如果在Javascript中不起作用

If总是给我最后的结果

检查这个片段

function calc(on, ep, am, ek, b1, b2, b3, b4) {
  var sum, mo;
  b1 = parseInt(b1);
  b2 = parseInt(b2);
  b3 = parseInt(b3);
  b4 = parseInt(b4);
  sum = b1 + b2 + b3 + b4;
  mo = sum / 4;
  if (mo < 5) {
    x = "Not Good";
  }
  if (mo < 6, 5 && mo > 4, 99) {
    x = "Good";
  }
  if (mo < 8, 5 && mo > 6, 49) {
    x = "Really Good";
  }
  if (mo > 8, 49) {
    x = "Perfect";
  }
  alert("O " + on + " " + ep + " " + am + " who is in " + ek + "th semester had avereged " + mo + " " + x);
}