Error: “missing ) after argument list”

form validation produces error in FireFox at first line at the “<": For(i=0; i<4; i++) { var j = (i+1); var sup1 = supArray[i]; var sup2 = supArray[j]; if((sup1 != "") || (sup2 != "")) { if(sup1 == sup2) { alert("You cannot select the same supplier more than once."); return false; } } }