js 给input输入框添加方向键事件移动光标

$("#lastname").keydown(function (event) {   
    var tobj = document.getElementById("lastname");
    var start = event.target.selectionStart;
   if (event.keyCode == 39) {/* event.keyCode==39判断键盘是按下了左方向键 */  
	   tobj.setSelectionRange(start+1,start+1);
   }  
   if (event.keyCode == 37) {/* event.keyCode==37判断键盘是按下了右方向键 */  
	   tobj.setSelectionRange(start-1,start-1);
   }  
});