前端面试常见的手写功能(常用代码)

前端大学

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2020-09-26 11:36

(给前端大学加星标,提升前端技能.

作者:iboying

https://juejin.im/post/6873513007037546510

1. 防抖

function debounce(func, ms = 1000) {  let timer;  return function (...args) {    if (timer) {      clearTimeout(timer)    }    timer = setTimeout(() => {      func.apply(this, args)    }, ms)  }}
// 测试const task = () => { console.log('run task') }const debounceTask = debounce(task, 1000)window.addEventListener('scroll', debounceTask)

2. 节流

function throttle(func, ms = 1000) {  let canRun = true  return function (...args) {    if (!canRun) return    canRun = false    setTimeout(() => {      func.apply(this, args)      canRun = true    }, ms)  }}
// 测试const task = () => { console.log('run task') }const throttleTask = throttle(task, 1000)window.addEventListener('scroll', throttleTask)

3. new

function myNew(Func, ...args) {  const instance = {};  if (Func.prototype) {    Object.setPrototypeOf(instance, Func.prototype)  }  const res = Func.apply(instance, args)  if (typeof res === "function" || (typeof res === "object" && res !== null)) {    return res  }  return instance}
// 测试function Person(name) { this.name = name}Person.prototype.sayName = function() { console.log(`My name is ${this.name}`)}const me = myNew(Person, 'Jack')me.sayName()console.log(me)

4. bind

Function.prototype.myBind = function (context = globalThis) {  const fn = this  const args = Array.from(arguments).slice(1)  const newFunc = function () {    const newArgs = args.concat(...arguments)    if (this instanceof newFunc) {      // 通过 new 调用,绑定 this 为实例对象      fn.apply(this, newArgs)    } else {      // 通过普通函数形式调用,绑定 context      fn.apply(context, newArgs)    }  }  // 支持 new 调用方式  newFunc.prototype = Object.create(fn.prototype)  return newFunc}
// 测试const me = { name: 'Jack' }const other = { name: 'Jackson' }function say() { console.log(`My name is ${this.name || 'default'}`);}const meSay = say.bind(me)meSay()const otherSay = say.bind(other)otherSay()

5. call

Function.prototype.myCall = function (context = globalThis) {  // 关键步骤,在 context 上调用方法,触发 this 绑定为 context,使用 Symbol 防止原有属性的覆盖  const key = Symbol('key')  context[key] = this  let args = [].slice.call(arguments, 1)  let res = context[key](...args)  delete context[key]  return res};
// 测试const me = { name: 'Jack' }function say() { console.log(`My name is ${this.name || 'default'}`);}say.myCall(me)

6. apply

Function.prototype.myApply = function (context = globalThis) {  // 关键步骤,在 context 上调用方法,触发 this 绑定为 context,使用 Symbol 防止原有属性的覆盖  const key = Symbol('key')  context[key] = this  let res  if (arguments[1]) {    res = context[key](...arguments[1])  } else {    res = context[key]()  }  delete context[key]  return res}
// 测试const me = { name: 'Jack' }function say() { console.log(`My name is ${this.name || 'default'}`);}say.myApply(me)

7. deepCopy

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,用于处理循环引用的情况 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}
// 测试const source = { name: 'Jack', meta: { age: 12, birth: new Date('1997-10-10'), ary: [1, 2, { a: 1 }], say() { console.log('Hello'); } }}source.source = sourceconst newObj = deepCopy(source)console.log(newObj.meta.ary[2] === source.meta.ary[2]);


8. 事件总线 | 发布订阅模式

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]) { // 创建副本,如果回调函数内继续注册相同事件,会造成死循环 const tasks = this.cache[name].slice() for (let fn of tasks) { fn(); } } }
emit(name, once = false) { if (this.cache[name]) { // 创建副本,如果回调函数内继续注册相同事件,会造成死循环 const tasks = this.cache[name].slice() for (let fn of tasks) { fn(); } if (once) { delete this.cache[name] } } }}
// 测试const eventBus = new EventEmitter()const task1 = () => { console.log('task1'); }const task2 = () => { console.log('task2'); }eventBus.on('task', task1)eventBus.on('task', task2)
setTimeout(() => { eventBus.emit('task')}, 1000)

