http://stackoverflow.com/questions/38070908/javascript-function-variable-always-undefined

by Aides

HTML

<div id="filterTitleTopic">

</div>

CSS

#filterTitleTopic
{
  width: 200px;
  height: 200px;
  background-color: red;
}

JavaScript

var CTT;
$("#filterTitleTopic").click(function () {  
    console.log(CTT);
    if (CTT == undefined || CTT == null || CTT == 2) {              
         $('input.topic').prop('checked', false);
         CTT = 1;
         console.log("unchecking boxes...");
    }
    else if (CTT == 1) {                
        $('input.topic').prop('checked', true);
        CTT = 2;
        console.log("checking boxes...");
    }
});