Array.prototype.map实现

function map(arr, callBack) {
  if (!Array.isArray(arr) || !arr.length || typeof callBack !== "function") {
    return [];
  }
  let newArr = [];
  for (let index = 0; index < arr.length; index++) {
    newArr.push(callBack(arr[index], index, arr));
  }
  return newArr;
}


var arr = [1,2,3,4,5]
map(arr,(item)=>item*2) // [2, 4, 6, 8, 10]