Array.prototype.myMap = function (fn, thisArg) {
let reuslts = []
this.reduce((pre, cur, index, array) => {
reuslts[index] = fn.call(thisArg, cur, index, array)
}, 0)
return reuslts
}
let arr = [3,4,6,2]
let newArr = arr.myMap((item, index, arr) => {
return item * 3
})
console.log(newArr)