Testing a React component that uses jQuery & window object

Question

My React component has to respond to resize event, for which I am using jQuery, ie:

//...
componentDidMount: function() {
  $(window).on("resize", function);
}
//..

However, this causes issues with my Jest test, specifically:

- TypeError: Cannot read property 'on' of undefined
    at RadiumEnhancer.React.createClass.componentDidMount (bundles/Opportunities/OpportunityPartial.jsx:21:14)

When stepping through the test it looks like window is defined in a jest test, but $(window) doesn't appear to return anything.

I was under the impression that Jest would examine the required module (jQuery)'s API to construct its mock, but if I'm understanding the problem correctly it seems like this isn't happening?

I can get around this by not mocking jQuery, but obviously that isn't totally preferable.

Answer 1

Here is the unit test solution using jestjs and enzyme:

index.tsx:

import React, { Component } from 'react';
import $ from 'jquery';

class SomeComponent extends Component {
  componentDidMount() {
    $(window).on('resize', this.resizeEventHandler);
  }

  resizeEventHandler() {
    console.log('resize event handler');
  }

  render() {
    return <div>some component</div>;
  }
}

export default SomeComponent;

index.spec.tsx:

import React from 'react';
import SomeComponent from './';
import { shallow } from 'enzyme';
import $ from 'jquery';

jest.mock('jquery', () => {
  const m$ = {
    on: jest.fn(),
  };
  return jest.fn(() => m$);
});

describe('36082197', () => {
  afterEach(() => {
    jest.restoreAllMocks();
    jest.resetAllMocks();
  });
  it('should pass', () => {
    const logSpy = jest.spyOn(console, 'log');
    const eventHandlerMap = {};
    ($().on as jest.MockedFunction<any>).mockImplementation((event, handler) => {
      eventHandlerMap[event] = handler;
    });
    const wrapper = shallow(<SomeComponent></SomeComponent>);
    const instance = wrapper.instance();
    expect(wrapper.text()).toBe('some component');
    expect($).toBeCalledWith(window);
    // tslint:disable-next-line: no-string-literal
    expect($(window).on).toBeCalledWith('resize', instance['resizeEventHandler']);
    eventHandlerMap['resize']();
    expect(logSpy).toBeCalledWith('resize event handler');
  });
});

Unit test result with 100% coverage:

 PASS  src/stackoverflow/36082197/index.spec.tsx (12.045s)
  36082197
    ✓ should pass (18ms)

  console.log node_modules/jest-mock/build/index.js:860
    resize event handler

-----------|----------|----------|----------|----------|-------------------|
File       |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
-----------|----------|----------|----------|----------|-------------------|
All files  |      100 |      100 |      100 |      100 |                   |
 index.tsx |      100 |      100 |      100 |      100 |                   |
-----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        14.267s, estimated 15s

Source code: https://github.com/mrdulin/jest-codelab/tree/master/src/stackoverflow/36082197