var arr=new Array(1,2,3,4,5,6,7,8,9,9,8,7,6,5,4,3,2,1); 
var new_arr=new Array();
for(var i=0;i<arr.length;i++)
{
    if(arr.indexOf(arr[i])==i)
    {
        new_arr.push(arr[i]);
    }
}
console.log(new_arr);