How to mock an asynchronous function call in another class

Question

I have the following (simplified) React component.

class SalesView extends Component<{}, State> {   state: State = {     salesData: null   };    componentDidMount() {     this.fetchSalesData();   }    render() {     if (this.state.salesData) {       return <SalesChart salesData={this.state.salesData} />;     } else {       return <p>Loading</p>;     }   }    async fetchSalesData() {     let data = await new SalesService().fetchSalesData();     this.setState({ salesData: data });   } } 

When mounting, I fetch data from an API, which I have abstracted away in a class called SalesService. This class I want to mock, and for the method fetchSalesData I want to specify the return data (in a promise).

This is more or less how I want my test case to look like:

• predefine test data
• import SalesView
• mock SalesService

• setup mockSalesService to return a promise that returns the predefined test data when resolved

• create the component

• await
• check snapshot

Testing the looks of SalesChart is not part of this question, I hope to solve that using Enzyme. I have been trying dozens of things to mock this asynchronous call, but I cannot seem to get this mocked properly. I have found the following examples of Jest mocking online, but they do not seem to cover this basic usage.

• Hackernoon: Does not use asychronous calls
• Wehkamp tech blog: Does not use asynchronous calls
• Agatha Krzywda: Does not use asynchronous calls
• GitConnected: Does not use a class with a function to mock
• Jest tutorial An Async Example: Does not use a class with a function to mock
• Jest tutorial Testing Asynchronous Code: Does not use a class with a function to mock
• SO question 43749845: I can't connect the mock to the real implementation in this way
• 42638889: Is using dependency injection, I am not
• 46718663: Is not showing how the actual mock Class is implemented

My questions are:

• How should the mock class look like?
• Where should I place this mock class?
• How should I import this mock class?
• How do I tell that this mock class replaces the real class?
• How do set up the mock implementation of a specific function of the mock class?
• How do I wait in the test case for the promise to be resolved?

One example that I have that does not work is given below. The test runner crashes with the error throw err; and the last line in the stack trace is at process._tickCallback (internal/process/next_tick.js:188:7)

# __tests__/SalesView-test.js import React from 'react'; import SalesView from '../SalesView';  jest.mock('../SalesService'); const salesServiceMock = require('../SalesService').default;  const weekTestData = [];  test('SalesView shows chart after SalesService returns data', async () => {   salesServiceMock.fetchSalesData.mockImplementation(() => {     console.log('Mock is called');     return new Promise((resolve) => {       process.nextTick(() => resolve(weekTestData));     });   });    const wrapper = await shallow(<SalesView/>);   expect(wrapper).toMatchSnapshot(); }); 

Answer 1

Sometimes, when a test is hard to write, it is trying to tell us that we have a design problem.

I think a small refactor could make things a lot easier - make SalesService a collaborator instead of an internal.

By that I mean, instead of calling new SalesService() inside your component, accept the sales service as a prop by the calling code. If you do that, then the calling code can also be your test, in which case all you need to do is mock the SalesService itself, and return whatever you want (using sinon or any other mocking library, or even just creating a hand rolled stub).

Answer 2

You could potentially abstract the new keyword away using a SalesService.create() method, then use jest.spyOn(object, methodName) to mock the implementation.

import SalesService from '../SalesService ';  test('SalesView shows chart after SalesService returns data', async () => {      const mockSalesService = {         fetchSalesData: jest.fn(() => {             return new Promise((resolve) => {                 process.nextTick(() => resolve(weekTestData));             });         })     };      const spy = jest.spyOn(SalesService, 'create').mockImplementation(() => mockSalesService);      const wrapper = await shallow(<SalesView />);     expect(wrapper).toMatchSnapshot();     expect(spy).toHaveBeenCalled();     expect(mockSalesService.fetchSalesData).toHaveBeenCalled();      spy.mockReset();     spy.mockRestore(); });