javascript數組的長度不是隻讀屬性

var arr = ['a', 'b', 'c', 'd', 'e', 'f']; 

// length:6, 0:a, 1:b, 2:c, 3:d, 4:e, 5:f

arr.length = 5;

// length:5, 0:a, 1:b, 2:c, 3:d, 4:e

看看這篇文章吧 

http://www.sitepoint.com/quick-tip-create-manipulate-arrays-in-javascript/