How to allow only letters in a text box with JavaScript

function lettersOnly(evt) {
       evt = (evt) ? evt : event;
       var charCode = (evt.charCode) ? evt.charCode : ((evt.keyCode) ? evt.keyCode :
          ((evt.which) ? evt.which : 0));
       if (charCode > 31 && (charCode < 65 || charCode > 90) &&
          (charCode < 97 || charCode > 122)) {
          alert("Enter letters only.");
          return false;
       }
       return true;
     }

onkeypress="return lettersOnly(event)" 

http://answers.oreilly.com/topic/134-how-to-allow-only-numbers-or-letters-in-a-text-box-with-javascript/