In this article I will explain with an example, how to validate ASP.Net CheckBoxList control and check whether at-least one CheckBox is checked using JavaScript.
HTML Markup
The HTML Markup consists of an ASP.Net CheckBoxList control and a Button control.
The ASP.Net Button has been assigned an OnClientClick event handler in which the name of JavaScript function has been specified which will perform the validation.
<asp:CheckBoxList ID="CheckBoxList1" runat="server">
    <asp:ListItem Text="One" Value="1"></asp:ListItem>
    <asp:ListItem Text="Two" Value="2"></asp:ListItem>
    <asp:ListItem Text="Three" Value="3"></asp:ListItem>
<br />
<asp:Button ID="Button1" runat="server" Text="Validate" OnClientClick="return Validate()" />
Implementing CheckBoxList validation using JavaScript
Inside the Validate JavaScript function, first the CheckBoxList and its internal CheckBoxes are referenced.
Note: The ID of the ASP.Net controls, tend to change when Master Pages or UserControls are used and hence the ClientID property is used to fetch the IDs of the controls. For more details, please refer ASP.Net: JavaScript document.getElementById returns NULL when using Master Page.
Then, a loop is executed over the CheckBoxes to check whether the CheckBox is checked.
If at-least one CheckBox is checked then the validation is successful else an error message is displayed using JavaScript Alert Message Box.
<script type="text/javascript">
    function Validate() {
        var checkBoxList = document.getElementById("<%=CheckBoxList1.ClientID%>");
        var checkBoxes = checkBoxList.getElementsByTagName("input");
        for (var i = 0; i < checkBoxes.length; i++) {
            if (checkBoxes[i].checked) {
                return true;
        alert("Please select atleast one item.");
        return false;
Validate ASP.Net CheckBoxList control (at least one checked) using JavaScript
Browser Compatibility

The above code has been tested in the following browsers.
Internet Explorer  FireFox  Chrome  Safari  Opera 

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