jest mock function return 无参

jest-mock-function-return-void.js

const mock = jest.fn();
mock.mockReturnValue(42);
mock(); // 42
mock.mockReturnValue(43);
mock(); // 43

// 或者
const myMockFn = jest
  .fn()
  .mockReturnValue('default')
  .mockReturnValueOnce('first call')
  .mockReturnValueOnce('second call');

// 'first call', 'second call', 'default', 'default'
console.log(myMockFn(), myMockFn(), myMockFn(), myMockFn());

如果 function 返回结果是根据输入计算出来的，使用 mockFn.mockImplementation(() => { })

const mockFn = jest.fn().mockImplementation(scalar => 42 + scalar);
// or: jest.fn(scalar => 42 + scalar);

const a = mockFn(0);
const b = mockFn(1);

a === 42; // true
b === 43; // true

mockFn.mock.calls[0][0] === 0; // true
mockFn.mock.calls[1][0] === 1; // true