Mock dynamic html element in jest

Question

I want to write a test for a utils method. In that method I get a html element by id and then change the color of the element. The problem is that element is only available after a button click. How can I mock the element?

UtilListItem.js

import variables from '../stylesheets/Variables.scss';

export function activeListItem(props){
 let listItem = document.getElementById(props.id);
 listItem.style.backgroundColor = variables.whiteGray;
 return listItem;
}

UtilListeItem.test.js

it('check if the correct color is set for the acitve list item', () => {
  let props = {id:'123'}
  const listItem = activeListItem(props);
  expect(listItem.style.backgroundColor).toBe('#ededed');
});

error

TypeError: Cannot read property 'style' of null

Answer 1

I'd suggest you to you jest.spyOn. It's a really handy way to spy on a function and/or attach some mock behaviour.

You can use it like this:

imoprt { activeListItem } from './utils';

let spy;
beforeAll(() => {
  spy = jest.spyOn(document, 'getElementById');
});

describe('activeListItem', () => {
  describe('with found element', () => {
    let mockElement;
    beforeAll(() => {
      // here you create the element that the document.createElement will return
      // it might be even without an id
      mockElement = document.createElement(....);
      spy.mockReturnValue(mockElement);
    });

    // and then you could expect it to have the background
    it('should have the background applied', () => {
      expect(mockElement.style.backgroundColor).toBe('#ededed');
    });
  });

  describe('without found element', () => {
    // and here you can create a scenario
    // when document.createElement returns null
    beforeAll(() => {
      spy.mockReturnValue(null);
    });

    // and expect you function not to throw an error
    it('should not throw an error', () => {
      expect(() => activeListItem({id:'123'})).not.toThrow();
    });
  });
});

It's also a good idea to mock the .scss file, since it's a dependency of your utility file, so that when it's change it won't affect your unit test.

Answer 2

There are two options I can think of, you can opt either of them:

1. Put a check on listItem of function activeListItem

   export function activeListItem(props) {
        let listItem = document.getElementById(props.id);
        if (listItem === null) {
             return;
        }
        listItem.style.backgroundColor = variables.whiteGray;
        return listItem;
    }
   

2. Add dummy element in your test case

   it('check if the correct color is set for the acitve list item', () => {
      /** Create and add dummy div **/
       let testId = "dummy-testId";
       let newDiv = document.createElement("div");
       newDiv.setAttribute("id", testId);
       document.body.appendChild(newDiv);
   
       let props = {id: testId}
       const listItem = activeListItem(props);
       expect(listItem.style.backgroundColor).toBe('#ededed');
   });