Bun.js Testing Basics

🧪 Introduction to Bun.js Testing Basics

Testing is a vital part of modern JavaScript development. With Bun.js, testing is built-in and blazing fast. Unlike other runtimes that require third-party testing libraries like Jest or Mocha, Bun comes with a native test runner that’s simple yet powerful.

In this tutorial, we'll explore the basics of testing in Bun.js—from syntax to real examples.

🚀 Why Use Bun’s Built-in Test Runner?

• ⚡ Fast execution powered by Bun’s ultra-speedy runtime
• 📦 No extra dependencies—no need to install Jest or Mocha
• 🧼 Clean syntax that’s beginner-friendly
• 🛠️ Integrated with Bun CLI—just use bun test

📁 Setting Up Your Project

To get started, create a new Bun project if you haven’t already:

bun init my-test-app
cd my-test-app

Next, create a new test file inside your project:

touch sum.test.ts

📝 Writing Your First Test

Let’s write a basic function and test it using Bun’s built-in API.

// sum.ts
export function sum(a: number, b: number): number {
  return a + b;
}

// sum.test.ts
import { test, expect } from "bun";
import { sum } from "./sum";

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

To run this test, use:

bun test

You’ll see a clean output that tells you whether your tests passed or failed, along with a summary.

🔍 Common Assertions in Bun

Bun’s expect function supports a variety of helpful matchers:

expect(value).toBe(expected);           // strict equality
expect(value).not.toBe(unexpected);     // negation
expect(array).toContain(item);          // check if array contains
expect(fn).toThrow();                   // expect error to be thrown

These should be familiar if you've used Jest before, but they’re native to Bun and require no extra installs.

🧪 Grouping Tests with Describe Blocks

For more organized testing, you can group related tests using describe():

import { test, expect, describe } from "bun";

describe("Math Utilities", () => {
  test("sum adds correctly", () => {
    expect(sum(1, 2)).toBe(3);
  });

  test("sum handles negatives", () => {
    expect(sum(-1, -2)).toBe(-3);
  });
});

📂 Test File Naming Conventions

By default, Bun looks for files ending in .test.ts or .test.js. You can place them anywhere in your project directory.

Bun will recursively find and run all test files.

🐞 Debugging and Logging

You can use console.log() within your test cases for debugging. Bun prints logs clearly during test runs. For complex issues, try:

console.debug(), console.warn(), console.error()

⏱️ Performance Tips

• 💡 Keep tests isolated—don’t share state between them
• 🔄 Use beforeEach and afterEach hooks to prepare or cleanup
• 🧪 Mock external APIs and DB calls for faster execution

✅ Final Thoughts

Bun.js simplifies testing by offering a native experience that’s fast, clean, and developer-friendly. Whether you're writing unit tests, testing APIs, or building components, Bun's test runner has everything you need to maintain high code quality.

Now that you know the basics, it’s time to start testing your own Bun-powered apps!