Add new file

cheatfiles/JestCheat.md

@@ -0,0 +1,114 @@
+# Jest cheat sheet
+
+## Basic test
+
+```js
+describe('makePoniesPink', () => {
+  it('should make each pony pink', () => {
+    const actual = fn(['Alice', 'Bob', 'Eve']);
+    expect(actual).toEqual(['Pink Alice', 'Pink Bob', 'Pink Eve']);
+  });
+});
+```
+
+## Matchers
+
+```js
+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](https://facebook.github.io/jest/docs/expect.html)
+
+## Mock functions
+
+```js
+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](https://facebook.github.io/jest/docs/mock-function-api.html)
+
+## Mock modules
+
+1. Create a file like `__mocks__/lodash/memoize.js`:
+
+   ```js
+   module.exports = a => a;
+   ```
+
+2. Add to your test:
+
+   ```js
+   jest.mock('lodash/memoize');
+   ```
+
+[Manual mocks docs](https://facebook.github.io/jest/docs/manual-mocks.html)
+
+## Resources
+
+* [Jest site](https://facebook.github.io/jest/)
+* [Testing React components with Jest and Enzyme](http://blog.sapegin.me/all/react-jest) by Artem Sapegin
+* [Testing React Applications](https://youtu.be/59Ndb3YkLKA) by Max Stoiber
+* [Migrating to Jest](https://medium.com/@kentcdodds/migrating-to-jest-881f75366e7e#.pc4s5ut6z) by Kent C. Dodds
+* [Migrating AVA to Jest](http://browniefed.com/blog/migrating-ava-to-jest/) by Jason Brown
+* [How to Test React and MobX with Jest](https://semaphoreci.com/community/tutorials/how-to-test-react-and-mobx-with-jest)
+
+***
+
+## You may also like
+
+* [Opinionated list of React components](https://github.com/sapegin/react-components)
+
+## Contributing
+
+Improvements are welcome! Open an issue or send a pull request.
+
+## Author and license
+
+[Artem Sapegin](http://sapegin.me/), a frontend developer at [Here](https://here.com/en) and the creator of [React Styleguidist](https://github.com/styleguidist/react-styleguidist). I also write about frontend at [my blog](http://blog.sapegin.me/).
+
+CC0 1.0 Universal license, see the included [License.md](/License.md) file.