Untitled

<script type="text/javascript">
    var id =0;
    class theObject{
        constructor(){
            this.id=id++;
        }
        writeOutput(){
            console.log("write output");
            $('<span>').appendTo('#output').html("id:"+id);

        }
    }

    var theObject1 = new theObject();
    var theObject2 = new theObject();

    $(document).ready(function(){

        theObject1.writeOutput();
        theObject2.writeOutput();
    })


    </script>
    <body>
    <div id="output"></div>

    </body>
    </html>

$('<span>').appendTo('#output').html("id:" + this.id);
// ------------------------------------------^^^^^

writeOutput()
{
    console.log("write output");
    $('<span>').appendTo('#output').html("id:"+this.id);
}