Why don't you try like this
function myfunction() {
if (Ruwani==dformat){
var name = "Ruwani Wickramaarachchi";
var bday = "it's "+ name +"'s Birthday";
return dformat;}
else {
var name = "Daisy Victor";
var bday = "it's "+ name +"'s Birthday";
return bday;}
<!-- return bday; -->
}