🧪 Jest

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For new projects, consider using Vitest instead. It’s faster, has better ESM support, and is compatible with most Jest APIs.

Basic test skeleton Jump to heading

describe('MyFunction', () => {
  it('should do something', () => {
    // Arrange
    const input = 'test'

    // Act
    const result = myFunction(input)

    // Assert
    expect(result).toBe('expected')
  })
})

    

Use Jest globals Jump to heading

import { expect, test, describe, beforeEach, jest } from '@jest/globals'

test('two plus two is four', () => {
  expect(2 + 2).toBe(4)
})

    

Mocking Jump to heading

Mock a function Jump to heading

const mockFn = jest.fn()
mockFn.mockReturnValue('mocked value')
mockFn.mockResolvedValue('async mocked value')

// Check calls
expect(mockFn).toHaveBeenCalled()
expect(mockFn).toHaveBeenCalledWith('arg1', 'arg2')
expect(mockFn).toHaveBeenCalledTimes(2)

    

Mock a module Jump to heading

jest.mock('./myModule', () => ({
  myFunction: jest.fn(() => 'mocked'),
}))

    

Spy on a method Jump to heading

const spy = jest.spyOn(object, 'method')
expect(spy).toHaveBeenCalled()
spy.mockRestore()

    

Async testing Jump to heading

test('async function', async () => {
  const result = await fetchData()
  expect(result).toEqual({ data: 'value' })
})

test('resolves correctly', async () => {
  await expect(fetchData()).resolves.toEqual({ data: 'value' })
})

test('rejects with error', async () => {
  await expect(failingFetch()).rejects.toThrow('Network error')
})

    

Testing throws Jump to heading

test('throws on invalid input', () => {
  expect(() => formatDate('invalid')).toThrow('Date string wrong format')
  expect(() => formatDate('invalid')).toThrow(Error)
  expect(() => formatDate('invalid')).toThrowErrorMatchingInlineSnapshot(
    `"Date string wrong format"`
  )
})

    

Matchers cheatsheet Jump to heading

// Equality
expect(value).toBe(exact)           // strict equality (===)
expect(value).toEqual(obj)          // deep equality
expect(value).toStrictEqual(obj)    // deep equality + undefined props

// Truthiness
expect(value).toBeTruthy()
expect(value).toBeFalsy()
expect(value).toBeNull()
expect(value).toBeUndefined()
expect(value).toBeDefined()

// Numbers
expect(value).toBeGreaterThan(3)
expect(value).toBeLessThan(5)
expect(value).toBeCloseTo(0.3, 5)   // for floats

// Strings
expect(string).toMatch(/pattern/)
expect(string).toContain('substring')

// Arrays
expect(array).toContain(item)
expect(array).toHaveLength(3)
expect(array).toContainEqual({ id: 1 })

// Objects
expect(object).toHaveProperty('key')
expect(object).toHaveProperty('key', 'value')
expect(object).toMatchObject({ subset: true })

    

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