ASP.NET–prevent button from form submit based on condition in javascript

Here is a javascript function that retutns true or false based on wheather a listbox is empty (select element in html does not have any option elements)

function WarnIfCleanersListboxEmpty() {
    if ($('#lboxCleaners').children().length == 0) {
        alert('list is empty');
        return false;
    } 
    return true;
}

in ASP.NET page there is a button that submits the form

 <asp:Button ID="btnGenerate" runat="server" Text="Generuj" OnClientClick="return WarnIfCleanersListboxEmpty()"/>

it’s the return in OnClientClick that I always forget and then wander why the form gets submitted even if my javascript function evaluates to false.

WRONG: OnClientClick=”WarnIfCleanersListboxEmpty()”

RIGHT: OnClientClick=”return WarnIfCleanersListboxEmpty()”

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