts

Unit Testing in Bun.js is a game-changer—minimal setup, built-in support, and extremely fast execution make it a perfect fit for modern JavaScript development. Whether you're building a full-stack app or a tiny utility library, Bun empowers you to write reliable, maintainable, and well-tested code 🧑‍💻✨.