理解并修复 JavaScript 中的“未捕获语法错误：无法在模块外使用导入语句”

现代 JavaScript 使开发人员能够使用 **ECMAScript 模块 (ES 模块)** 编写更简洁、模块化的代码。这些模块通过启用“import”和“export”语句简化了代码组织，从而更轻松地管理依赖项并在大型应用程序中保持可伸缩性。但是，过渡到 ES 模块并不总是无缝的。开发人员经常会遇到以下错误：

Uncaught SyntaxError: Cannot use import statement outside a module

此错误通常发生在 JavaScript 文件未被正确识别为模块时，即使它使用了 ES6 语法。虽然错误消息很简单，但根本原因往往涉及配置错误或对 JavaScript 模块工作原理的误解。

本指南旨在通过探索错误原因并为浏览器和 Node.js 环境提供实用解决方案来揭开此错误的神秘面纱。最后，您将掌握在 JavaScript 项目中充分利用 ES 模块所需的知识。

问题

在未正确设置以识别 ES6 模块语法的环境中使用 ES6 模块语法（“import”和“export”）时，经常会出现错误“Uncaught SyntaxError: Cannot use import statement outside a module”。例如：

// main.js
import { greet } from './helper.js';
greet();

并且 `helper.js` 文件包含：

// helper.js
export function greet() {
  console.log("Hello, world!");
}

在浏览器或 Node.js 环境中运行上述代码时，如果这些环境不将 `helper.js` 视为模块，则会遇到错误。问题在于 JavaScript 如何确定文件是否应被视为 ES 模块。

对于从旧 JavaScript 实践过渡或在混合环境（例如使用 CommonJS 的遗留系统）中工作的开发人员来说，这个问题尤其令人沮丧。

为什么会发生这种情况？

此错误源于 JavaScript 环境解释文件及其配置的方式不同：

浏览器环境

现代浏览器原生支持 ES 模块，但你必须明确将脚本声明为模块。这可以通过将 `type="module"` 属性添加到 `

For example:

Modules in Browsers

Deferred Execution: ES Modules are deferred by default, meaning they don’t block the HTML parsing process.

Strict Mode: All ES Modules automatically run in strict mode, helping catch common errors.

Node.js Environments

Node.js traditionally used **CommonJS** as its module system, which relies on `require` and `module.exports`. While ES Modules are supported starting from Node.js version `12.x`, they require explicit configuration. If this configuration is missing, Node.js will default to treating `.js` files as CommonJS, causing the error.

What Are ES Modules?

ES Modules, introduced in ECMAScript 6 (ES6), provide a standardized way to manage dependencies in JavaScript. They allow developers to:

Export specific parts of a file, such as functions, variables, or classes, using the export keyword.

Import these parts into other files using the import keyword.

This system promotes encapsulation and reduces issues caused by global variables. For example:

// helper.js
export function greet() {
  console.log("Hello, world!");
}

// main.js
import { greet } from './helper.js';
greet();

With ES Modules, sharing functionality between files becomes explicit and maintainable, especially in large projects. Modules are also an essential component of modern build tools like Webpack, Vite, and Rollup.

CommonJS vs. ES Modules

Before ES Modules, Node.js used **CommonJS**, which is still widely supported for backward compatibility. Here’s a comparison:

Node.js Support

Versions prior to 12.x use CommonJS exclusively.

From version 12.x onwards, ES Modules are supported but require configuration (e.g., using .mjs files or adding "type": "module" to package.json).

Browser Support

Modern browsers natively support ES Modules. However, the `type="module"` attribute must be added to `

Key Benefits

Organized Code: Break your application into reusable modules.

Native Support: Modern browsers like Chrome, Firefox, and Safari handle ES Modules without additional tools.

For Node.js Environments

Option 1: Update package.json

Add `"type": "module"` to your `package.json` file:

{
  "name": "es-modules-example",
  "type": "module"
}

This tells Node.js to treat all `.js` files in the project as ES Modules.

Option 2: Use the .mjs Extension

Rename your JavaScript files to have the `.mjs` extension, which explicitly marks them as ES Modules:

mv main.js main.mjs
mv helper.js helper.mjs
node main.mjs

Option 3: Use an ESM Loader for Older Versions

For Node.js versions without native ES Module support, use the `esm` loader:

npm install esm
node -r esm main.js

Advanced Tips and Best Practices

Combining CommonJS and ES Modules

If mixing modules is unavoidable, use **dynamic imports** to load CommonJS modules in ES Modules:

const { readFile } = await import('fs');

Dynamic imports enable conditional and asynchronous loading, which is useful in complex projects.

Default vs. Named Exports

Default Exports: Use when exporting a single main function or class:

export default function greet() {
    console.log("Hello!");
  }

Named Exports: Use when exporting multiple items:

export const name = "JavaScript";
  export function version() {
    return "ES6";
  }

Cross-Browser Compatibility

For older browsers:

Use Babel to transpile ES Module syntax into compatible JavaScript.

Use Webpack to bundle modules for consistent delivery.

Conclusion

ES Modules offer a powerful and standardized framework for managing dependencies, improving code organization, and promoting maintainability. However, their adoption requires attention to configuration and compatibility.

By understanding the causes of errors like `Uncaught SyntaxError: Cannot use import statement outside a module` and following the solutions outlined here, you can avoid common pitfalls and fully leverage the benefits of ES Modules. Embracing best practices—such as sticking to a single module system and ensuring cross-environment compatibility—will enable you to build scalable, future-proof applications.

For more insights on JavaScript module systems, check out our related articles:

CommonJS vs. ES Modules: Key Differences Explained

How to Use Babel for Cross-Browser JavaScript Compatibility

Mastering JavaScript Imports and Exports for Clean Code

These resources can further enhance your understanding of modular JavaScript development. Happy coding!

CLIS.CC