Complete Guide to ES6 Module Imports in Node.js: Transitioning from CommonJS to ESM

Current State and Challenges of ES6 Module Imports in Node.js

With the continuous evolution of JavaScript, the ES6 module system has gradually become the standard for modern frontend development. However, when developers attempt to use ES6 import statements like import fs from 'fs' directly in Node.js environments, they often encounter the SyntaxError: Unexpected token import error. The root cause of this issue lies in Node.js's long-standing foundation on the CommonJS module system, with ES6 module support being relatively new and still experimental in certain versions.

Relationship Between Node.js Versions and Module Support

In Node.js 10 and earlier versions, ES6 module support must be explicitly enabled using the --experimental-modules flag. Even in newer Node.js versions, ES6 module functionality may require specific configurations to work properly. In contrast, the CommonJS syntax const fs = require('fs') works reliably across all Node.js versions, making it the most dependable module import method currently available.

Using Experimental Modules

For developers wishing to use ES6 modules, experimental support can be enabled in Node.js 10 through the following approach:

node --experimental-modules main.mjs

It's important to note that when using ES6 modules, file extensions must be .mjs, as traditional .js files will not correctly parse ES6 import statements. This requirement ensures backward compatibility while providing clear identification for the new module system.

CommonJS vs ES6 Module Syntax Comparison

The CommonJS module system uses the require() function to import modules, while ES6 modules use import statements. While functionally similar, the two syntaxes exhibit significant differences in implementation details and loading mechanisms:

// CommonJS syntax
const fs = require('fs');
const output = fs.readFileSync('someData.txt');

// ES6 module syntax
import * as fs from 'fs';
const output = fs.readFileSync('someData.txt');

Special Considerations in Next.js Environments

In modern frontend frameworks like Next.js, module imports must account for differences between client-side and server-side rendering. As shown in the reference article, Node.js native modules can be safely used in functions like getStaticProps that execute exclusively on the server:

export async function getStaticProps() {
  const fs = require('fs/promises');
  const filePath = path.join(process.cwd(), 'data', 'dummy-data.json');
  const fileData = await fs.readFile(filePath);
  const data = JSON.parse(fileData);
  
  return {
    props: { products: data.products }
  };
}

Best Practices for Module Imports

In practical development, it's recommended to choose the appropriate module system based on project requirements and target environments: for projects requiring broad compatibility, prioritize CommonJS syntax; for new projects targeting modern environments, consider using ES6 modules while ensuring runtime environment support. Additionally, conditional imports or dynamic require() can enable flexible module loading strategies across different environments.

Future Outlook and Migration Strategies

As Node.js continues to improve its ES6 module support, future versions are expected to provide more stable and complete ES module functionality. For existing projects, a gradual migration strategy can be adopted: begin by experimenting with ES6 syntax in non-critical modules, progressively replacing original CommonJS code. Meanwhile, leveraging build tools like Webpack or Babel allows developers to enjoy the development conveniences of ES6 modules in current environments.