用javascript如何删除一个数组中的重复元素

<script type="text/javascript">
Array.prototype.unique = function()
{
var a = {};//一个空的数组;
for(var i=0; i<this.length; i++)
{
if(typeof a[this[i]] == "undefined")
a[this[i]] = 1;
}
this.length = 0;
for(var i in a)
this[this.length] = i;
return this;
}

var a = [1,2,4,12,5,3,5,4,8,5,2,8,8,1,5];
alert(a +"/n"+ a.unique());
</script>