js 去除数组重复的值

Array.prototype.unique = function(){
  var a = {};
  var len = this.length;
  for(var i=0; i<len; 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 arr = [1,1,1,2,2,2,3,3,4,4];
arr.unique();
alert(arr.toString());