TypeError: Failed to load plugin 'jest' declared in '.eslintrc': Class extends value undefined is not a constructor or null

3 min read 05-10-2024

Debugging "TypeError: Failed to load plugin 'jest' declared in '.eslintrc': Class extends value undefined is not a constructor or null"

Let's break down the common error message "TypeError: Failed to load plugin 'jest' declared in '.eslintrc': Class extends value undefined is not a constructor or null" and understand how to fix it.

This error usually occurs when you're using ESLint, the popular JavaScript linting tool, with the 'jest' plugin, which is designed to enforce best practices for writing tests using the Jest testing framework. The error message essentially tells us that ESLint is unable to find the 'jest' plugin, resulting in a failed attempt to load it.

Understanding the Problem

In simpler terms: Imagine you're building a car, and you need a specific part (the 'jest' plugin) to complete the engine (ESLint). This error message indicates that you've specified the part in your instructions (.eslintrc file), but for some reason, it's missing.

Replicating the Scenario

Here's a typical scenario where this error might occur:

// .eslintrc.js
module.exports = {
  plugins: ['jest'],
  // ... other ESLint configurations
};

The plugins array in the .eslintrc.js file tells ESLint to load the 'jest' plugin. However, if the plugin is not properly installed, or if there's an issue with its configuration, the error arises.

Why is this happening?

The error message points to a core problem: ESLint can't find the necessary 'jest' plugin to extend its functionalities. This typically boils down to:

Missing Plugin: You haven't installed the 'jest' plugin, so ESLint doesn't have access to it.
Incorrect Installation: The plugin might be installed, but ESLint doesn't recognize it due to incorrect installation steps or version conflicts.
Configuration Errors: There might be mistakes in your .eslintrc.js file regarding the plugin's configuration, such as incorrect paths or naming conventions.

Fixing the Issue

Here's a breakdown of solutions to resolve this error:

Install the 'jest' plugin:

npm install --save-dev eslint-plugin-jest

Check Plugin Installation: Double-check that the plugin is installed correctly by running:
```
npm ls eslint-plugin-jest
```
If the plugin is not listed, re-install it using the steps in step 1.
Verify Plugin Configuration: Ensure that the plugin name is correctly spelled in your .eslintrc.js file. Additionally, consider the following:
- Plugin Path: If you've installed the 'jest' plugin in a custom location, update the plugins array in your .eslintrc.js to reflect the correct path.
- Version Compatibility: Make sure your ESLint version is compatible with the 'jest' plugin version. You can check the plugin's documentation for compatibility requirements.
Restart ESLint: After making any changes to your configuration, restart your development server or IDE.
Clear Cache (if applicable): If the error persists, clearing your ESLint cache might help.
```
eslint --init
```

Additional Tips

Use the --verbose flag: Running ESLint with the --verbose flag can provide more detailed error messages that might help pinpoint the source of the problem.
Consult Plugin Documentation: Refer to the 'jest' plugin's documentation for detailed guidance on configuration and potential issues.
Check Project Dependencies: Review your project's package.json file and ensure that all dependencies, including ESLint and the 'jest' plugin, are correctly listed and updated.

Resources

ESLint Documentation: https://eslint.org/
'jest' Plugin Documentation: https://www.npmjs.com/package/eslint-plugin-jest

By understanding the underlying reasons for this error and following these troubleshooting steps, you can effectively fix the "TypeError: Failed to load plugin 'jest' declared in '.eslintrc': Class extends value undefined is not a constructor or null" and ensure your ESLint setup is working flawlessly with the 'jest' plugin for robust testing.