I have a React 16 application created from create-react-app (which uses react-scripts 1.1.4) with the following component we created:

import React, {Component} from 'react';

import './ListNav.css';



const tabs = {

    previousIndex: 0

};



function styleStringGenerator(index) {

    let styleString = {

        leftBase: 'left: ',

        widthBase: 'width: '

    }



    if (index === 0) {

        styleString.aggregate = `${styleString.leftBase} 0;         ${styleString.widthBase}${tabs.widths[0]}px;`;

} else {

    styleString.aggregate =     `${styleString.leftBase}${tabs.distanceFromOrigin[index]}px;     ${styleString.widthBase}${tabs.widths[index]}px;`;

}



    return styleString.aggregate;

}  



class ListNav extends Component{

    constructor(props){

        super(props);

        this.handleDataTypeSelection =     this.handleDataTypeSelection.bind(this);



        this.tabScrollWidth = null;



        this.setInputRef = element => {

            this.tabScrollWidth = element;

        };

    }



    render(){

        const dataTypeSelection = (s) => () => this.handleDataTypeSelection(s);



        return(

            <div className="tab" ref={this.setInputRef}>

                <div className="tab__header" onClick={dataTypeSelection("Addresses")}>

                    <span className="tab__title">Addresses</span>

                </div>

                <div className="tab__header" onClick={dataTypeSelection("Hotspots")}>

                    <span className="tab__title">Hotspot Data</span>

                </div>

                <div className="tab__header" onClick={dataTypeSelection("PSRs")}>

                    <span className="tab__title">PSRs</span>

                </div>

                <div className="tab__underline"></div>

            </div>

        );

    }



    componentDidMount(){

        tabs.elements = document.querySelectorAll('.tab__header');

        tabs.length = tabs.elements.length;

        tabs.finalIndex = tabs.length - 1;

        tabs.totalWidth = document.querySelector('.tab').scrollWidth;

        console.log(document);



        tabs.widths = 

        tabs.elements.forEach((v, index, array) => {

            tabs.widths.push(v.scrollWidth);

        });



        tabs.distanceFromOrigin = [0];

        tabs.widths.forEach((v, index, array) => {

            if (index > 0) {

                tabs.distanceFromOrigin.push(array[index-1] + tabs.distanceFromOrigin[index-1]);

            }

        });



        let styleString = styleStringGenerator(0);

                document.querySelector('.tab__underline').setAttribute('style', styleString);

        document.querySelector('.tab__title').setAttribute('class', 'tab__title tab__title--active');    



        document.querySelectorAll('.tab__header').forEach((v, index, array) => v.addEventListener('click', function(){

            const currentIndex = index;



            if (tabs.previousIndex !== currentIndex) {



                const styleString = styleStringGenerator(index);



                document.querySelector('.tab__underline').setAttribute('style', styleString);

                document.querySelector('.tab__title--active').setAttribute('class', 'tab__title');

                this.querySelector('.tab__title').setAttribute('class', 'tab__title tab__title--active');



                tabs.previousIndex = (function(){return currentIndex})();

            }



        }, index));        

    }



    handleDataTypeSelection(s){

        this.props.getData(s);

    }

}



export default ListNav;

I am using Jest 20.0.4, Enzyme 3.3.0 and enzyme-adapter-react-16 1.1.1 and created the following test:

import React from 'react';

import Enzyme from 'enzyme';

import {shallow, mount} from 'enzyme';

import Adapter from 'enzyme-adapter-react-16';



import ListNav from '../components/map-list/list-nav/ListNav';



Enzyme.configure({

  adapter: new Adapter()

});



const listNav = shallow(<ListNav/>);



describe('ListNav', () => {

  it('ListNav renders without crashing', () => {

    expect(listNav).toMatchSnapshot(); 

  });

});

When I run my test, I get the following error:

TypeError: Cannot read property 'scrollWidth' of null

The line in question is in the component, in the componentDidMount() call. The code fails on the line:

tabs.totalWidth = document.querySelector('.tab').scrollWidth;

because tabs.totalWidth = document.querySelector('.tab') evaluates to null so scrollWidth can't be read. I am using shallow(<ListNav/>) and can see "classname": "tab" in my snapshot, but the test cannot seem to find it. Any ideas as to how to either better implement my test or better construct my code?

Solution 1:

Make your document dependency swappable using a closure. This way, in your unit tests you can provide a mock.

Usage in your real code would be:

import ListNav from "./ListNav";

...

render(){

  return <ListNav/>;

}

Usage in your tests:

import { create } from "./ListNav";



it('should...', ()=>{

  const documentMock = { title: "mock title" };

  const ListNavWithMock = create(documentMock);

  const component = shallow(<ListNavWithMock />);

});

In order to support that your module will have to be modified like this:

import React from "react";

export const create = documentInstance => {

  return class ListNav extends React.Component {

    render() {

      return <div>{documentInstance.title}</div>;

    }

  };

};



export default create(document);

See an exemple here where both ListNav and ListNavWithMock are loaded.

Solution 2 (if you use webpack)

1. Abstract away the code that relies on the document api by creating a new module called documentHelper.js
   


2. In your component, import documentHelper
   


3. In your unit test, swap the documentHelper module with a mock using https://github.com/plasticine/inject-loader.

Example:

describe('ListNav', () => {

  let ListNav ;

  let documentHelperMock;



  beforeEach(() => {

    documentHelperMock= { title: "mock title" };

    ListNav= require('inject-loader!./ListNav')({

      '.documentHelperMock': {documentHelperMock},

    });

  });



  it('should ...', () => {

    const wrapper = shallow(<ListNav/>)

  });

});

Note: make sure you don't import the module under test (ListNav) at the top of your file. The require call does that part.

This approach is less intrusive because component code does not have to be modified in a way that makes it obvious that it's for testing purpose. It just makes the code cleaner by moving document specific code out of your component.

This approach is also easier because the APIs you will have to mock will be your own (documentHelper.UpdateTabs). In the first solution your mock might have to be complex (querySelector and what it returns).