Mocking Components for Testing
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This module looks at testing in React. You’ll learn how to set up a test environment, Jest, and other useful tools to test your React App.   

Learning Objectives 

The objectives of this module are to provide you with an understanding of: 

  • How to set up the test environment  
  • Jest  
  • Snapshot testing  
  • How to test with Props  
  • How to mock components for testing  
  • Mock functions  
  • How to test components asynchronously  
  • How to test components with routing  
  • How to test custom hooks 

Intended Audience  

This learning path is aimed at all who wish to learn how to use the ReactJS framework.  


It is essential you understand the face of contemporary web development to attend this course. We insist upon JavaScript experience, along with good HTML and CSS skills. 


We welcome all feedback and suggestions - please contact us at to let us know what you think. 


- [Narrator] If you think about unit testing that can appear to pose some problems, components inherently render other components that could raise difficulties as we depend on the render of these other components to create the tree. And this doesn't sit well with unit testing. We aren't interested in the rendered components implementation or output, as that should be tested independently. However, to maintain the usability of our component, it needs to re render something when it's called for. Jess has a mock function that will help here. The function takes a string that should be the path to the component to mock. The options allows us to specify a callback that returns a function. The function will return to JSX expression to represent the component we are mocking in the tree. The just mocked functions are written at the top of the test code. Anytime the unit on the test calls for the dependent file, the mock willing to septer and insert the mock code. Our App component now renders component with props and we've run the test for the original snapshot as the reference. The test has failed, that's because the snapshot generated now renders the new version of the App component with the component we props in it. Pressing New Updates to snapshot, and the test will pass again. If we examine the app.test.JS.snapfile, we can see that the file contains the mark of the component with props. Obviously this could cause issues if this component changes in the future. And as it receives props that's highly likely. To get around this, we can mark the component with props in the app.test.JSfile. Anytime it's needed, it will use this mock implementation. And it doesn't matter what the actual component with props does. If that's the case then what we'll do is mock this component using the jest mock function. We'll add the jest mock function now. It's being supplied with the path to the actual component with props file. We provide a callback that returns a function to represent this mock component. We'll allow it to receive props but just return a simple paragraph that contains the text mocked component with props. Once we save this test, it reruns. We'll need to update the snapshot obviously. If we do the that and then check the snapshot we can see that the mock component has been rendered. There's the mock component. So now we've unitized the app component tests and we move the implementation of the component with props from it. We can implement any number of Jess mock functions. So now you can create discrete unit tests for any component that renders other components.


Introduction to Testing React with Jest - How to set up the Test Environment - Jest - The What and How of Testing in React - Snapshot Testing - Testing Components with Props - Testing State Events Interactions - Mocking Functions - Testing Components Asynchronously - Testing Components with Routing - Testing Custom Hooks  

About the Author
Learning Paths

Ed is an Outstanding Trainer in Software Development, with a passion for technology and its uses and holding more than 10 years’ experience.

Previous roles have included being a Delivery Manager, Trainer, ICT teacher, and Head of Department. Ed continues to develop existing and new courses, primarily in web design using: PHP, JavaScript, HTML, CSS, SQL, and OOP (Java), Programming Foundations (Python), and DevOps (Git, CI/CD, etc). Ed describes himself as practically minded, a quick learner, and a problem solver who pays great attention to detail. 

Ed’s specialist area is training in Emerging Technologies, within Web Development. Ed mainly delivers courses in JavaScript covering vanilla JS, ES2015+, TypeScript, Angular, and React (the latter is authored by Ed) and has delivered on behalf of Google for PWAs. Ed has also developed a new suite of PHP courses and has extensive experience with HTML/CSS and MySQL. 

Ed is responsible for delivering QA’s Programming Foundations course using the Eclipse IDE. His skillset extends into the DevOps sphere, where he is able to deliver courses based around Agile/Scrum practices, version control, and CI/CD.

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