ES Modules (ESM) is the official standard module system of JavaScript, which implements code dependency management through import and export. 1. It provides better code organization and reuse, makes functional modules clearly separated, improves readability and collaboration efficiency, and supports cross-project reuse and avoids naming conflicts; 2. It supports Tree-shaking optimization performance, and build tools can remove unused code and reduce packaging volume; 3. Native support brings a faster development experience, such as Vite uses ESM to enable instant startup of the development server. When using it, you should pay attention to the complete path, the local server environment is required, compatibility issues, and dynamic import should be used. Mainstream frameworks such as React, Vue, and Svelte have fully supported ESM, and recommend modular development methods. Mastering ESM is crucial to improving development efficiency and quality.

ES Modules (ECMAScript Modules, ESM) plays a core role in modern front-end development. It is not only a way of code organization, but also the basis for modularity, maintainability and performance optimization.

What are ES Modules?

ES Modules is an official JavaScript standard module system, introduced from ES6 (ES2015). Its core idea is to manage code dependencies and exposure through import and export .

• Modularity : Each file is an independent module that can export variables, functions, or classes.
• Static structure : Unlike CommonJS, the import and export of ESM is determined during the code parsing stage, not determined at the runtime.
• Browser native support : mainstream browsers already support ESM and can be used directly without additional packaging tools.

For example:

 // math.js
export function add(a, b) {
  return ab;
}

// main.js
import { add } from './math.js';
console.log(add(2, 3));

Why is ESM so important in the modern front-end?

1. Better code organization and reuse

ESM allows developers to split functional modules in a clearer way. For example, putting API requests, tool functions, and component logic in different modules not only improves the readability of the code, but also facilitates team collaboration.

• Each module has a single responsibility
• Can be reused across projects
• Avoid naming conflicts

2. Support Tree-shaking to improve performance

The static import feature of ESM allows building tools (such as Webpack, Rollup, Vite) to perform tree-shaking, that is, automatically remove unused code. This is very critical to reducing the final packaging volume.

For example:

 // utils.js
export function formatTime() { /* ... */ }
export function deepClone() { /* ... */ }

// main.js
import { formatTime } from './utils.js';

In this case, the packaging tool can recognize that deepClone is not being used, thus excluding it from the final output.

3. Native support brings faster development experience

Modern build tools like Vite use browser native support for ESM to achieve "instant startup" development servers without waiting for the entire project to be packaged. This saves a lot of development time for large projects.

What should I pay attention to in actual use?

Although ESM is very powerful, there are still several easy places to get stuck in actual use:

• The path must be complete : including the extension .js , otherwise the browser may not load correctly.
• Requires a local server environment : You cannot use ESM directly through file:// to open HTML files, and an error CORS will be reported.
• Compatibility issues : If an old project is still using CommonJS, it will cost to migrate to ESM.
• Dynamic import uses the import() function : If you need to load modules on demand, you can use import('./module.js') , which returns a promise.

The relationship between ESM and modern frameworks

Currently, mainstream front-end frameworks such as React, Vue and Svelte all embrace ESM. Most of their official documents and examples are written based on ESM, and developers are recommended to use modular development methods.

For example, the Vue Single File Component (SFC) also organizes logic, templates and styles in a modular way, and finally processes it into a format that is available to the browser through the build tool.

Basically that's it. ES Modules does not seem complicated, but has a profound impact in modern front-end engineering. Mastering it is a key step in improving development efficiency and quality.

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