Testing in JavaScript

Write automated tests with Jest or Vitest for reliability and refactoring safety.

Project setup (Vitest example)

npm install -D vitest

Add script:

{ "scripts": { "test": "vitest" } }

A simple test

import { describe, it, expect } from 'vitest'
import { sum } from './sum.js'

describe('sum', () => {
  it('adds numbers', () => {
    expect(sum(2, 3)).toBe(5)
  })
})

Async tests

it('fetches data', async () => {
  const data = await fetchData()
  expect(data.ok).toBe(true)
})

Mocks and spies

vi.mock('./net.js', () => ({ fetchData: vi.fn().mockResolvedValue({ ok: true }) }))
const spy = vi.spyOn(console, 'log')

Jest differences

Jest APIs are similar; configure jest.config.js. For ESM, use native support or transforms.

Coverage

vitest run --coverage

Best practices

• Keep tests deterministic and isolated
• Use factories/fixtures; mock external I/O
• Test behavior and contract; avoid coupling to implementation details

Summary

• Set up Vitest/Jest; write unit and async tests
• Use mocks/spies for isolation; measure coverage