判断键盘输入的字符

function keyDown() {
　　 var keycode = event.keyCode
　　 var realkey = String.fromCharCode(event.keyCode)
if(keycode==13)
{
  //return false;
}
　　 else
{
  alert("keycode: " + keycode + " realkey: " + realkey);
}
}
     document.onkeydown = keyDown