如何提高前端应用的性能？

# 前端性能优化实战指南

## 1. 资源加载优化

### 1.1 代码拆分与懒加载
```javascript
// 动态导入实现懒加载
const LazyComponent = React.lazy(() => import('./LazyComponent'));

function MyComponent() {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <LazyComponent />
    </Suspense>
  );
}

1.2 资源预加载

<!-- 预加载关键资源 -->
<link rel="preload" href="critical.css" as="style">
<link rel="prefetch" href="next-page.js" as="script">

1.3 图片优化

• 使用WebP格式替代JPEG/PNG
• 实现响应式图片：

<picture>
  <source srcset="image.webp" type="image/webp">
  <source srcset="image.jpg" type="image/jpeg"> 
  <img src="image.jpg" alt="Description">
</picture>

2. 渲染性能优化

2.1 减少重排重绘

// 批量DOM操作
const container = document.getElementById('container');
const fragment = document.createDocumentFragment();

items.forEach(item => {
  const el = document.createElement('div');
  el.textContent = item;
  fragment.appendChild(el);
});

container.appendChild(fragment);

2.2 使用CSS硬件加速

.transform-element {
  transform: translateZ(0);
  will-change: transform;
}

2.3 虚拟列表实现

// React示例
import { FixedSizeList as List } from 'react-window';

<List
  height={400}
  itemCount={1000}
  itemSize={50}
  width={300}
>
  {({ index, style }) => (
    <div style={style}>Row {index}</div>
  )}
</List>

3. JavaScript优化

3.1 防抖与节流

// 防抖实现
function debounce(func, delay) {
  let timer;
  return function(...args) {
    clearTimeout(timer);
    timer = setTimeout(() => func.apply(this, args), delay);
  };
}

// 节流实现
function throttle(func, limit) {
  let lastFunc;
  let lastRan;
  return function(...args) {
    if (!lastRan) {
      func.apply(this, args);
      lastRan = Date.now();
    } else {
      clearTimeout(lastFunc);
      lastFunc = setTimeout(() => {
        if ((Date.now() - lastRan) >= limit) {
          func.apply(this, args);
          lastRan = Date.now();
        }
      }, limit - (Date.now() - lastRan));
    }
  };
}

3.2 Web Worker使用

// 主线程
const worker = new Worker('worker.js');
worker.postMessage(data);
worker.onmessage = (e) => {
  console.log('Result:', e.data);
};

// worker.js
self.onmessage = (e) => {
  const result = heavyCalculation(e.data);
  self.postMessage(result);
};

4. 缓存策略

4.1 Service Worker缓存

// service-worker.js
self.addEventListener('install', (event) => {
  event.waitUntil(
    caches.open('v1').then((cache) => {
      return cache.addAll([
        '/',
        '/index.html',
        '/styles/main.css',
        '/scripts/main.js'
      ]);
    })
  );
});

self.addEventListener('fetch', (event) => {
  event.respondWith(
    caches.match(event.request).then((response) => {
      return response || fetch(event.request);
    })
  );
});

4.2 HTTP缓存头设置

Cache-Control: public, max-age=31536000
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"

5. 监控与分析

5.1 性能指标采集

// 核心Web指标监控
const observer = new PerformanceObserver((list) => {
  for (const entry of list.getEntries()) {
    console.log(entry.name, entry.startTime, entry.duration);
  }
});

observer.observe({ type: 'largest-contentful-paint', buffered: true });
observer.observe({ type: 'first-input-delay', buffered: true });

5.2 内存泄漏检测