不妨试试这13个手写代码？(面试高频)

1. promise.all

Promise.all = function(promises) {
  return new Promise(function(resolve, reject) {
    var result = []
    var count = 0
    var promiseNum = promises.length
    if (promises.length === 0) resolve([])

    for (let i = 0; i < promiseNum; i++) {
      Promise.resolve(promises[i]).then(function(val) {
        count++
        result[i] = val
        if (count === promiseNum) resolve(result)
      }, function(error){
        reject(error)
      })
    }
  })
}

2. promise.race

Promise.race = function(promises) {
  return new Promise(function(resolve, reject){
    for (var i; i < promises.length; i++) {
      Promise.resolve(promises[i]).then(function(data){
        resolve(data)
      }, function(error) {
        reject(error)
      })
    }
  })
}

3. Object.create

_create = function(proto, properties) {
  // 新申明一个函数
  var F = function() {}
  // 将函数的原型指向proto
  F.prototype = proto
  // 返回这个函数的实力化对象
  var o = new F()
  if (typeof properties === 'object') {
    Object.defineProperties(o, properties)
  }
  return o
}

4. new

function _new(Con, ...args) {
  //1.创建一个空的简单JavaScript对象（即car = {}）；
  let obj = {}
  // 2.将构造函数的作用域赋给新对象（因此this就指向了这个对象）；
  Object.setPrototypeOf(obj, Con.prototype)
  // obj.constructor.prototype === obj.__proto__ === Con.prototype
  // 3.相当于调用构造函数Car，并拿到构造函数返回的结果（执行构造函数中的代码（为这个新对象添加属性)）
  let result = Con.apply(obj, args)

  // 4. 如果构造函数返回的结果是object，则用构造函数的结果作为返回值；否则返回这个临时对象(也就是this)，这个临时对象的原型链指向该构造函数
  return result instanceof Object ? result : obj
}

5. map

Array.prototype.map = function(fn) {
  if(this===null || underfined){
    throw '输入不能为null或者underfinded'
  }
  
  let res = []
  
  for (let i=0; i<this.length; i++) {
    res.push(fn(this[i], i, this))
  }

  return res
}

6. filter

Array.prototype.filter = function(fn) {
  if(this===null || underfined){
    throw '输入不能为null或者underfinded'
  }

  let res = []

  for (let i = 0; i <this.length; i++) {
    if (fn(this[i],i,this)) {
      res.push(this[i])
    }
  }

  return res
}

7. reduce

Array.prototype.reduce = function(fn, base) {
  let initialArr = this;
  let arr = initialArr.concat();
  if(base) arr.unshift(base)
  let newValue;

  while(arr.length > 1) {
    newValue = fn.call(null, arr[0], arr[1]);
    arr.splice(0,2,newValue)
  }

  return newValue
}

8. 深拷贝

function deepCopy(obj, cache = new Map()) {
  if (cache.get(obj)) {
    return cache.get(obj)
  }

  if (typeof obj !== 'object') return obj
  if (typeof obj === null) return obj
  let cloneObj

  if (obj instanceof Date) return new Date(obj.getTime())
  if (obj instanceof RegExp) return new RegExp(obj.source, obj.flags)
  if (obj instanceof Array) {
    cloneObj = []
  } else {
    const p = obj.constructor.prototype
    cloneObj = Object.create(p) // 将对象的原型链也复制，如果用{}代替p，则对象构造函数(constructor)都将为Object
  }
  cache.set(obj)
  for (let key in obj) {
    // 只克隆该属性，不克隆其原型属性
    if (obj.hasOwnProperty(key)) {
      cloneObj[key] = deepCopy(obj[key], cache)
    }
  }
  return cloneObj
}

9. 防抖

function debounce(fn, delay) {
  let ts = null

  return function() {
    clearTimeout(ts)
    ts = setTimeout(() =>  {
      fn.apply(this, arguments) // 在这个示例中是否用apply作用不大
    }, delay)
  }
}

10. 节流

function throttle(fn, delay) {
  let ts = null
  return function() {
    if (!ts) {
      ts = setTimeout(() => {
        fn.call(this, ...arguments)
        ts = null
      }, delay)
    }
  }
}

11. call

Function.prototype.call = function(context) {
  if(typeof this !== 'function') {
    throw new TypeError(`${this} is not a function`)
  }

  context = context || window;
  context.fn = this
  const args = [...arguments].slice(1)
  const result = context.fn(...args)
  delete context.fn

  return result
}

12. apply

Function.prototype.apply = function(context) {
  if (typeof this !== 'function') {
    throw new TypeError(`${this} is not a function`)
  }

  context = context || window;
  context.fn = this

  let result
  if (arguments[1]) {
    result = context.fn(...arguments[1])
  } else {
    result = context.fn()
  }

  delete context.fn
  return result
}

13. bind

Function.prototype.bind = function(context) {
  if (typeof this !== 'function') {
    throw new TypeError(`${this} is not a function`)
  }
  context = context || window

  context.fn = this
  const args = [...arguments].slice(1)

  return function F() {
    // 如果调用了new
    if (this instanceof F) {
      return new context.fn(...args, ...arguments)
    }

    return context.fn(args.concat(...arguments))
  }
}