Mocking clientHeight and scrollHeight in React + Enzyme for test

Question

We have a React component called ScrollContainer than calls a prop function when its content is scrolled to the bottom.

Basically:

componentDidMount() {
  const needsToScroll = this.container.clientHeight != this.container.scrollHeight

  const { handleUserDidScroll } = this.props

  if (needsToScroll) {
    this.container.addEventListener('scroll', this.handleScroll)
  } else {
    handleUserDidScroll()
  }
}

componentWillUnmount() {
  this.container.removeEventListener('scroll', this.handleScroll)
}

handleScroll() {
  const { handleUserDidScroll } = this.props
  const node = this.container
  if (node.scrollHeight == node.clientHeight + node.scrollTop) {
    handleUserDidScroll()
  }
}

this.container is set as follows in the render method:

<div ref={ container => this.container = container }>
  ...
</div>

I want to test this logic using Jest + Enzyme.

I need a way to force the clientHeight, scrollHeight and scrollTop properties to be values of my choosing for the test scenario.

With mount instead of shallow I can get these values but they are always 0. I have yet to find any way to set them to anything non zero. I can set the container on wrapper.instance().container = { scrollHeight: 0 } and etc, but this only modifies the test context not the actual component.

Any suggestions would be appreciated!

Answer 1

Jest spyOn can be used to mock getter and setter from version 22.1.0+. See jest docs

I used below code to mock implementation of document.documentElement.scrollHeight

const scrollHeightSpy = jest
                    .spyOn(document.documentElement, 'scrollHeight', 'get')
                    .mockImplementation(() => 100);

It returns 100 as scrollHeight value.

Answer 2

JSDOM doesn't do any actual rendering - it just simulates the DOM structure - so things like element dimensions aren't calculated as you might expect. If you grab dimensions via method calls there are ways to mock these within your test. For example:

 beforeEach(() => {
    Element.prototype.getBoundingClientRect = jest.fn(() => {
          return { width: 100, height: 10, top: 0, left: 0, bottom: 0, right: 0 };
        });
  });

This obviously won't work in your example. It may be possible to override these properties on the elements and mock changes to them; but I suspect that wouldn't lead to particularly meaningful/useful tests.

See also this thread

Answer 3

In a scenario where you are storing the value as ref, either with createRef or useRef, you might be able to get away with mocking createRef or useRef directly:

import { useRef } from 'react';

jest.mock('react', () => ({
  ...jest.requireActual('react'),
  useRef: jest.fn(),
}));

test('test ref', () => {
  useRef.mockReturnValue(/* value of scrollHeight */);
  ...
});