try 
{  
     if(typeof(eval(funcName))=="function"){

         funcName();

     }

}catch(e){

    //alert("not function"); 

} 

<script type="text/javascript">
    function calculateSum() {
    }  

    // using jQuery
    if ($.isFunction(window.calculateSum))
        calculateSum();
    else
        document.writeln('calculateSum does not exist');
</script>