在js中判断checkboxlist(.net控件客户端id)是否有选中

var CheckBox = document.getElementById("<%=cblWeek.ClientID %>").getElementsByTagName("INPUT"); 
if (CheckBox != undefined) { 
var i = 0; 
var j = 0; 
for (i = 0; i < CheckBox.length; i++) { 
if (CheckBox[i].checked) { 
j += 1; 
} 
} 
if (j == 0) { 
alert("请选择星期！"); 
return false; 
} 
}