jquery二级下拉框联动

$(document).ready(function(){
 var options={"1":["a1","a2"],"2":["b1","b2"]};

$("#sel1").change(function(){
	$("#sel2").empty();
if(typeof options[$(this).val()]=="undefined"){
	return;
}
;
for(i=0;i<options[$(this).val()].length;i++){
	$("<option>"+options[$(this).val()][i]+"</option>").appendTo($("#sel2"));	
	}
	
});
});

<select id="sel1"> <option value="a"> a </option> <option value="b"> b </option> </select> <select id="sel2"> <option> a1 </option> <option> b1 </option> </select>