How to allow only numbers in a text box with JavaScript

  function numeralsOnly(evt) {
       evt = (evt) ? evt : event;
        var charCode = (evt.charCode) ? evt.charCode : ((evt.keyCode) ? evt.keyCode :
           ((evt.which) ? evt.which : 0));
        if (charCode > 31 && (charCode < 48 || charCode > 57)) {
           alert("Enter numerals only in this field.");
           return false;
          }
           return true;
   }

onkeypress="return numeralsOnly(event)"