while(i--)循环使用javascript

Question

我通常使用while循环:

while (i<some_value) 

我看到while(i--)语法并且认为它更短更凉爽并且在google-chrome中尝试了以下内容.

var num_arr= [4,8,7,1,3];


var length_of_num_arr=num_arr.length;


while(length_of_num_arr--) console.log(num_arr);
 [4, 8, 7, 1, 3]
 [4, 8, 7, 1, 3]
 [4, 8, 7, 1, 3]
 [4, 8, 7, 1, 3]
 [4, 8, 7, 1, 3] **// THIS IS EXPECTED RESULT**

但是当我尝试......

while((num_arr.length)--) console.log(num_arr);
[4, 8, 7, 1]
[4, 8, 7]
[4, 8]
[4]
[] // WHY IS THIS HAPPENING??

使用此语法需要了解一些隐藏的内容吗？

Answer 1

数组的length属性是可写的,并且在设置时会切断它们的元素或添加空插槽.

var items = [1, 2, 3, 4, 5];
items.length = 3; // items is now [1, 2, 3]
items.length = 6; // items is now a sparse array: [1, 2, 3, undefined × 3]

所以,不要这样做.