Can't run the cpde

<script>
        var firstname = "John";
        var lastname = "Doe";
        var totalname = firstname +lastname ;
        var d = new day();
        document.getElementById("something").innerHTML  = d ;
        document.getElementById("something").innerHTML = totalname;

Nothing is shown , Only if I remove

var d = new day();
        document.getElementById("something").innerHTML  = d ;

JohnDoe can be shown then .Why this happen?Help appreciated