CJS in ESM Project

Integrating a CommonJS (CJS) package into an ECMAScript Module (ESM) project can be achieved using the import statement, with attention to how the CJS module exports its content. Here's how you can approach this:

1. Default Export Handling:

   • If the CJS module assigns a function or object directly to module.exports, you can import it as a default import in your ESM code:

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     // CJS module (example.cjs)
     module.exports = function exampleFunction() {
       // function implementation
     };
     

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     // ESM module
     import exampleFunction from './example.cjs';
     exampleFunction();
     

     In this scenario, the entire module is treated as the default export.

2. Named Exports Handling:

   • If the CJS module adds properties to exports or module.exports, you can import them using named imports:

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     // CJS module (example.cjs)
     module.exports.namedFunction = function namedFunction() {
       // function implementation
     };
     

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     // ESM module
     import { namedFunction } from './example.cjs';
     namedFunction();
     

     Here, each property on the exports object becomes a named export.

3. Namespace Import:

   • Alternatively, you can import the entire module as a namespace to access all its exports:

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     // ESM module
     import * as exampleModule from './example.cjs';
     exampleModule.namedFunction();
     

     This approach is useful when you want to access multiple exports from the CJS module.

Considerations:

• File Extensions: Ensure that the CJS module file has the appropriate extension (e.g., .cjs) if your project is configured to distinguish between module types based on extensions.

• Asynchronous Imports: In some cases, especially when dealing with dynamic imports or certain module systems, you might need to use asynchronous import() to load CJS modules.

• TypeScript Projects: If you're using TypeScript, enabling the esModuleInterop compiler option can simplify the import process by allowing default imports from CJS modules.

  typescriptlang.org

By following these methods, you can effectively incorporate CJS packages into your ESM project.

Integrating an ECMAScript Module (ESM) into a CommonJS (CJS) project requires the use of dynamic import() statements, as the static require() function is not compatible with ESM's asynchronous nature. Here's how you can achieve this:

1. Dynamic Import in CommonJS:

   • Within your CommonJS module, utilize the dynamic import() function to load the ESM module asynchronously:

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     // CommonJS module (example.cjs)
     (async () => {
       const { default: esmFunction, namedExport } = await import('./example.mjs');
       esmFunction();
       console.log(namedExport);
     })();
     

     In this example, esmFunction is the default export from the ESM module, and namedExport is a named export.

2. Handling Default and Named Exports:

   • When importing an ESM module dynamically, the returned module object contains a default property for the default export and properties corresponding to named exports:

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     // ESM module (example.mjs)
     export const namedExport = 'This is a named export';
     export default function esmFunction() {
       console.log('This is the default export');
     }
     

     The CommonJS module can access these exports as shown in the previous example.

Considerations:

• Asynchronous Nature: Dynamic imports return a promise, so ensure that your CommonJS code handles this asynchronous behavior appropriately, often using async functions and the await keyword.

• File Extensions: Use the .mjs extension for ESM files and .cjs for CommonJS files to clearly distinguish between the two module types.

• Node.js Version: Ensure you're using Node.js version 12 or later, as earlier versions do not support ESM.

By following these steps, you can effectively incorporate ESM modules into your CommonJS project.

In Node.js, the createRequire function from the module module allows you to use CommonJS require functionality within ECMAScript Modules (ESM). This is particularly useful when you need to import CommonJS modules into an ESM context. Here's how you can use it:

1. Import createRequire:

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   import { createRequire } from 'module';
   

2. Create a require Function:

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   const require = createRequire(import.meta.url);
   

   Here, import.meta.url provides the URL of the current module, ensuring that the require function behaves as if it were called in a CommonJS module at the same location.

3. Use the require Function:

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   const commonjsModule = require('./commonjs-module');
   

   This allows you to import CommonJS modules using the require function within an ESM module.

Example:

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// ESM module (example.mjs)
import { createRequire } from 'module';
const require = createRequire(import.meta.url);

// Import a CommonJS module
const fs = require('fs');

fs.readFile('./example.txt', 'utf8', (err, data) => {
  if (err) throw err;
  console.log(data);
});

Considerations:

• Node.js Version Compatibility: The createRequire function was introduced in Node.js v12.2.0. If you're working with an earlier version, you can use the create-require package as a polyfill.

  github.com

• Dynamic Imports: While createRequire is useful for synchronously importing CommonJS modules into ESM, you can also use dynamic import() statements to asynchronously load modules. However, createRequire provides a synchronous alternative that can be more straightforward in certain scenarios.

By using createRequire, you can seamlessly integrate CommonJS modules into your ESM projects, maintaining compatibility across different module systems.