手写forEach

    Array.prototype.myforEach = function (fn, thisValue) {
        let index = 0;
        let arr = thisValue || this;
        if (typeof fn !== 'function') {
            throw new TypeError(fn + 'is not a function')
        }
        while (index < arr.length) {
            if (index in arr) {
                fn.call (thisValue, arr[index], index, arr)
            }
            index ++
        }
    };
    
    let arr = [1, 3, 5, 7]
    arr.myforEach((item, i , arr) =>{
        console.log('item:' + item + 'i:' + i)
    })

手写map方法

    Array.prototype.mymap = function(fn,thisValue){
        let arr = thisValue || this
        let index = 0
        let newArr = [];
        if(typeof fn !== 'function'){
            throw new TypeError(fn + 'is not function')
        }
        while(index<arr.length){
            if(index in arr){
                let result = fn.call(thisValue, arr[index], index, arr)
                newArr[index] = result //返回一个新数组
            }
            index++
        }
        return newArr
    }
    
    let arr = [1,3,5,7]
    let newArr = arr.mymap((item)=>{
        return item*3
    })
    console.log(newArr)

手写filter方法

Array.prototype.myFilter=function(callback, context=window){

  let len = this.length
      newArr = [],
      i=0

  for(; i < len; i++){
    if(callback.apply(context, [this[i], i , this])){
      newArr.push(this[i]);
    }
  }
  return newArr;
}

手写every方法：
every方法参数：

Array.prototype.myEvery=function(callback, context = window){
    var len=this.length,
        flag=true,
        i = 0;

    for(;i < len; i++){
      if(!callback.apply(context,[this[i], i , this])){
        flag=false;
        break;
      }
    }
    return flag;
  }