前端手写代码原理实现

let obj = {
  name: 'JoJo'
}
function foo(){
  console.log(this.name)
}
obj.fn = foo
obj.fn() // log: JoJo

Function.prototype.mycall = function () {
  if(typeof this !== 'function'){
    throw 'caller must be a function'
  }
  let othis = arguments[0] || window
  othis._fn = this
  let arg = [...arguments].slice(1)
  let res = othis._fn(...arg)
  Reflect.deleteProperty(othis, '_fn') //删除_fn属性
  return res
}

let obj = {
  name: 'JoJo'
}
function foo(){
  console.log(this.name)
}
foo.mycall(obj) // JoJo

Function.prototype.mybind = function (oThis) {
  if(typeof this != 'function'){
    throw 'caller must be a function'
  }
  let fThis = this
  //Array.prototype.slice.call 将类数组转为数组
  let arg = Array.prototype.slice.call(arguments,1)
  let NOP = function(){}
  let fBound = function(){
    let arg_ = Array.prototype.slice.call(arguments)
    // new 绑定等级高于显式绑定
    // 作为构造函数调用时，保留指向不做修改
    // 使用 instanceof 判断是否为构造函数调用
    return fThis.apply(this instanceof fBound ? this : oThis, arg.concat(arg_))
  }
  // 维护原型
  if(this.prototype){
    NOP.prototype = this.prototype
    fBound.prototype = new NOP()
  }
  return fBound
}

let obj = {
  msg: 'JoJo'
}
function foo(msg){
  console.log(msg + '' + this.msg)
}
let f = foo.mybind(obj)
f('hello') // hello JoJo

function new_(){
  let fn = Array.prototype.shift.call(arguments)
  if(typeof fn != 'function'){
    throw fn + ' is not a constructor'
  }
  let obj = {}
  obj.__proto__ = fn.prototype
  let res = fn.apply(obj, arguments)
  return typeof res === 'object' ? res : obj
}

function instanceof_(left, right){
  left = left.__proto__
  while(left !== right.prototype){
    left = left.__proto__ // 查找原型，再次while判断
    if(left === null){
      return false
    }
  }
  return true
}

function objectCreate_(proto, propertiesObject = {}){
  if(typeof proto !== 'object' || typeof proto !== 'function' || proto !== null){
    throw('Object prototype may only be an Object or null:'+proto)
  }
  let res = {}
  res.__proto__ = proto
  Object.defineProperties(res, propertiesObject)
  return res
}

function deepClone(origin) {
  let toStr = Object.prototype.toString
  let isInvalid = toStr.call(origin) !== '[object Object]' && toStr.call(origin) !== '[object Array]'
  if (isInvalid) {
    return origin
  }
  let target = toStr.call(origin) === '[object Object]' ? {} : []
  for (const key in origin) {
    if (origin.hasOwnProperty(key)) {
      const item = origin[key];
      if (typeof item === 'object' && item !== null) {
        target[key] = deepClone(item)
      } else {
        target[key] = item
      }
    }
  }
  return target
}

class EventBus{
  constructor(){
    this.task = {}
  }

  on(name, cb){
    if(!this.task[name]){
      this.task[name] = []
    }
    typeof cb === 'function' && this.task[name].push(cb)
  }

  emit(name, ...arg){
    let taskQueue = this.task[name]
    if(taskQueue && taskQueue.length > 0){
      taskQueue.forEach(cb=>{
        cb(...arg)
      })
    }
  }

  off(name, cb){
    let taskQueue = this.task[name]
    if(taskQueue && taskQueue.length > 0){
      if(typeof cb === 'function'){
        let index = taskQueue.indexOf(cb)
        index != -1 && taskQueue.splice(index, 1)
      }
      if(typeof cb === 'undefined'){
        this.task[name].length = 0
      }
    }
  }

  once(name, cb){
    let callback = (...arg) => {
      this.off(name, callback)
      cb(...arg)
    }
    typeof cb === 'function' && this.on(name, callback)
  }
}

let bus = new EventBus()
bus.on('add', function(a,b){
  console.log(a+b)
})
bus.emit('add', 10, 20) //30