[рони]

Demath,
ок!

function findElem(A,Arr)
{
    var i=Arr.length;
    while (i--&&Arr[i]!=A) ;
    alert(i<0?'Not found':i);
}
findElem(-5,[1, 2, 3, 4, 5, 6, 7, 8, 19, 9, 16, 15, 65, 0])