Differences Between CommonJS and ES Modules

In JavaScript development, modular systems play a crucial role in managing and organizing code. CommonJS and ES Modules are two major modular standards, and they differ significantly in module loading, export mechanisms, and compatibility. This article provides a detailed comparison of the main features and differences between these two module systems.

1. Module Loading

CommonJS:

• Synchronous Loading: CommonJS uses synchronous module loading. This means that when using the require() function, the module is loaded and executed immediately. This approach is well-suited for server-side environments (such as Node.js) but can lead to performance issues in the browser, as the browser needs to complete the module loading before proceeding with subsequent code.

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  // Importing a module
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  const math = require("./math");
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  console.log(math.add(2, 3));

ES Modules:

• Static and Asynchronous Loading: ES Modules support both static and asynchronous loading. Module imports are statically analyzed at compile-time, allowing the JavaScript engine to optimize them. ES Modules also support asynchronous loading through the import syntax (using dynamic import()), which is particularly useful in browser environments.

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  // Static import
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  import { add } from "./math.js";
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  console.log(add(2, 3));
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  // Dynamic import
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  import("./math.js").then((module) => {
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    console.log(module.add(2, 3));
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  });

2. Module Exports and Imports

CommonJS:

• Module Exports: Use module.exports or exports object to export module functionality. You can export objects, functions, or other values.

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  // Exporting a module
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  module.exports = {
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    add: function (a, b) {
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      return a + b;
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    },
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  };

• Module Imports: Use the require() function to import modules.

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  // Importing a module
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  const math = require("./math");
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  console.log(math.add(2, 3));

ES Modules:

• Module Exports: Use the export keyword to export module functionality, supporting named and default exports.

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  // Named export
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  export function add(a, b) {
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    return a + b;
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  }
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  // Default export
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  export default function add(a, b) {
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    return a + b;
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  }

• Module Imports: Use the import keyword to import modules, supporting importing part of a module or the whole module.

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  // Importing named export
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  import { add } from "./math.js";
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  console.log(add(2, 3));
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  // Importing default export
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  import add from "./math.js";
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  console.log(add(2, 3));

3. Module Resolution

CommonJS:

• Dynamic Resolution: Module paths are dynamically resolved, allowing the calculation of module paths at runtime. This enables conditional imports or dynamic loading of modules.

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  const math = require("./math-" + someCondition + ".js");

ES Modules:

• Static Resolution: Module paths are statically resolved at compile-time, allowing the compiler to determine dependencies before runtime. This enables static analysis and optimization but does not support dynamic path resolution.

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  import { add } from "./math.js";

4. Compatibility

CommonJS:

• Primarily for Node.js: CommonJS is the default module system for Node.js. Browser environments do not natively support CommonJS modules but can use tools like Browserify or Webpack to work with CommonJS modules.

ES Modules:

• Standardized: ES Modules are part of the ECMAScript standard and are widely supported in modern browsers and Node.js. They are recommended for both frontend and backend development.

5. Runtime Behavior

CommonJS:

• Dynamic Loading and Caching: CommonJS modules are loaded and executed the first time require is called and are then cached. Subsequent require calls for the same module return the cached module instance.

ES Modules:

• Static Loading and Circular Dependencies: ES Modules support static analysis, allowing dependencies to be resolved at compile-time. For circular dependencies, ES Modules allow partial loading, meaning modules can reference parts of other modules even if they are still loading.

6. Export Values vs. Export References

CommonJS:

• Export Values Are Copies: In CommonJS, exported values are a copy of the export object. Modifying the exported object does not affect the values seen by other modules.

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  let count = 0;
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  module.exports = {
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    add: function (a, b) {
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      return a + b;
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    },
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    getCount: function () {
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      return count;
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    },
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  };
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  // app.js
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  const math = require("./math");
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  console.log(math.getCount()); // Output: 0
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  math.count = 10; // Does not change the value returned by math.getCount()
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  console.log(math.getCount()); // Still outputs: 0

ES Modules:

• Export Values Are References: In ES Modules, exported values are references to the original object. Modifying the exported object will be reflected in other modules.

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  export let count = 0;
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  export function add(a, b) {
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    return a + b;
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  }
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  // app.js
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  import { count, add } from "./math.js";
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  console.log(count); // Output: 0
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  count = 10; // Changes the count in math.js
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  import { count as newCount } from "./math.js";
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  console.log(newCount); // Still outputs: 10

Conclusion

• CommonJS is suited for server-side (Node.js) use, with synchronous module loading that is simple and easy to use but requires additional tools for browser support.
• ES Modules are the standardized module system in ECMAScript, supporting static analysis and optimization, suitable for modern browsers and Node.js with better performance and flexibility.

Choosing the right module system can help improve code maintainability and performance. It is essential to select based on project requirements and environment.