9. 柯里化:只传递给函数一部分参数来调用它,让它返回一个函数去处理剩下的参数

function curry(func) {  return function curried(...args) {    // 关键知识点:function.length 用来获取函数的形参个数    // 补充:arguments.length 获取的是实参个数    if (args.length >= func.length) {      return func.apply(this, args)    }    return function (...args2) {      return curried.apply(this, args.concat(args2))    }  }}
// 测试function sum (a, b, c) { return a + b + c}const curriedSum = curry(sum)console.log(curriedSum(1, 2, 3))console.log(curriedSum(1)(2,3))console.log(curriedSum(1)(2)(3))

10. es5 实现继承

function create(proto) {  function F() {}  F.prototype = proto;  return new F();}
// Parentfunction Parent(name) { this.name = name}
Parent.prototype.sayName = 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.sayAge = function () { console.log(this.age)}
// 测试const child = new Child(18, 'Jack')child.sayName()child.sayAge()

11. instanceof

function isInstanceOf(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__  }}
// 测试class Parent {}class Child extends Parent {}const child = new Child()console.log(isInstanceOf(child, Parent), isInstanceOf(child, Child), isInstanceOf(child, Array))

12. 异步并发数限制

/** * 关键点 * 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(2, [1000, 1000, 1000, 1000], timeout).then((res) => { console.log(res)})

13. 异步串行 | 异步并行

// 字节面试题,实现一个异步加法function asyncAdd(a, b, callback) {  setTimeout(function () {    callback(null, a + b);  }, 500);}
// 解决方案// 1. promisifyconst promiseAdd = (a, b) => new Promise((resolve, reject) => { asyncAdd(a, b, (err, res) => { if (err) { reject(err) } else { resolve(res) } })})
// 2. 串行处理async function serialSum(...args) { return args.reduce((task, now) => task.then(res => promiseAdd(res, now)), Promise.resolve(0))}
// 3. 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 () => { console.log('Running...'); 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) console.log('Done');})()

14. vue reactive

// 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 (val && typeof val === 'object') { 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)

15. promise

// 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); throw new Error('a error') }).catch((err) => { console.log('==>', err); })

16. 数组扁平化

// 方案 1function recursionFlat(ary = []) {  const res = []  ary.forEach(item => {    if (Array.isArray(item)) {      res.push(...recursionFlat(item))    } else {      res.push(item)    }  })  return res}// 方案 2function reduceFlat(ary = []) {  return ary.reduce((res, item) => res.concat(Array.isArray(item) ? reduceFlat(item) : item), [])}
// 测试const source = [1, 2, [3, 4, [5, 6]], '7']console.log(recursionFlat(source))console.log(reduceFlat(source))

17. 对象扁平化

function objectFlat(obj = {}) {  const res = {}  function flat(item, preKey = '') {    Object.entries(item).forEach(([key, val]) => {      const newKey = preKey ? `${preKey}.${key}` : key      if (val && typeof val === 'object') {        flat(val, newKey)      } else {        res[newKey] = val      }    })  }  flat(obj)  return res}
// 测试const source = { a: { b: { c: 1, d: 2 }, e: 3 }, f: { g: 2 } }console.log(objectFlat(source));

18. 图片懒加载

// function isVisible(el) {  const position = el.getBoundingClientRect()  const windowHeight = document.documentElement.clientHeight  // 顶部边缘可见  const topVisible = position.top > 0 && position.top < windowHeight;  // 底部边缘可见  const bottomVisible = position.bottom < windowHeight && position.bottom > 0;  return topVisible || bottomVisible;}
function imageLazyLoad() { const images = document.querySelectorAll('img') for (let img of images) { const realSrc = img.dataset.src if (!realSrc) continue if (isVisible(img)) { img.src = realSrc img.dataset.src = '' } }}
// 测试window.addEventListener('load', imageLazyLoad)window.addEventListener('scroll', imageLazyLoad)// orwindow.addEventListener('scroll', throttle(imageLazyLoad, 1000))

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