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Fixing Jest ModuleNameMapper: Unexpected Token In CJS Folder

9 min read 11-15- 2024
Fixing Jest ModuleNameMapper: Unexpected Token In CJS Folder

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When working with Jest in a modern JavaScript project, especially when using TypeScript or Babel, you might run into the frustrating error: "Unexpected token in CJS folder." This error often arises due to incorrect configurations in your Jest setup, particularly when using the moduleNameMapper option in your Jest configuration. This article will guide you through the common reasons for this error and how to fix it effectively. Let's delve into the intricacies of Jest's configuration to ensure a smooth testing experience.

Understanding the Error

The error "Unexpected token in CJS folder" typically occurs when Jest encounters a syntax it doesn't understand in CommonJS (CJS) modules. This can happen when you're using ES6 features (like import/export syntax) in files that Jest expects to be in CommonJS format. Additionally, this issue can arise if there is a misconfiguration in your Jest settings, particularly in the moduleNameMapper.

What is moduleNameMapper?

moduleNameMapper is a powerful configuration option in Jest that allows you to map module paths to specific file paths or mock modules. This is particularly useful for aliasing directories, mocking static assets, or handling stylesheets, among other use cases. A typical moduleNameMapper setup looks like this:

module.exports = {
  moduleNameMapper: {
    '^@components/(.*)

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: '/src/components/$1', '^@styles/(.*)

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: '/src/styles/$1', }, };

Common Causes of the "Unexpected Token" Error

  1. Incorrect moduleNameMapper Entries:

  2. Babel Configuration Issues:

  3. TypeScript Misconfigurations:

  4. Jest Configuration Not Set for ES Modules:

Steps to Fix the Issue

Step 1: Review Your moduleNameMapper

Ensure that all entries in your moduleNameMapper are pointing to valid CJS paths. The values should correctly reference the expected module path and should not point to files that contain unsupported syntax.

Here's an example of a proper moduleNameMapper setup:

module.exports = {
  moduleNameMapper: {
    '^@/(.*)
                                          
                                       
                                    
                                 
                              
                           
                        
                     
                     

                        

                     

                  
                  

                     

                        

                           
                        

                     

                  

                  

                  
                  
                  

                     

                        
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: '/src/$1',
    '\\.(css|less|scss)
                                          
                                       
                                    
                                 
                              
                           
                        
                     
                     

                        

                     

                  
                  

                     

                        

                           
                        

                     

                  

                  

                  
                  
                  

                     

                        
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: 'identity-obj-proxy', // Mocking styles
  },
};

Step 2: Check Your Babel Configuration

If you are using Babel, ensure your .babelrc or babel.config.js is correctly set up to transpile the code properly. For instance, you might need to include the following presets:

module.exports = {
  presets: ['@babel/preset-env', '@babel/preset-react'],
};

Also, ensure that Jest can find the Babel configuration. You may need to install babel-jest to enable Jest to use Babel to transpile your code:

npm install --save-dev babel-jest

Step 3: Configure TypeScript Properly

For TypeScript users, make sure that your tsconfig.json is set to output CommonJS modules. Here's how a simple tsconfig.json might look:

{
  "compilerOptions": {
    "target": "es6",
    "module": "commonjs",
    "outDir": "./dist",
    "strict": true,
    "esModuleInterop": true
  }
}

Step 4: Use Jest with ES Modules

If you're using ES modules, you'll need to update your Jest configuration to recognize ES syntax. For example:

module.exports = {
  transform: {
    '^.+\\.jsx?
                                          
                                       
                                    
                                 
                              
                           
                        
                     
                     

                        

                     

                  
                  

                     

                        

                           
                        

                     

                  

                  

                  
                  
                  

                     

                        
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: 'babel-jest',
    '^.+\\.tsx?
                                          
                                       
                                    
                                 
                              
                           
                        
                     
                     

                        

                     

                  
                  

                     

                        

                           
                        

                     

                  

                  

                  
                  
                  

                     

                        
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: 'ts-jest',
  },
  moduleFileExtensions: ['js', 'jsx', 'ts', 'tsx'],
};

Step 5: Update Jest and Dependencies

Sometimes, the issue can arise from outdated dependencies. Ensure that your Jest and its related packages are up-to-date. You can update them using:

npm install --save-dev jest @types/jest ts-jest

Step 6: Utilize ts-jest for TypeScript Projects

If you're working with TypeScript, make sure to utilize ts-jest as your transformer. Add it to your Jest configuration:

module.exports = {
  preset: 'ts-jest',
  testEnvironment: 'node',
};

Example Configuration

Here’s a comprehensive example of a Jest configuration that correctly sets up module mapping, Babel, and TypeScript:

module.exports = {
  preset: 'ts-jest',
  testEnvironment: 'node',
  moduleNameMapper: {
    '^@/(.*)
                                          
                                       
                                    
                                 
                              
                           
                        
                     
                     

                        

                     

                  
                  

                     

                        

                           
                        

                     

                  

                  

                  
                  
                  

                     

                        
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: '/src/$1',
    '\\.(css|less|scss)
                                          
                                       
                                    
                                 
                              
                           
                        
                     
                     

                        

                     

                  
                  

                     

                        

                           
                        

                     

                  

                  

                  
                  
                  

                     

                        
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: 'identity-obj-proxy',
  },
  transform: {
    '^.+\\.jsx?
                                          
                                       
                                    
                                 
                              
                           
                        
                     
                     

                        

                     

                  
                  

                     

                        

                           
                        

                     

                  

                  

                  
                  
                  

                     

                        
Featured Posts

                        

                        
                     

                  

                  

               
            
         
      
	  
      
  
   
: 'babel-jest',
    '^.+\\.tsx?
                                          
                                       
                                    
                                 
                              
                           
                        
                     
                     

                        

                     

                  
                  

                     

                        

                           
                        

                     

                  

                  

                  
                  
                  

                     

                        
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: 'ts-jest',
  },
  moduleFileExtensions: ['js', 'jsx', 'ts', 'tsx', 'json', 'node'],
};

Additional Tips

jest --clearCache

Conclusion

Navigating the complexities of Jest’s configuration can sometimes be daunting, especially when you encounter errors like "Unexpected token in CJS folder." However, by following the structured approach outlined in this article, you should be able to troubleshoot and resolve the issue effectively. Remember to validate your moduleNameMapper, check your Babel and TypeScript configurations, and ensure your dependencies are up-to-date. With the right setup, Jest will become an invaluable tool in your testing toolkit, allowing for efficient and reliable testing of your JavaScript applications. Happy testing! 🎉

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