what is mjs file?

By admin | 1 year ago

javascriptnodeECMAScript Moduleesm

An `.mjs` file extension refers to an ECMAScript Module (ESM) file used with JavaScript programming. This designation allows developers to use the module system introduced in ECMAScript 2015 (ES6) in environments that support this standard, such as modern web browsers and Node.js.

Key Characteristics of `.mjs` Files:

  • **Module Syntax:** `.mjs` files utilize the `import` and `export` statements to include and share JavaScript code across different files, enabling more organized, maintainable, and reusable code.

  • **Strict Mode:** Code in `.mjs` files is automatically executed in strict mode, which imposes a stricter parsing and error handling model on JavaScript code. This avoids common pitfalls and introduces better error-checking features.

  • **Node.js Support:** Initially, Node.js introduced the `.mjs` extension to distinguish between CommonJS modules (using `.js` extension and `require` syntax) and ECMAScript Modules (using `.mjs` extension and `import/export` syntax). This distinction helps Node.js to correctly handle both module types, which coexist in the Node.js ecosystem.

  • **Browser Support:** Modern browsers also support ECMAScript Modules, allowing developers to use `import` and `export` directly in the browser without needing tools like Webpack or Babel for module bundling or transpilation. However, browsers do not specifically require the `.mjs` extension, as they rely on the `type="module"` attribute in the `

Usage Example

In an `.mjs` file, you could have something like this:


// math.mjs

export function sum(x, y) {

    return x + y;

}

export const pi = 3.14159;

And in another file, you can import the `sum` function and the `pi` constant:


// app.mjs

import { sum, pi } from './math.mjs';

console.log(sum(10, 5));  // 15

console.log(pi);          // 3.14159

Conclusion

The `.mjs` extension is a convention that explicitly marks a file as an ECMAScript Module, providing clear semantics on how the file should be processed and executed. While its use is not mandatory in all environments (as the distinction can also be handled via package.json or server MIME types), it serves as a helpful indicator of the module system in use, especially in Node.js applications.