JavaScriptの一般的な手書き機能

function debounce(func, ms = 500) { let timer; return function (...args) { if (timer) { clearTimeout(timer); } timer = setTimeout(() => { func.apply(this, args); }, ms); }; }复制代码

function throttle(func, ms) { let canRun = true; return function (...args) { if (!canRun) return; canRun = false; setTimeout(() => { func.apply(this, args); canRun = true; }, ms); }; }复制代码

function myNew(Func) { const instance = {}; if (Func.prototype) { Object.setPrototypeOf(instance, Func.prototype); } const res = Func.apply(instance, [].slice.call(arguments, 1)); if (typeof res === "function" || (typeof res === "object" && res !== null)) { return res; } return instance; }复制代码

Function.prototype.myBind = function (context = globalThis) { const fn = this; const args = Array.from(arguments).slice(1); const newFunc = function () { if (this instanceof newFunc) { // 通过 new 调用，绑定 this 为实例对象 fn.apply(this, args); } else { // 通过普通函数形式调用，绑定 context fn.apply(context, args); } }; // 支持 new 调用方式 newFunc.prototype = fn.prototype; return newFunc; };复制代码

Function.prototype.myCall = function (context = globalThis) { // 关键步骤，在 context 上调用方法，触发 this 绑定为 context context.fn = this; let args = [].slice.call(arguments, 1); let res = context.fn(...args); delete context.fn; return res; };复制代码

Function.prototype.myApply = function (context = globalThis) { // 关键步骤，在 context 上调用方法，触发 this 绑定为 context context.fn = this; let res; if (arguments[1]) { res = context.fn(...arguments[1]); } else { res = context.fn(); } delete context.fn; return res; };复制代码

function deepCopy(obj, cache = new WeakMap()) { if (!obj instanceof Object) return obj; // 防止循环引用 if (cache.get(obj)) return cache.get(obj); // 支持函数 if (obj instanceof Function) { return function () { obj.apply(this, arguments); }; } // 支持日期 if (obj instanceof Date) return new Date(obj); // 支持正则对象 if (obj instanceof RegExp) return new RegExp(obj.source, obj.flags); // 还可以增加其他对象，比如：Map, Set等，根据情况判断增加即可，面试点到为止就可以了 // 数组是 key 为数字素银的特殊对象 const res = Array.isArray(obj) ? [] : {}; // 缓存 copy 的对象，用于出来循环引用的情况 cache.set(obj, res); Object.keys(obj).forEach((key) => { if (obj[key] instanceof Object) { res[key] = deepCopy(obj[key], cache); } else { res[key] = obj[key]; } }); return res; }复制代码

class EventEmitter { constructor() { this.cache = {}; } on(name, fn) { if (this.cache[name]) { this.cache[name].push(fn); } else { this.cache[name] = [fn]; } } off(name, fn) { const tasks = this.cache[name]; if (tasks) { const index = tasks.findIndex((f) => f === fn || f.callback === fn); if (index >= 0) { tasks.splice(index, 1); } } } emit(name) { if (this.cache[name]) { for (let fn of this.cache[name]) { fn(); } } } emit(name, once = false) { if (this.cache[name]) { // 创建事件副本，如果回调函数内继续注册相同事件，触发时，会造成死循环 const tasks = this.cache[name].slice() for (let fn of tasks) { fn(); } if (once) { delete this.cache[name] } } } }复制代码

function curry(func) { return function curried(...args) { if (args.length >= func.length) { func.apply(this, args); } else { return function (...args2) { curried.apply(this, args.concat(args2)); }; } }; }复制代码

function create(proto) { function F() {} F.prototype = proto; return new F(); }// Parentfunction Parent(name) { this.name = name; } Parent.prototype.say = function () { console.log(this.name); };// Childfunction Child(age, name) { Parent.call(this, name); this.age = age; } Child.prototype = create(Parent.prototype); Child.prototype.constructor = Child; Child.prototype.say = function () { console.log(this.age); };复制代码

function instanceOf(instance, klass) { let proto = instance.__proto__; let prototype = klass.prototype; while (true) { if (proto === null) return false; if (proto === prototype) return true; proto = proto.__proto__; } }复制代码

