JavaScript模塊上的確定JS綜述：ES模塊與COMPORJS

ES模块和CommonJS的主要区别在于加载方式和使用场景。1. CommonJS是同步加载，适用于Node.js服务器端环境；2. ES模块是异步加载，适用于浏览器等网络环境；3. 语法上，ES模块使用import/export，且必须位于顶层作用域，而CommonJS使用require/module.exports，可在运行时动态调用；4. CommonJS广泛用于旧版Node.js及依赖它的库如Express，ES模块则适用于现代前端框架和Node.js v14+；5. 虽然可混合使用，但容易引发问题，建议项目中统一使用一种方式；6. 使用ES模块时需注意文件扩展名、路径处理等细节问题。

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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