How to mock imported named function in Jest when m

问题:

I have the following module I'm trying to test in Jest:

// myModule.js

export function otherFn() {
  console.log('do something');
}

export function testFn() {
  otherFn();

  // do other things
}

As shown above, it exports some named functions and importantly testFn uses otherFn.

In Jest when I'm writing my unit test for testFn, I want to mock the otherFn function because I don't want errors in otherFn to affect my unit test for testFn. My issue is that I'm not sure the best way to do that:

// myModule.test.js
jest.unmock('myModule');

import { testFn, otherFn } from 'myModule';

describe('test category', () => {
  it('tests something about testFn', () => {
    // I want to mock "otherFn" here but can't reassign
    // a.k.a. can't do otherFn = jest.fn()
  });
});

Any help/insight is appreciated.

回答1:

Use jest.requireActual() inside jest.mock()

jest.requireActual(moduleName)

Returns the actual module instead of a mock, bypassing all checks on whether the module should receive a mock implementation or not.

Example

I prefer this concise usage where you require and spread within the returned object:

// myModule.test.js

jest.mock('./myModule.js', () => (
  {
    ...(jest.requireActual('./myModule.js')),
    otherFn: () => {}
  }
))

describe(...)

This method is also referenced in Jest's Manual Mocks documentation (near the end of Examples):

To ensure that a manual mock and its real implementation stay in sync, it might be useful to require the real module using jest.requireActual(moduleName) in your manual mock and amending it with mock functions before exporting it.

回答2:

import m from '../myModule';

Does not works for me, I did use:

import * as m from '../myModule';

m.otherFn = jest.fn();

回答3:

Looks like I'm late to this party, but yes, this is possible.

testFn just needs to call otherFn using the module.

If testFn uses the module to call otherFn then the module export for otherFn can be mocked and testFn will call the mock.

---

Here is a working example:

myModule.js

import * as myModule from './myModule';  // import myModule into itself

export function otherFn() {
  return 'original value';
}

export function testFn() {
  const result = myModule.otherFn();  // call otherFn using the module

  // do other things

  return result;
}

myModule.test.js

import * as myModule from './myModule';

describe('test category', () => {
  it('tests something about testFn', () => {
    const mock = jest.spyOn(myModule, 'otherFn');  // spy on otherFn
    mock.mockReturnValue('mocked value');  // mock the return value

    expect(myModule.testFn()).toBe('mocked value');  // SUCCESS

    mock.mockRestore();  // restore otherFn
  });
});

回答4:

The transpiled code will not allow babel to retrieve the binding that otherFn() is referring to. If you use a function expession, you should be able to achieve mocking otherFn().

// myModule.js
exports.otherFn = () => {
  console.log('do something');
}

exports.testFn = () => {
  exports.otherFn();

  // do other things
}

 

// myModule.test.js
import m from '../myModule';

m.otherFn = jest.fn();

But as @kentcdodds mentioned in the previous comment, you probably would not want to mock otherFn(). Rather, just write a new spec for otherFn() and mock any necessary calls it is making.

So for example, if otherFn() is making an http request...

// myModule.js
exports.otherFn = () => {
  http.get('http://some-api.com', (res) => {
    // handle stuff
  });
};

Here, you would want to mock http.get and update your assertions based on your mocked implementations.

// myModule.test.js
jest.mock('http', () => ({
  get: jest.fn(() => {
    console.log('test');
  }),
}));

回答5:

I solved my problem with a mix of the answers that I found here:

myModule.js

import * as myModule from './myModule';  // import myModule into itself

export function otherFn() {
  return 'original value';
}

export function testFn() {
  const result = myModule.otherFn();  // call otherFn using the module

  // do other things

  return result;
}

myModule.test.js

import * as myModule from './myModule';

describe('test category', () => {
  let otherFnOrig;

  beforeAll(() => {
    otherFnOrig = myModule.otherFn;
    myModule.otherFn = jest.fn();
  });

  afterAll(() => {
    myModule.otherFn = otherFnOrig;
  });

  it('tests something about testFn', () => {
    // using mock to make the tests
  });
});

回答6:

On top of the first answer here, you can use babel-plugin-rewire to mock imported named function too. You can check out the section superficially for named function rewiring.

One of the immediate benefits for your situation here is that you do not need to change how you call the other function from your function.

回答7:

I know this was asked a long time ago, but I just ran into this very situation and finally found a solution that would work. So I thought I'd share here.

For the module:

// myModule.js

export function otherFn() {
  console.log('do something');
}

export function testFn() {
  otherFn();

  // do other things
}

You can change to the following:

// myModule.js

export const otherFn = () => {
  console.log('do something');
}

export const testFn = () => {
  otherFn();

  // do other things
}

exporting them as a constants instead of functions. I believe the issue has to do with hoisting in JavaScript and using const prevents that behaviour.

Then in your test you can have something like the following:

import * as myModule from 'myModule';


describe('...', () => {
  jest.spyOn(myModule, 'otherFn').mockReturnValue('what ever you want to return');

  // or

  myModule.otherFn = jest.fn(() => {
    // your mock implementation
  });
});

Your mocks should now work as you would normally expect.