Checking number using isNaN function

<script>
document.write(isNaN(2) + " : "+" 2 " ); // Output false
document.write("<br>");
document.write(isNaN(2.4) + " : "+ " 2.4 "); // Output false
document.write("<br>");
document.write(isNaN('2.4') + " : "+ "\'2.4\'" ); // Output false
document.write("<br>");
document.write(isNaN('Hello') +" : "+ "\'Hello\'"); // Output true
document.write("<br>");
document.write(isNaN('5ab')  + " : " + "\'5ab\' ") // Output true
document.write("<br>");
document.write(isNaN() + " : " ); // Output true
document.write("<br>");
</script>

<script language='JavaScript' type='text/JavaScript'>
<!--
function disp_data(){
var my_string=	document.getElementById('t1').value;
if(isNaN(my_string)){
var str= " :This is Not a number. ";	
}else{
var str=" : This is a number. " ;	
}
document.getElementById("a2").innerHTML=my_string + str +  ", Output of  of isNaN = " + isNaN(my_string);
}
//-->
</script>

Empty string and isNaN

 my_string = parseInt(my_string); 

var my_string = prompt("Please enter a number","");

document.write(my_string)

if(isNaN(my_string)){

document.write ("this is not a number ");

}else{document.write ("this is  a number ");

}