What does jest.fn() do and how can I use it?

Question

Can anyone explain how jest.fn() actually works, with a real world example, as I'm confused on how to use it and where it has to be used.

For example if I have the component Countries which fetches country List on click of a button with help of the Utils Function

export default class Countries extends React.Component {   constructor(props) {     super(props)      this.state = {       countryList:''     }   }    getList() {     //e.preventDefault();     //do an api call here     let list = getCountryList();     list.then((response)=>{ this.setState({ countryList:response }) });   }    render() {      var cListing = "Click button to load Countries List";      if(this.state.countryList) {       let cList = JSON.parse(this.state.countryList);       cListing = cList.RestResponse.result.map((item)=> { return(<li key={item.alpha3_code}> {item.name} </li>); });     }      return (       <div>         <button onClick={()=>this.getList()} className="buttonStyle"> Show Countries List </button>         <ul>           {cListing}         </ul>       </div>     );    } } 

Utils function used

const http = require('http');       export function getCountryList() {       return new Promise(resolve => {         let url = "/country/get/all";         http.get({host:'services.groupkt.com',path: url,withCredentials:false}, response => {           let data = '';           response.on('data', _data => data += _data);           response.on('end', () => resolve(data));         });       });               } 

Where could I use Jest.fn() or how can I test for getList function is called when I click on the button?

Answer 1

Jest Mock Functions

Mock functions are also known as "spies", because they let you spy on the behavior of a function that is called indirectly by some other code, rather than just testing the output. You can create a mock function with jest.fn().

Check the documentation for jest.fn()

Returns a new, unused mock function. Optionally takes a mock implementation.

  const mockFn = jest.fn();   mockFn();   expect(mockFn).toHaveBeenCalled(); 

With a mock implementation:

  const returnsTrue = jest.fn(() => true);   console.log(returnsTrue()) // true; 

---

So you can mock getList using jest.fn() as follows:

jest.dontMock('./Countries.jsx'); const React = require('react/addons'); const TestUtils = React.addons.TestUtils; const Countries = require('./Countries.jsx');  describe('Component', function() {   it('must call getList on button click', function() {     var renderedNode = TestUtils.renderIntoDocument(<Countries />);     renderedNode.prototype.getList = jest.fn()      var button = TestUtils.findRenderedDOMComponentWithTag(renderedNode, 'button');      TestUtils.Simulate.click(button);      expect(renderedNode.prototype.getList).toBeCalled();   }); });