//將數反轉??也就是倒序??[1,2,3]?變成?[3,2,1]
function?reverse(arr)?{
????//先定義一個空的數組?這時他的length?===?0
????var?newArr?=?[];
????//遍歷傳進來的數組?從后往前遍歷?
????for(var?i=?arr.length-1;?i>=0?;?i--){
????????//把遍歷出來的值放到新數組中??以下代碼就相當于?newArr.push(arr[i]);
????????newArr[newArr.length]?=?arr[i];??//newArr.length?的值會隨著newArr里的元素增多而遞增?0，1，2，3....
????}
????return?newArr;
}

console.log(reverse([1,2,3]));