/** * 关键点说明 * 1. new promise 一经创建，立即执行 * 2. 使用 Promise.resolve().then 可以把任务加到微任务队列，防止立即执行迭代方法 * 3. 微任务处理过程中，产生的新的微任务，会在同一事件循环内，追加到微任务队列里 * 4. 使用 race 在某个任务完成时，继续添加任务，保持任务按照最大并发数进行执行 * 5. 任务完成后，需要从 doingTasks 中移出 */function limit(count, array, iterateFunc) { const tasks = []; const doingTasks = []; let i = 0; const enqueue = () => { if (i === array.length) { return Promise.resolve(); } const task = Promise.resolve().then(() => iterateFunc(array[i++])); tasks.push(task); const doing = task.then(() => doingTasks.splice(doingTasks.indexOf(doing), 1)); doingTasks.push(doing); const res = doingTasks.length >= count ? Promise.race(doingTasks) : Promise.resolve(); return res.then(enqueue); }; return enqueue().then(() => Promise.all(tasks)); }// testconst timeout = i => new Promise(resolve => setTimeout(() => resolve(i), i)); limit(4, [1000, 1000, 1000, 1000], timeout).then((res) => { console.log(res); })复制代码

// 大厂面试题，实现一个异步加法function asyncAdd(a, b, callback) { setTimeout(function () { callback(null, a + b); }, 1000); }// 0. promisifyconst promiseAdd = (a, b) => new Promise((resolve, reject) => { asyncAdd(a, b, (err, res) => { if (err) { reject(err) } else { resolve(res) } }) })// 1. 串行处理async function serialSum(...args) { return args.reduce((task, now) => task.then(res => promiseAdd(res, now)), Promise.resolve(0)) }// 2. 并行处理async function parallelSum(...args) { if (args.length === 1) return args[0] const tasks = [] for (let i = 0; i < args.length; i += 2) { tasks.push(promiseAdd(args[i], args[i + 1] || 0)) } const results = await Promise.all(tasks) return parallelSum(...results) }// 测试(async () => { const res1 = await serialSum(1, 2, 3, 4, 5, 8, 9, 10, 11, 12) console.log(res1) const res2 = await parallelSum(1, 2, 3, 4, 5, 8, 9, 10, 11, 12) console.log(res2) })()复制代码

// Dep moduleclass Dep { static stack = [] static target = null deps = null constructor() { this.deps = new Set() } depend() { if (Dep.target) { this.deps.add(Dep.target) } } notify() { this.deps.forEach(w => w.update()) } static pushTarget(t) { if (this.target) { this.stack.push(this.target) } this.target = t } static popTarget() { this.target = this.stack.pop() } }// reactivefunction reactive(o) { if (o && typeof o === 'object') { Object.keys(o).forEach(k => { defineReactive(o, k, o[k]) }) } return o }function defineReactive(obj, k, val) { let dep = new Dep() Object.defineProperty(obj, k, { get() { dep.depend() return val }, set(newVal) { val = newVal dep.notify() } }) if (isObj(val)) { reactive(val) } }// watcherclass Watcher { constructor(effect) { this.effect = effect this.update() } update() { Dep.pushTarget(this) this.value = this.effect() Dep.popTarget() return this.value } }// 测试代码const data = reactive({ msg: 'aaa'})new Watcher(() => { console.log('===> effect', data.msg); }) setTimeout(() => { data.msg = 'hello'}, 1000)复制代码

// 建议阅读 [Promises/A+ 标准](https://promisesaplus.com/)class MyPromise { constructor(func) { this.status = 'pending' this.value = null this.resolvedTasks = [] this.rejectedTasks = [] this._resolve = this._resolve.bind(this) this._reject = this._reject.bind(this) try { func(this._resolve, this._reject) } catch (error) { this._reject(error) } } _resolve(value) { setTimeout(() => { this.status = 'fulfilled' this.value = value this.resolvedTasks.forEach(t => t(value)) }) } _reject(reason) { setTimeout(() => { this.status = 'reject' this.value = reason this.rejectedTasks.forEach(t => t(reason)) }) } then(onFulfilled, onRejected) { return new MyPromise((resolve, reject) => { this.resolvedTasks.push((value) => { try { const res = onFulfilled(value) if (res instanceof MyPromise) { res.then(resolve, reject) } else { resolve(res) } } catch (error) { reject(error) } }) this.rejectedTasks.push((value) => { try { const res = onRejected(value) if (res instanceof MyPromise) { res.then(resolve, reject) } else { reject(res) } } catch (error) { reject(error) } }) }) } catch(onRejected) { return this.then(null, onRejected); } }// 测试new MyPromise((resolve) => { setTimeout(() => { resolve(1); }, 500); }) .then((res) => { console.log(res); return new MyPromise((resolve) => { setTimeout(() => { resolve(2); }, 500); }); }) .then((res) => { console.log(res); }, err => { console.log('==>', err); });复制代码