var tab = [1, 2, 'a', 'b', 5, 6];
var suma = 0;
var i = 0;

while(i < tab.length){
  try{
    if(isNaN(tab[i])){
      continue;
    }
	suma += tab[i];
  }
  finally{
    i++;
  }
}

alert("Suma komórek tablicy to " + suma);
