257 lines
6.2 KiB
Markdown
257 lines
6.2 KiB
Markdown
# 实际场景与逻辑题
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## 1. 并发控制（限制并发请求数）
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> **场景**：批量上传文件（如100张图片每次最多5个并发）、网络爬虫控制请求频率、避免压垂服务器。
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> **解决**：大量异步任务同时发起导致浏览器卡顿或服务端拒绝服务。
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```js
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async function limitConcurrency(tasks, limit) {
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const results = [];
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const executing = [];
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for (const [index, task] of tasks.entries()) {
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const p = Promise.resolve().then(() => task()).then(res => {
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results[index] = res;
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executing.splice(executing.indexOf(p), 1);
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});
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executing.push(p);
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if (executing.length >= limit) {
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await Promise.race(executing);
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}
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}
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await Promise.all(executing);
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return results;
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}
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// 使用示例
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const tasks = urls.map(url => () => fetch(url));
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await limitConcurrency(tasks, 3); // 最多3个并发
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```
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### 异步任务调度器
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```js
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class Scheduler {
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constructor(limit) {
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this.limit = limit;
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this.queue = [];
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this.running = 0;
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}
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add(promiseCreator) {
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return new Promise((resolve, reject) => {
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this.queue.push({ promiseCreator, resolve, reject });
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this.run();
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});
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}
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run() {
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while (this.running < this.limit && this.queue.length) {
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const { promiseCreator, resolve, reject } = this.queue.shift();
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this.running++;
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promiseCreator()
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.then(resolve, reject)
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.finally(() => { this.running--; this.run(); });
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}
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}
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}
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```
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---
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## 2. 缓存函数（Memoization）
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> **场景**：计算密集型函数结果缓存（如斐波那契）、API 请求缓存、React useMemo 原理。
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> **解决**：相同输入重复计算浪费性能，用空间换时间。
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```js
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function memoize(fn) {
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const cache = new Map();
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return function(...args) {
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const key = JSON.stringify(args);
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if (cache.has(key)) return cache.get(key);
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const result = fn.apply(this, args);
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cache.set(key, result);
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return result;
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};
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}
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// 支持过期时间
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function memoizeWithTTL(fn, ttl = 60000) {
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const cache = new Map();
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return function(...args) {
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const key = JSON.stringify(args);
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const cached = cache.get(key);
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if (cached && Date.now() - cached.time < ttl) {
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return cached.value;
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}
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const result = fn.apply(this, args);
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cache.set(key, { value: result, time: Date.now() });
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return result;
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};
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}
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```
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---
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## 3. 解析 URL 参数
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> **场景**：从链接提取分享参数、跟踪渠道来源（utm_source）、路由参数解析。
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> **解决**：将 URL 查询字符串转换为结构化对象，方便业务使用。
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```js
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function parseQuery(url) {
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const query = url.split('?')[1] || '';
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return query.split('&').reduce((acc, pair) => {
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const [key, value] = pair.split('=').map(decodeURIComponent);
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if (key) {
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acc[key] = acc[key]
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? [].concat(acc[key], value)
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: value;
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}
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return acc;
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}, {});
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}
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// 使用正则
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function parseQueryRegex(url) {
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const result = {};
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url.replace(/[?&]([^=&#]+)=([^&#]*)/g, (_, key, value) => {
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result[decodeURIComponent(key)] = decodeURIComponent(value);
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});
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return result;
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}
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// 示例: "https://example.com?a=1&b=2&a=3"
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// => { a: ['1', '3'], b: '2' }
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```
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---
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## 4. DOM 查找最近公共祖先
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> **场景**：事件委托的目标元素判断、富文本编辑器选区处理、拖拽边界计算。
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> **解决**：在 DOM 树中找到两个节点的最近共同父级。
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```js
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function findCommonAncestor(node1, node2) {
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const ancestors = new Set();
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// 收集 node1 的所有祖先
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let current = node1;
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while (current) {
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ancestors.add(current);
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current = current.parentNode;
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}
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// 查找 node2 的祖先中第一个在 ancestors 中的节点
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current = node2;
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while (current) {
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if (ancestors.has(current)) return current;
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current = current.parentNode;
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}
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return null;
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}
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// 原生方法（现代浏览器）
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// node1.compareDocumentPosition(node2)
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```
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---
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## 5. 懒加载/无限滚动
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> **场景**：电商商品列表、社交信息流、图片画廊、新闻列表。
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> **解决**：首屏加载慢、一次性加载大量数据卡顿的问题。
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```js
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// IntersectionObserver 实现懒加载
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function lazyLoad(selector) {
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const observer = new IntersectionObserver((entries) => {
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entries.forEach(entry => {
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if (entry.isIntersecting) {
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const img = entry.target;
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img.src = img.dataset.src;
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observer.unobserve(img);
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}
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});
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});
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document.querySelectorAll(selector).forEach(img => observer.observe(img));
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}
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// 无限滚动
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function infiniteScroll(container, loadMore) {
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const observer = new IntersectionObserver(([entry]) => {
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if (entry.isIntersecting) loadMore();
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});
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// 监听底部占位元素
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const sentinel = document.createElement('div');
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container.appendChild(sentinel);
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observer.observe(sentinel);
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}
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```
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---
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## 6. LRU 缓存
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> **场景**：浏览器缓存淘汰、图片缓存池、Redis 内存管理、keep-alive 组件缓存。
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> **解决**：内存有限时如何淘汰最久未使用的数据，保留热点数据。
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```js
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class LRUCache {
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constructor(capacity) {
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this.capacity = capacity;
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this.cache = new Map();
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}
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get(key) {
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if (!this.cache.has(key)) return -1;
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const value = this.cache.get(key);
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this.cache.delete(key);
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this.cache.set(key, value);
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return value;
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}
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put(key, value) {
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if (this.cache.has(key)) this.cache.delete(key);
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this.cache.set(key, value);
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if (this.cache.size > this.capacity) {
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this.cache.delete(this.cache.keys().next().value);
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}
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}
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}
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```
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---
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## 7. 大数相加
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> **场景**：订单号/交易号处理、金融精确计算、超出 JS Number 范围的计算。
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> **解决**：JS 数字最大安全整数 2^53-1 限制，大数运算精度丢失问题。
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```js
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function addBigNumbers(a, b) {
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const maxLen = Math.max(a.length, b.length);
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a = a.padStart(maxLen, '0');
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b = b.padStart(maxLen, '0');
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let carry = 0, result = '';
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for (let i = maxLen - 1; i >= 0; i--) {
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const sum = +a[i] + +b[i] + carry;
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result = (sum % 10) + result;
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carry = Math.floor(sum / 10);
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}
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return carry ? carry + result : result;
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}
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// 示例: addBigNumbers("12345678901234567890", "98765432109876543210")
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```
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