usefulfiles/cheatfiles/JestCheat.md

Jest cheat sheet

Basic test

describe('makePoniesPink', () => {
  it('should make each pony pink', () => {
    const actual = fn(['Alice', 'Bob', 'Eve']);
    expect(actual).toEqual(['Pink Alice', 'Pink Bob', 'Pink Eve']);
  });
});

Matchers

expect.assertions(28)

expect(42).toBe(42)
expect(42).not.toBe(3)
expect([1, 2]).toEqual([1, 2])
expect('foo').toBeTruthy()
expect('').toBeFalsy()
expect(null).toBeNull()
expect(undefined).toBeUndefined()
expect(7).toBeDefined()

expect('long string').toMatch('str')
expect(result).toMatch(/regexp/)

expect({a: 1, b: 2}).toMatchObject({a: 1})

expect(2).toBeGreaterThan(1)
expect(1).toBeGreaterThanOrEqual(1)
expect(1).toBeLessThan(2)
expect(1).toBeLessThanOrEqual(1)
expect(0.2 + 0.1).toBeCloseTo(0.3, 5)

expect(['Alice', 'Bob', 'Eve']).toHaveLength(3)
expect(['Alice', 'Bob', 'Eve']).toContain('Alice')
expect([{a: 1}, {a: 2}]).toContainEqual({a: 1})
expect(['Alice', 'Bob', 'Eve']).toEqual(expect.arrayContaining(['Alice', 'Bob']))

expect(new A()).toBeInstanceOf(A)

expect(node).toMatchSnapshot()

expect(fn).toThrow()
expect(fn).toThrow('Out of cheese')
expect(fn).toThrowErrorMatchingSnapshot()

expect(fn).toBeCalled()
expect(fn).toHaveBeenCalledTimes(1)
expect(fn).toBeCalledWith(expect.stringContaining('foo'))
expect(fn).toBeCalledWith(expect.stringMatching(/^[A-Z]\d+$/))
expect(fn).toBeCalledWith(expect.objectContaining({x: expect.any(Number), y: expect.any(Number)}))
expect(fn).toHaveBeenLastCalledWith(expect.anything())

Matchers docs

Mock functions

it('should call the callback', () => {
  const callback = jest.fn();
  fn(callback);
  expect(callback).toBeCalled();
  expect(callback.mock.calls[0][1].baz).toBe('pizza'); // Second argument of the first call
});

Mock functions docs

Mock modules

1. Create a file like __mocks__/lodash/memoize.js:

   module.exports = a => a;
   

2. Add to your test:

   jest.mock('lodash/memoize');
   

Manual mocks docs

Resources

• Jest site
• Testing React components with Jest and Enzyme by Artem Sapegin
• Testing React Applications by Max Stoiber
• Migrating to Jest by Kent C. Dodds
• Migrating AVA to Jest by Jason Brown
• How to Test React and MobX with Jest

---

You may also like

• Opinionated list of React components

Contributing

Improvements are welcome! Open an issue or send a pull request.

Author and license

Artem Sapegin, a frontend developer at Here and the creator of React Styleguidist. I also write about frontend at my blog.

CC0 1.0 Universal license, see the included License.md file.