How to mock/replace getter function of object with Jest?

Question

In Sinon I can do the following:

var myObj = {
    prop: 'foo'
};

sinon.stub(myObj, 'prop').get(function getterFn() {
    return 'bar';
});

myObj.prop; // 'bar'

But how can I do the same with Jest? I can't just overwrite the function with something like jest.fn(), because it won't replace the getter

"can't set the value of get"

Solution 1

You could use Object.defineProperty

Object.defineProperty(myObj, 'prop', {
  get: jest.fn(() => 'bar'),
  set: jest.fn()
});

Solution 2

For anyone else stumbling across this answer, Jest 22.1.0 introduced the ability to spy on getter and setter methods.

Edit: like in scieslak's answer below, because you can spy on getter and setter methods, you can use Jest mocks with them, just like with any other function:

class MyClass {
  get something() {
    return 'foo'
  }
}

jest.spyOn(MyClass, 'something', 'get').mockReturnValue('bar')
const something = new MyClass().something

expect(something).toEqual('bar')

Solution 3

If you care about spying only, go for @Franey 's answer. However if you actually need to stub a value for the getter, this is how you can do it:

class Awesomeness {
  get isAwesome() {
    return true
  }
}

describe('Awesomeness', () => {
  it('is not always awesome', () => {
    const awesomeness = new Awesomeness
    jest.spyOn(awesomeness, 'isAwesome', 'get').mockReturnValue(false)

    expect(awesomeness.isAwesome).toEqual(false)
  })
})

Comments

[I_like_foxes]

Thank you a lot, that works. Sometimes I notice I don't need more knowledge about some library/framework but the fundamental workings of JavaScript. May I ask one follow up question: How would I mock this function if I wanted my mock to replace the original function just one time but the next call should handled by the original function again?

[shoke]

This works, but when I try to do it in my beforeEach() call it fails with Cannot redefine property. Why is this?

[cgat]

@clu add configurable: true to your the defineProperty function. That should do the trick

[Misu]

Notice that - in contrast with mocks - it won't "unmock" your getter at the end of each test case, so - if other tests rely on the original return value of the getter - you may get different results running one test and the whole suite.

[Sandeep vashisth]

getting cannot redefine property.

[Franey]

@taystack Being able to spy on the getters/setters was a necessary step toward being able to mock them like a regular function. I updated my answer to be more explicit about that and added a mock to the example code.

[Tomasz Kula]

Scroll down to Franey answer. It is more recent.

[Fabrizio Bertoglio]

something property does not exist with [email protected]with ["module:metro-react-native-babel-preset", while it works fine with scieslak solution which sets the spy exactly on the awesomeness instance. In rspec I would use a method called any_instance and I would mock/stub any_instance of that class. There is a clear difference in syntax between the jest official spyOn documentation and your example. In that example jest spys on an object literal

[Fabrizio Bertoglio]

const video = { get play() { return true; },};

[Ran Lottem]

I got an error that the property does not exist. It also looks like the other answer calls spyOn on the class whereas you call it on an instance.

[Mike P.]

@fabrizo jest.spyOn(MyClass, 'something', 'get').mockReturnValue('bar') should be jest.spyOn(MyClass.prototype, 'something', 'get').mockReturnValue('bar')

[mdhvn]

I'm seeing the same thing - 'property does not exist' - when calling on an instance.

[Valentine Shi]

This is really good as this allows working with the complex object's fixture - its POJO representation. This especially useful when you have to supply that object as dependency and would not like to bother creating the actual object - actually keeping yourself away from integration tesing when it is not yet desirable.