Switch case with conditions

Am I writing the correct switch case with conditions?

var cnt = $("#div1 p").length;
alert(cnt);

switch (cnt) {
  case (cnt >= 10 && cnt <= 20):
    alert('10');
    break;
  case (cnt >= 21 && cnt <= 30):
    alert('21');
    break;
  case (cnt >= 31 && cnt <= 40):
    alert('31');
    break;
  default:
    alert('>41');
}

For some reason, the alert does not occur when the conditions are matched!