Jest / Enzyme - How to test at different viewports?

Question

I am trying to run a test on a component at a certain viewport width. I am doing the following, but this doesn't seem to change it:

test('Component should do something at a certain viewport width.', () => {
    global.innerWidth = 2000;
    const component = mount(<SomeComponent />);
    // ...
});

I also found an article that explains how to do it using JSDom, but as Jest now ships with JSDom, I wondered if there was a native solution.

https://www.codementor.io/pkodmad/dom-testing-react-application-jest-k4ll4f8sd

Answer 1

Background Information:

• jsdom does not implement window.resizeBy() or window.resizeTo()
• jsdom defines the window innerWidth and innerHeight to be 1024 x 768
• It is possible to simulate a window resize using jsdom by manually setting window.innerWidth and window.innerHeight and firing the resize event

Here is an example:

---

comp.js

import * as React from 'react';

export default class Comp extends React.Component {
  constructor(...args) {
    super(...args);
    this.state = { width: 0, height: 0 }
  }
  updateDimensions = () => {
    this.setState({ width: window.innerWidth, height: window.innerHeight });
  }
  componentDidMount() {
    this.updateDimensions();
    window.addEventListener("resize", this.updateDimensions);
  }
  componentWillUnmount() {
    window.removeEventListener("resize", this.updateDimensions);
  }
  render() {
    return <div>{this.state.width} x {this.state.height}</div>;
  }
}

---

comp.test.js

import * as React from 'react';
import { shallow } from 'enzyme';

import Comp from './comp';

const resizeWindow = (x, y) => {
  window.innerWidth = x;
  window.innerHeight = y;
  window.dispatchEvent(new Event('resize'));
}

describe('Comp', () => {
  it('should display the window size', () => {
    const component = shallow(<Comp />);

    expect(component.html()).toEqual('<div>1024 x 768</div>');

    resizeWindow(500, 300);
    expect(component.html()).toEqual('<div>500 x 300</div>');

    resizeWindow(2880, 1800);
    expect(component.html()).toEqual('<div>2880 x 1800</div>');
  });
});

Notes:

• As of Enzyme v3 shallow calls React lifecycle methods like componentDidMount() so it can be used in place of mount
• This answer borrows heavily from the information here, here, here, and @JoeTidee's own answer here.

Answer 2

The below test works for me. Source code is no longer marked as uncovered.

it('resize event listener changes the state', () => {
  const wrapper = shallow(<Component />);
  const instance = wrapper.instance();

  instance.setState({
    mobileMode: true
  });

  global.innerWidth = 800;
  window.dispatchEvent(new Event('resize'));
  expect(instance.state.mobileMode).toBeFalsy();

  global.innerWidth = 600;
  window.dispatchEvent(new Event('resize'));
  expect(instance.state.mobileMode).toBeTruthy();
});

Resize listener inside my component:

// ...
  resizeListener = () => {
      if (window.innerWidth < 768) {
        this.setState({
          mobileMode: true
        });
      } else {
        this.setState({
          mobileMode: false
        });
      }
    };
    window.addEventListener('resize', resizeListener);
// ...

Answer 3

If you're using TypeScript it will complain that window.innerWidth / innerHeight are read-only. You can get around this by either redeclaring the property:

Object.defineProperty(window, 'innerWidth', {writable: true, configurable: true, value: 105})

Or using the Object.assign method:

window = Object.assign(window, { innerWidth: 105 });

Both are not extremely nice solutions, but they work.

Answer 4

Having written helpers similar to the other answers in my tests a few times, I decided to create window-resizeto as a very tiny library of reusable logic.

window-resizeto is a tiny polyfill for window.resizeTo for test environments (as JSDOM does not implement it).

Per the Usage section of the README, you can add it to your jest.config.js:

module.exports = {
  setupFilesAfterEnv: [
    // polyfill window.resizeTo
    'window-resizeto/polyfill'
  ]
}

and then use it in a spec file:

window.resizeTo(500, 500) // window is now resized to 500x500

It's also written in TypeScript, so typings are provided out-of-the-box, and has 100% test coverage to ensure it keeps working correctly.