Jest unit testing a function that calls a function that returns a promise

Question

If I have code something like this:

import externalThing from 'externalThing'
import anotherThing from 'anotherThing'

function functionIWantToTest()
{    
    externalThing.doSomething()
    .then(data=>{
        anotherThing.do(data.Id)
    })
}

What is the best practice way to unit test this?

Ideally the test would be something like this:

import externalThing from 'externalThing'
import anotherThing from 'anotherThing'

jest.mock('externalThing',()=>jest.fn())
jest.mock('anotherThing',()=>jest.fn())

describe('when calling functionIWantToTest',()=>{

    beforeEach(()=>{     
       anotherThing.do=jest.fn()
       //mock external thing somehow so that it still the 'then' code

       functionIWantToTest()
    })

    it('anotherThing should be called',()=>{
        expect(anotherThing.do).toHaveBeenCalledWith(1)
    });
});

But when I try I just end up building a chain of mocked functions with jest.fn() and no code actually gets executed.

Hey @guest271314 its in the test :), for anotherThing.do to have been called with a param. — Dan
– Dan, Commented Aug 13, 2017 at 19:07
Have not tried jest. What does jest.fn() return? Are there two different assignments of values to anotherThing.do? Note that no values are actually returned from functionIWantToTest() call — guest271314
– guest271314, Commented Aug 13, 2017 at 19:10
jest.fn() returns a mocked function, you can provide mock implementation by doing jest.fn(()=>{...} ). Yeah its a void function, I am expecting it do call something else(there is more logic in the real code) — Dan
– Dan, Commented Aug 13, 2017 at 19:13
Ok. Though still no value is returned from functionIWantToTest() call. Why do you anotherThing.do=jest.fn() if anotherThing is a function that is already defined at import anotherThing from 'anotherThing'? — guest271314
– guest271314, Commented Aug 13, 2017 at 19:15

guest271314 · Accepted Answer · 2017-08-13 19:43:58Z

1

Have not tried jest. You can use console.assert() to test the function calls and Promise values returned. Note, no value is actually returned from either of the function calls, if that matter to the expected result, see Why is value undefined at .then() chained to Promise?.

function functionIWantToTest() {
  return Promise.resolve({
      Id: 1
    })
    .then(data => {
      console.assert(data.Id === 1, data, [`${data.Id} is not equal to 1`]);
      console.assert(data.Id !== 1, data, [`${data.Id} is equal to 1`]);
    })
}

functionIWantToTest()

answered Aug 13, 2017 at 19:43

guest271314

1

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