Bun Testing
Use this skill when writing tests with Bun's built-in test runner, which provides Jest-compatible APIs with significantly faster execution.
Key Concepts
Test Runner Basics
Bun includes a built-in test runner that works out of the box:
import { test, expect, describe, beforeAll, afterAll } from "bun:test";
describe("Math operations", () => {
test("addition", () => {
expect(1 + 1).toBe(2);
});
test("subtraction", () => {
expect(5 - 3).toBe(2);
});
});Running Tests
# Run all tests
bun test
# Run specific test file
bun test ./src/utils.test.ts
# Run with coverage
bun test --coverage
# Watch mode
bun test --watchMatchers and Assertions
Bun supports Jest-compatible matchers:
import { test, expect } from "bun:test";
test("matchers", () => {
// Equality
expect(42).toBe(42);
expect({ a: 1 }).toEqual({ a: 1 });
// Truthiness
expect(true).toBeTruthy();
expect(false).toBeFalsy();
expect(null).toBeNull();
expect(undefined).toBeUndefined();
// Numbers
expect(10).toBeGreaterThan(5);
expect(3).toBeLessThan(5);
expect(3.14).toBeCloseTo(3.1, 1);
// Strings
expect("hello world").toContain("hello");
expect("test@example.com").toMatch(/^[\w-\.]+@([\w-]+\.)+[\w-]{2,4}$/);
// Arrays
expect([1, 2, 3]).toContain(2);
expect([1, 2, 3]).toHaveLength(3);
// Objects
expect({ a: 1, b: 2 }).toHaveProperty("a");
expect({ a: 1, b: 2 }).toMatchObject({ a: 1 });
// Errors
expect(() => {
throw new Error("Test error");
}).toThrow("Test error");
});Best Practices
Organize Tests with describe/test
Structure tests in a clear hierarchy:
import { describe, test, expect } from "bun:test";
describe("UserService", () => {
describe("createUser", () => {
test("creates user with valid data", () => {
// Test implementation
});
test("throws error with invalid email", () => {
// Test implementation
});
});
describe("findUser", () => {
test("finds existing user by id", () => {
// Test implementation
});
test("returns null for non-existent user", () => {
// Test implementation
});
});
});Use Setup and Teardown Hooks
Clean up state between tests:
import { describe, test, beforeAll, afterAll, beforeEach, afterEach } from "bun:test";
describe("Database tests", () => {
beforeAll(() => {
// Run once before all tests
console.log("Setting up test database");
});
afterAll(() => {
// Run once after all tests
console.log("Tearing down test database");
});
beforeEach(() => {
// Run before each test
console.log("Resetting test data");
});
afterEach(() => {
// Run after each test
console.log("Cleaning up test data");
});
test("example test", () => {
expect(true).toBe(true);
});
});Mocking with Bun
Use Bun's built-in mocking:
import { test, expect, mock } from "bun:test";
test("mocking functions", () => {
const mockFn = mock((x: number) => x * 2);
mockFn(2);
mockFn(3);
expect(mockFn).toHaveBeenCalledTimes(2);
expect(mockFn).toHaveBeenCalledWith(2);
expect(mockFn).toHaveBeenCalledWith(3);
expect(mockFn.mock.results[0].value).toBe(4);
});
test("mocking modules", async () => {
// Mock a module
mock.module("./api", () => ({
fetchData: mock(() => Promise.resolve({ data: "mocked" })),
}));
const { fetchData } = await import("./api");
const result = await fetchData();
expect(result).toEqual({ data: "mocked" });
});Async Testing
Handle asynchronous code properly:
import { test, expect } from "bun:test";
test("async function", async () => {
const data = await fetchData();
expect(data).toBeDefined();
});
test("promises", () => {
return fetchData().then((data) => {
expect(data).toBeDefined();
});
});
test("async/await with error", async () => {
await expect(async () => {
await fetchInvalidData();
}).toThrow("Invalid data");
});Common Patterns
Testing HTTP Endpoints
import { describe, test, expect } from "bun:test";
describe("API endpoints", () => {
test("GET /api/users returns users list", async () => {
const response = await fetch("http://localhost:3000/api/users");
const users = await response.json();
expect(response.status).toBe(200);
expect(Array.isArray(users)).toBe(true);
});
test("POST /api/users creates new user", async () => {
const newUser = { name: "Alice", email: "alice@example.com" };
const response = await fetch("http://localhost:3000/api/users", {
method: "POST",
headers: { "Content-Type": "application/json" },
body: JSON.stringify(newUser),
});
expect(response.status).toBe(201);
const user = await response.json();
expect(user).toMatchObject(newUser);
expect(user.id).toBeDefined();
});
});Testing File Operations
import { test, expect, beforeEach, afterEach } from "bun:test";
import { unlink } from "fs/promises";
describe("File operations", () => {
const testFile = "./test-output.txt";
afterEach(async () => {
try {
await unlink(testFile);
} catch {}
});
test("writes file successfully", async () => {
await Bun.write(testFile, "test content");
const file = Bun.file(testFile);
expect(await file.exists()).toBe(true);
const content = await file.text();
expect(content).toBe("test content");
});
});Snapshot Testing
import { test, expect } from "bun:test";
test("snapshot test", () => {
const data = {
id: 1,
name: "Alice",
email: "alice@example.com",
};
expect(data).toMatchSnapshot();
});Parameterized Tests
import { test, expect } from "bun:test";
const testCases = [
{ input: 1, expected: 2 },
{ input: 2, expected: 4 },
{ input: 3, expected: 6 },
];
testCases.forEach(({ input, expected }) => {
test(`double(${input}) should equal ${expected}`, () => {
expect(double(input)).toBe(expected);
});
});Testing with Timers
import { test, expect } from "bun:test";
test("delayed execution", async () => {
let executed = false;
setTimeout(() => {
executed = true;
}, 100);
await new Promise((resolve) => setTimeout(resolve, 150));
expect(executed).toBe(true);
});Anti-Patterns
Don't Use External Test Runners
// Bad - Installing Jest or other test runners
// package.json
{
"devDependencies": {
"jest": "^29.0.0"
}
}
// Good - Use Bun's built-in test runner
bun testDon't Forget to Clean Up
// Bad - Test pollution
test("test 1", () => {
globalState.value = 10;
expect(globalState.value).toBe(10);
});
test("test 2", () => {
// May fail due to test 1's state
expect(globalState.value).toBe(0);
});
// Good - Clean state
import { beforeEach } from "bun:test";
beforeEach(() => {
globalState.value = 0;
});Don't Test Implementation Details
// Bad - Testing private methods
test("private method", () => {
const instance = new MyClass();
expect(instance._privateMethod()).toBe(true);
});
// Good - Test public API
test("public behavior", () => {
const instance = new MyClass();
const result = instance.publicMethod();
expect(result).toBe(expectedValue);
});Don't Write Flaky Tests
// Bad - Timing-dependent test
test("flaky test", () => {
setTimeout(() => {
expect(value).toBe(10);
}, 50); // May fail on slow systems
});
// Good - Deterministic test
test("reliable test", async () => {
await performAsyncOperation();
expect(value).toBe(10);
});Related Skills
- bun-runtime: Core Bun runtime APIs and functionality
- bun-package-manager: Managing test dependencies
- bun-bundler: Building test files for different environments