Mock Document for Jest Testing

Question

I have a function like so inside a react component:

    handleOnScroll = () => {
        const {navigationSections, setNavigationSectionActive} = this.props;

        const reversedSections = this.getReversedNavigationSections();

        const OFFSET_TOP = 32;
        const st = window.pageYOffset || document.documentElement.scrollTop; 

        if (st > lastScrollTop) {
            for (let i = 0; i < navigationSections.length; i += 1) {
                if(document.getElementById(navigationSections[i].id).getBoundingClientRect().top <= OFFSET_TOP) {
                    setNavigationSectionActive(navigationSections[i].id);
                }
            }

        } else if (st < lastScrollTop) {
            for (let y = 0; y < reversedSections.length; y += 1) {
                if(document.getElementById(navigationSections[y].id).getBoundingClientRect().top <= OFFSET_TOP) {
                    setNavigationSectionActive(navigationSections[y].id);
                }
            }
        }

        lastScrollTop = st <= 0 ? 0 : st;

    }

and some of the tests like so:

    it('should handle handleOnScroll', () => {
        instance.handleOnScroll();
        expect(instance.getReversedNavigationSections()).toEqual(props.navigationSections.reverse());
    });

    props.navigationSections.forEach(navSection => {
        it('should call setNavigationSectionActive', () => {
            instance.handleOnScroll();
            expect(props.setNavigationSectionActive).toHaveBeenCalledWith(navSection.id);
        });
    });

the first test passes but the second one ('should call setNavigationSectionActive') fails as you can see:

I think the reason is because the document is not mocked therefore the if fails. However, in the actual implementation when this gets executed:

document.getElementById(navigationSections[i].id).getBoundingClientRect().top 

the DIVs that have these IDs are in another section (not in the wrapper component used for the test in question).

should I mock the document to mimic the actual structure for the if statement to pass or am I completely wrong?

MY ATTEMPT SO FAR

    it('should handle custom handleOnScroll', () => {
        document.body.innerHTML = '<div><div id="id">my div</div><div id="id-1">my div</div></div>';

        const div = document.getElementById('id');
        div.getBoundingClientRect = () => ({ top: 100 });  // <= mock getBoundingClientRect
        instance.handleOnScroll();
        props.navigationSections.forEach(() => {
            if (global.document.getElementById('id').getBoundingClientRect().top <= global.OFFSET_TOP) {
                expect(props.setNavigationSectionActive).toHaveBeenCalledWith('id');
            }
        });
    });

Answer 1

The default test environment for Jest is jsdom which provides a browser-like environment.

If your test requires specific content in document then you can set the document body by using something like document.body.innerHTML.

jsdom implements a lot of browser functionality, but not everything. In this case getBoundingClientRect is stubbed to always return 0 so if you want it to return something else you'll have to mock it.

Here is a simple working example to get you started:

const OFFSET_TOP = 5;

const func = () => 
  document.getElementById('theid').getBoundingClientRect().top <= OFFSET_TOP ?
    'less' :
    'more';

test('func', () => {
  document.body.innerHTML = '<div id="theid">my div</div>';
  expect(func()).toBe('less');  // Success!

  const div = document.getElementById('theid');
  div.getBoundingClientRect = () => ({ top: 10 });  // <= mock getBoundingClientRect
  expect(func()).toBe('more');  // Success!
});