Accepting only Numeric data in a text field.

<script>

function isNumber(evt)

{
   var charCode = (evt.which) ? evt.which : event.keyCode
  
   if(parseInt(charCode) >= 48)
   {
         if(!(parseInt(charCode) >= 57))
         {
           return true;
         }
         return false;
   } 
     
      //Accept backspace in case u want to deleted the entered data.

   else if(parseInt(charCode) == 8)

   {
     return true;
   }
   else
   {
     return false;
   }
}

</script>

<input type="text" name="zipCode" id="zipCode" onkeypress="return isNumber(event);" />