ES模塊和CommonJS的主要區(qū)別在于加載方式和使用場景。1. CommonJS是同步加載，適用于Node.js服務(wù)器端環(huán)境；2. ES模塊是異步加載，適用于瀏覽器等網(wǎng)絡(luò)環(huán)境；3. 語法上，ES模塊使用import/export，且必須位于頂層作用域，而CommonJS使用require/module.exports，可在運行時動態(tài)調(diào)用；4. CommonJS廣泛用于舊版Node.js及依賴它的庫如Express，ES模塊則適用于現(xiàn)代前端框架和Node.js v14+；5. 雖然可混合使用，但容易引發(fā)問題，建議項目中統(tǒng)一使用一種方式；6. 使用ES模塊時需注意文件擴展名、路徑處理等細節(jié)問題。

If you're working with JavaScript and have run into modules, you've probably seen terms like ES Modules and CommonJS floating around. They’re both ways to organize and share code across files, but they work differently and are used in different environments. Here’s a straightforward breakdown of the main differences and when to use each.

What's the big difference between ES Modules and CommonJS?

The core difference is when things happen — CommonJS loads modules synchronously, while ES Modules load them asynchronously. That might sound abstract, but it affects how your code behaves and where you can use each system.

CommonJS was designed for server-side JavaScript (like Node.js), where files are read from disk quickly, so waiting for one module to finish loading before moving on isn't a problem.

ES Modules, on the other hand, were built with browsers in mind. Since fetching files over the network takes time, ES Modules are structured to handle that better by loading in a way that doesn’t block everything else.

How do import and export syntax differ?

This is where most developers notice the biggest difference day-to-day.

In ES Modules:

You use import and export like this:

// math.js
export const add = (a, b) => a + b;

// main.js
import { add } from './math.js';

These statements are static, meaning you can’t put them inside conditionals or functions. This helps tools like bundlers optimize your code during build time.

In CommonJS:

You use require() and module.exports:

// math.js
exports.add = (a, b) => a + b;

// or
module.exports = {
  add: (a, b) => a + b,
};

// main.js
const math = require('./math');
const add = math.add;

Here, require() can be called anywhere — even inside loops or if statements — because it runs at runtime.

Which one works where?

Use CommonJS when:

• You're writing Node.js code that doesn't use "type": "module" in package.json
• You're using older versions of Node.js (before ES Module support)
• You're working with tools like Express or Mongoose that historically rely on CommonJS

Use ES Modules when:

• You're building frontend apps (React, Vue, etc.)
• You're using modern Node.js versions (v14+) and want to write cleaner, future-proof code
• You're building libraries that need to be tree-shakable or used in both browser and Node.js environments

Note: If you're using a bundler like Webpack or Vite, you're likely already using ES Modules under the hood.

Can you mix ES Modules and CommonJS?

Technically yes, but it gets messy fast. Node.js allows you to mix them using certain flags or file extensions (mjs vs cjs), but doing so can lead to confusion and bugs.

For example, importing a CommonJS module into an ES Module works fine most of the time, but exporting from an ES Module and trying to use it in CommonJS might give you unexpected results unless you use await import().

So unless you have a very good reason, stick to one style per project.

A few gotchas to watch out for

• File extensions matter more in ES Modules — you usually need to include .js when importing.
• __dirname and path usage is trickier in ES Modules; you’ll often need to use import.meta.url and new URL() instead.
• Some older packages on npm still use CommonJS, which may cause issues if you're enforcing strict ESM mode.

That’s basically it. It’s not rocket science, but understanding these differences helps avoid confusion — especially as more tools shift toward ES Modules by default.

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