prefer-mock-return-shorthand.md

jest/prefer-mock-return-shorthand

📝 Prefer mock return shorthands.

🔧 This rule is automatically fixable by the --fix CLI option.

When working with mocks of functions that return simple values, Jest provides some API sugar functions to reduce the amount of boilerplate you have to write.

These methods should be preferred when possible.

Rule details

The following patterns are warnings:

jest.fn().mockImplementation(() => 'hello world');

jest
  .spyOn(fs.promises, 'readFile')
  .mockImplementationOnce(() => Promise.reject(new Error('oh noes!')));

myFunction
  .mockImplementationOnce(() => 42)
  .mockImplementationOnce(() => Promise.resolve(42))
  .mockReturnValue(0);

The following patterns are not warnings:

jest.fn().mockResolvedValue(123);

jest
  .spyOn(fs.promises, 'readFile')
  .mockReturnValue(Promise.reject(new Error('oh noes!')));
jest.spyOn(fs.promises, 'readFile').mockRejectedValue(new Error('oh noes!'));

jest.spyOn(fs, 'readFileSync').mockImplementationOnce(() => {
  throw new Error('oh noes!');
});

myFunction
  .mockResolvedValueOnce(42)
  .mockResolvedValueOnce(42)
  .mockReturnValue(0);