[Alexej]

думаю так вернее

var arr = [1,2,null,3,,4,5];
function s(x){
        return sum += x*x;
        }; 
var sum = 0;
for(var keys = 0;keys<arr.length;keys++){
    if(!arr[keys]) continue;
    var xx = arr[keys];
    s(xx);       
    };
  alert('Summa = ' + sum);