The Ultimate Guide to Resolving “Jest Issue in Angular: TypeError: Class extends value undefined is not a constructor or null”

Are you tired of encountering the frustrating “TypeError: Class extends value undefined is not a constructor or null” error when running Jest tests in your Angular application? You’re not alone! This issue has been a thorn in the side of many developers, but fear not, dear reader, for we’re about to dive into the most comprehensive guide on resolving this pesky problem once and for all.

What’s Causing the Jest Issue?

Before we dive into the solutions, let’s take a step back and understand the root cause of this issue. When you execute the command `jest xxxx`, Jest attempts to load and run your tests. However, in some cases, Jest might encounter a broken or misconfigured setup, resulting in the dreaded “TypeError: Class extends value undefined is not a constructor or null” error.

Possible Causes:

• Invalid or Corrupted Jest Configuration: A misconfigured Jest setup can lead to this error. Make sure your Jest configuration file (jest.config.js or jest.config.ts) is correctly set up and doesn’t contain any syntax errors.
• Mismatched Dependencies: Version conflicts between Jest and other dependencies, such as TypeScript or Angular, can cause this issue. Ensure that all dependencies are up-to-date and compatible.
• Broken or Missing Dependencies: If a required dependency is missing or corrupted, Jest might fail to execute. Verify that all necessary dependencies are installed and functioning correctly.
• Incorrect Project Structure: Jest relies on a specific project structure to function correctly. Make sure your project adheres to the standard Angular project structure.

Solution 1: Verify and Update Dependencies

Let’s start with the most common solution: updating and verifying dependencies. Follow these steps:

1. Run the following command to update all dependencies to their latest versions:

   npm update

2. Verify that Jest and other dependencies are installed correctly by checking the package.json file:

   npm ls jest

3. If you’re using TypeScript, ensure that the TypeScript version is compatible with Jest:

   npm ls typescript

Solution 2: Review and Correct Jest Configuration

Next, let’s take a closer look at your Jest configuration file:

1. Open the jest.config.js or jest.config.ts file in your editor.
2. Verify that the configuration file is correctly set up and doesn’t contain any syntax errors.
3. Check that the moduleFileExtensions property includes the necessary file extensions (e.g., .js, .ts, .jsx, and .tsx):

   
   module.exports = {
     // ...
     moduleFileExtensions: ['js', 'ts', 'jsx', 'tsx'],
     // ...
   };
   

4. If you’re using a custom configuration, ensure that it’s correctly set up and compatible with Jest.

Solution 3: Check for Broken or Missing Dependencies

Time to investigate broken or missing dependencies:

1. Run the following command to check for any missing dependencies:

   npm ls --depth 0

2. Review the output to identify any missing or broken dependencies.
3. Install any missing dependencies using npm or yarn.

Solution 4: Verify Project Structure

Let’s ensure that your project structure is correct:

1. Verify that your project follows the standard Angular project structure:

   
   PROJECT_ROOT
   node_modules
   dist
   src
   app
   components
   directive
   module
   pipe
   service
   ...
   tsconfig.json
   tslint.json
   jest.config.js
   ...
   package.json
   

2. Make sure that the tsconfig.json file is correctly set up and points to the correct source files.

Solution 5: Try a Fresh Jest Installation

As a last resort, let’s try a fresh Jest installation:

1. Uninstall Jest using npm or yarn:

   npm uninstall jest

2. Reinstall Jest using npm or yarn:

   npm install jest

3. Retry running your Jest tests.

Conclusion

There you have it – the ultimate guide to resolving the “Jest Issue in Angular: TypeError: Class extends value undefined is not a constructor or null” error. By following these solutions, you should be able to identify and fix the root cause of the issue.

Remember to:

• Verify and update dependencies
• Review and correct Jest configuration
• Check for broken or missing dependencies
• Verify project structure
• Try a fresh Jest installation (if all else fails)

With these solutions, you’ll be back to writing and running seamless Jest tests in your Angular application in no time!

Solution	Description
Verify and Update Dependencies	Update all dependencies to their latest versions and verify correct installation.
Review and Correct Jest Configuration	Verify that Jest configuration file is correctly set up and doesn’t contain syntax errors.
Check for Broken or Missing Dependencies	Identify and install any missing dependencies.
Verify Project Structure	Ensure that project follows standard Angular project structure.
Try a Fresh Jest Installation	Uninstall and reinstall Jest as a last resort.

Happy debugging, and may the Jest be with you!

Frequently Asked Question

Are you tired of encountering the infamous “TypeError: Class extends value undefined is not a constructor or null” error when running Jest in your Angular project? Worry no more! We’ve got the solutions to your Jest woes.