mock a specific function in a module

Question

I have two functions in one file

file1.ts:

const function1 = () => {
  return 123;
};
const function2 = () => {
  return function1() + 2;
};

export { function1, function2 };

I'm writing unit tests using jest for function 2. but I'm unable to mock function1

I just tried to use jest.spyOn to mock function1

import * as helperFunctions from 'file1';
describe('test function2 ', () => {
  let functionSpy: any;
  beforeAll(() => {
    functionSpy = jest.spyOn(helperFunctions, 'function1 ');
  });

  afterAll(() => {
    functionSpy.mockReset();
  });
  test('test', () => {
    functionSpy.mockReturnValue(1);
    expect(helperFunctions.function2()).toEqual(3);
  });
});

in my test, function1 is not mocked, it still calls the actual implementation. Any help is appreciated.

it is not possible to mock module partially. you should consider splitting module into two or just compose your tests in a way when you don't need to mock another function — skyboyer, May 30 '19 at 18:30
Possible duplicate of [Jest mock inner function](https://stackoverflow.com/questions/51269431/jest-mock-inner-function) — Brian Adams, May 31 '19 at 01:53
It isn't possible to mock the call to `function1` within `function2` the way the code is currently written...see [my answer here](https://stackoverflow.com/a/55193363/10149510) for details — Brian Adams, May 31 '19 at 01:54

score 0 · Answer 1 · answered Sep 08 '19 at 12:17

Here is the solution:

index.ts:

const function1 = () => {
  return 123;
};
const function2 = () => {
  return exports.function1() + 2;
};

exports.function1 = function1;
exports.function2 = function2;

Unit test:

describe('test function2 ', () => {
  const helperFunctions = require('./');
  helperFunctions.function1 = jest.fn();
  test('test', () => {
    helperFunctions.function1.mockReturnValueOnce(1);
    expect(helperFunctions.function2()).toEqual(3);
  });
});

Unit test result:

 PASS  src/stackoverflow/56354252/index.spec.ts
  test function2 
    ✓ test (8ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        2.696s, estimated 3s

You can find this issue for more detail: https://github.com/facebook/jest/issues/936#issuecomment-214939935

mock a specific function in a module

1 Answers1