JavaScript实现文本框仅允许输入数字的封装方法

function getEvent() {
 if (document.all) {
  return window.event; //for ie
 }
 func = getEvent.caller;
 while (func != null) {
  var arg0 = func.arguments[0];
  if (arg0) {
  if ((arg0.constructor == Event || arg0.constructor == MouseEvent) || (typeof (arg0) == "object" && arg0.preventDefault && arg0.stopPropagation)) {
     return arg0;
   }
  }
  func = func.caller;
 }
 return null;
 };
 
 function doit(){
  var ev = getEvent();
  if(ev.keyCode < 8) {return false;}
  else if(ev.keyCode > 8 && ev.keyCode < 48) {return false;}
  else if(ev.keyCode > 57 && ev.keyCode <96) {return false;}
  else if(ev.keyCode > 105) {return false;}
  else {return true;}
 };