什麼是樹木搖晃，它如何工作？

Tree shaking是一种通过移除未使用代码来优化JavaScript应用的技术。它基于ES6模块的静态结构，使构建工具能分析并排除未引用的函数、变量或类。具体步骤包括：1. 构建依赖图；2. 标记使用的导出项；3. 排除未使用的代码。其优势在于减少加载时间、降低内存占用并提升响应速度。要实现有效tree shaking，需使用ES6 import/export语法、启用生产环境模式，并避免副作用。正确使用可显著减小最终包体积，提高性能。

Tree shaking is a method used in JavaScript bundlers like Webpack or Rollup to eliminate dead code — that is, code that's written but never actually used in the application. The goal is simple: smaller bundles, faster load times, and better performance.

The term “tree shaking” comes from the idea of shaking a tree and having the dead leaves fall off. Similarly, unused code "falls off" during the build process, leaving only what’s needed.

How Tree Shaking Works

Tree shaking relies on ES6 module syntax (import and export). Because these modules are static (their structure is known at compile time), tools can analyze which exports are actually used and which ones aren't.

Here's how it typically works:

• The bundler builds a dependency graph starting from your entry file.
• It walks through all the imported modules and marks which exports are used.
• Any functions, variables, or classes that aren’t referenced anywhere get excluded from the final bundle.

It doesn’t matter if you import an entire library — if you only use one function from it, only that function might end up in the final output (assuming the library is structured properly with ES modules).

Why Tree Shaking Matters for Performance

Smaller JavaScript bundles mean less data needs to be downloaded, parsed, and executed by the browser. That translates directly into faster page loads and improved user experience — especially on mobile networks or low-powered devices.

A few reasons why this matters:

• Reduces initial load time
• Lowers memory usage
• Improves overall app responsiveness

If you're using a large utility library like Lodash or React, even small savings per component can add up quickly.

What You Need for Tree Shaking to Work

Not every setup will benefit automatically from tree shaking. Here are some conditions that help make it effective:

• Use ES6 import/export syntax instead of CommonJS (require).
• Make sure your bundler (like Webpack, Rollup, or Vite) supports tree shaking.
• Set your environment to production mode — many tools skip dead code removal in development.
• Avoid side effects where possible, or explicitly mark them in package.json with "sideEffects": false or an array of files that have intentional side effects.

Also, keep in mind that tree shaking isn’t magic. If you write something like:

import * as utils from './utils';

and then never use any of the utils, modern bundlers should remove them. But if you dynamically access properties or do things at runtime, the tooling might not be able to tell what’s unused.

Real-World Examples and Tips

Let’s say you have a file like this:

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

In another file, you only use add:

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

With tree shaking enabled, the final bundle may include only the add function and leave out subtract.

Some quick tips:

• Prefer named exports over default exports when possible (makes it easier to track usage).
• Use libraries that are built with ES modules and optimized for tree shaking (e.g., React 17 ).
• Be cautious with higher-order components or dynamic imports — they can sometimes prevent proper shaking.

So yeah, tree shaking helps cut down your app size by removing unused code, and it works best with clean ES module structures. It’s not complicated, but it does require paying attention to how you import and structure your code.

基本上就这些。

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