var arr = [1,2,3,4,6,7,8,9,12,3,25,63,100]

var arr2 = arr.map(item => item += 1)

console.log(arr)   // 原数组

console.log(arr2) // 返回一个每项加1的新数组

const myMap = function(fn, context){

    let arr = Array.prototype.slice.call(this)

    let mapArr = []

    for(let i = 0 ; i<arr.length ; i++ ){

        if(!arr.hasOwnProperty(i)) continue

        mapArr.push(fn.call(context,arr[i],i,this))

    }

    return mapArr

}

Array.prototype.myMap = myMap

var arr = [1,2,3,4,5]

var arr2 = arr.myMap(item=>item += 1)

console.log(arr)  // [1,2,3,4,5]

console.log(arr2) // [2,3,4,5,6]