Mocking a property using getters and setters

January 19, 2018

2 min read

This is the getter/setter pattern explained on MDN that I only recently learned existed.

const test = {
  set val(val) {
    console.log('setting');
    this._val = val;
  },
  get val() {
    console.log('getting');
    return this._val;
  },
  _val: 0,
};

Leveraging this pattern its possible to detect when a variable has been set or gotten. By passing in a mocked function as the getter or setter of a property you're interested in, its possible to test for assignment of the property by checking if the mocked function was called.

#examples

it('mocks the value property on originalObject', () => {
  const originalObject = {
    value: '',
  };

  // add new property to hold the value of the property
  originalObject._val = '';

  // create a mocked function in jest
  const mockedSetter = jest.fn();

  // implement setter behavior on the mock
  mockedSetter.mockImplementation((val) => {
    originalObject._val = val;
  });

  // override the value property definition with our mocked setter
  Object.defineProperty(originalObject, 'value', {
    set: mockedSetter,
    configurable: true,
  });

  // assign to the newly mocked proprty
  originalObject.value = 'test';

  // expect our value property was set exactly once
  expect(mockedSetter.mock.calls.length).toBe(1);
});

#sources

https://j11y.io/snippets/fun-with-getters-setters/ https://github.com/jquery/jquery/blob/master/src/attributes/val.js#L46 https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty