JS - Mocking an uninitialized class method inside a function

Question

Lets say I have this code below and I want to test someFunc() but I want to mock the doSomething() function? How can I do it in a test when the SomeClass is initialized only inside the someFunc() itself and not in the test?

class SomeClass {
  constructor(element){
    this.element = element
  }
  
  doSomething(element){
    ...
  }
}

function someFunc() {
  const newClass = new SomeClass()
  newClass.doSomething() // I want this to get to the mocked function
}

I am using jest, please help!

score 0 · Answer 1 · answered Mar 22 '22 at 13:54

0

Try some testing frameworks, most of them provide spies which allow you to mock specific functions for given classes.

For example, you can try Jasmine - https://jasmine.github.io/api/edge/Spy.html Where you would do something like

spyOn(newClass, 'doSomething').and.returnValue(42);

answered Mar 22 '22 at 13:54

Pavel Pazderník

37
1
10

But the newClass is not initialized in the test. Will it still work? – Danielle Vardi Mar 22 '22 at 13:58
@DanielleVardi then I would maybe consider mocking your named function completely, something like this https://stackoverflow.com/questions/39755439/how-to-mock-imported-named-function-in-jest-when-module-is-unmocked – Pavel Pazderník Mar 22 '22 at 14:01

score 0 · Answer 2 · answered Mar 22 '22 at 13:58

0

There are multiple ways to mock es6 class in Jest: https://jestjs.io/docs/es6-class-mocks

answered Mar 22 '22 at 13:58

hulufei

737
1
6
17

But my problem is different. I dont have a problem mocking a class that was initialized in test. My problem is mocking an uninitiliazed class – Danielle Vardi Mar 22 '22 at 14:07
Then `newClass.doSomething = jest.fn()` should work. – hulufei Mar 23 '22 at 07:53

JS - Mocking an uninitialized class method inside a function

2 Answers2