Dissable charactor on text box by javascript ,only allowing numbers at the text box

Its is used to disabled the charactor key pressing on javascript.
java script function
<--------------------------
 function isnumky(evt)
      {
         var char1 = (evt.which) ? evt.which : event.keycode
         if (char1 > 31 && (char1 < 48 || char1 > 57))
            return false;

         return true;
      }
 ---------------------------------------->
html  form
<input name="textfield" type="text" onkeypress="return isnumky(evnt)" size="50" />