# Javascript Pro — Code Examples

## Example 1

```javascript
// Debouncing and throttling
const debounce = (fn, delay) => {
  let timeoutId;
  return (...args) => {
    clearTimeout(timeoutId);
    timeoutId = setTimeout(() => fn(...args), delay);
  };
};

const throttle = (fn, limit) => {
  let inThrottle;
  return (...args) => {
    if (!inThrottle) {
      fn.apply(this, args);
      inThrottle = true;
      setTimeout(() => inThrottle = false, limit);
    }
  };
};

// Memoization
const memoize = (fn) => {
  const cache = new Map();
  return (...args) => {
    const key = JSON.stringify(args);
    if (cache.has(key)) return cache.get(key);
    const result = fn(...args);
    cache.set(key, result);
    return result;
  };
};

// Virtual scrolling implementation
class VirtualScroller {
  constructor(container, items, itemHeight) {
    this.container = container;
    this.items = items;
    this.itemHeight = itemHeight;
    this.visibleItems = Math.ceil(container.clientHeight / itemHeight);
    this.render();
  }
  
  render() {
    const scrollTop = this.container.scrollTop;
    const startIndex = Math.floor(scrollTop / this.itemHeight);
    const endIndex = startIndex + this.visibleItems;
    
    const visibleItems = this.items.slice(startIndex, endIndex);
    // Render only visible items
  }
}
```

## Example 2

```javascript
// Event delegation
document.addEventListener('click', (event) => {
  const button = event.target.closest('[data-action]');
  if (!button) return;
  
  const action = button.dataset.action;
  const handlers = {
    delete: () => deleteItem(button.dataset.id),
    edit: () => editItem(button.dataset.id),
    save: () => saveItem(button.dataset.id)
  };
  
  handlers[action]?.();
});

// Custom event system
class EventEmitter {
  constructor() {
    this.events = {};
  }
  
  on(event, listener) {
    if (!this.events[event]) this.events[event] = [];
    this.events[event].push(listener);
    
    return () => this.off(event, listener);
  }
  
  off(event, listener) {
    if (!this.events[event]) return;
    this.events[event] = this.events[event].filter(l => l !== listener);
  }
  
  emit(event, ...args) {
    if (!this.events[event]) return;
    this.events[event].forEach(listener => listener(...args));
  }
  
  once(event, listener) {
    const wrapper = (...args) => {
      listener(...args);
      this.off(event, wrapper);
    };
    this.on(event, wrapper);
  }
}
```

## Example 3

```javascript
// Stream processing
const { Transform, pipeline } = require('stream');

const upperCaseTransform = new Transform({
  transform(chunk, encoding, callback) {
    this.push(chunk.toString().toUpperCase());
    callback();
  }
});

// Worker threads
const { Worker, isMainThread, parentPort } = require('worker_threads');

if (isMainThread) {
  const worker = new Worker(__filename);
  worker.on('message', (result) => console.log(result));
  worker.postMessage({ cmd: 'calculate', data: [1, 2, 3] });
} else {
  parentPort.on('message', ({ cmd, data }) => {
    if (cmd === 'calculate') {
      const result = data.reduce((a, b) => a + b, 0);
      parentPort.postMessage(result);
    }
  });
}

// Cluster for multi-core utilization
const cluster = require('cluster');
const numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }
  
  cluster.on('exit', (worker, code, signal) => {
    console.log(`Worker ${worker.process.pid} died`);
    cluster.fork();
  });
} else {
  // Worker process
  require('./app');
}
```

## Example 4

```javascript
// Intersection Observer for lazy loading
const lazyLoad = () => {
  const images = document.querySelectorAll('img[data-src]');
  
  const imageObserver = new IntersectionObserver((entries) => {
    entries.forEach(entry => {
      if (entry.isIntersecting) {
        const img = entry.target;
        img.src = img.dataset.src;
        img.removeAttribute('data-src');
        imageObserver.unobserve(img);
      }
    });
  });
  
  images.forEach(img => imageObserver.observe(img));
};

// Web Workers for heavy computations
const worker = new Worker('worker.js');
worker.postMessage({ cmd: 'process', data: largeDataSet });
worker.onmessage = (e) => {
  console.log('Result:', e.data);
};

// IndexedDB for client storage
class IndexedDBStore {
  constructor(dbName, storeName) {
    this.dbName = dbName;
    this.storeName = storeName;
  }
  
  async open() {
    return new Promise((resolve, reject) => {
      const request = indexedDB.open(this.dbName, 1);
      
      request.onerror = () => reject(request.error);
      request.onsuccess = () => resolve(request.result);
      
      request.onupgradeneeded = (event) => {
        const db = event.target.result;
        if (!db.objectStoreNames.contains(this.storeName)) {
          db.createObjectStore(this.storeName, { keyPath: 'id' });
        }
      };
    });
  }
  
  async get(key) {
    const db = await this.open();
    const transaction = db.transaction([this.storeName], 'readonly');
    const store = transaction.objectStore(this.storeName);
    return new Promise((resolve, reject) => {
      const request = store.get(key);
      request.onsuccess = () => resolve(request.result);
      request.onerror = () => reject(request.error);
    });
  }
}
```