Determine whether checkboxlist (.net control client ID) is selected in js_javascript skills

When submitting content to be added or modified, the key data needs to be checked. How to determine whether the checkboxlist has options in js?
The specific operation is as follows:

Copy the code The code is as follows:

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("Please select a day of the week!");
return false;
}
}