isNaN function to check a number if string or not in JavaScript

Checking number using isNaN function

JavaScript has one important function isNaN() to check any data is number or string. This function isNaN() is remembered as short form is Is Not a Number.

<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>

Output is here

This function returns true if the data is string and returns false if the data is a number. Here is one simple example to check one string. We will use one if else condition check to display a message accordingly.

<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

JavaScript treats empty string as zero , hence isNaN returns True. We can add parseInt to convert string to number before to make isNaN function returns false.

 my_string = parseInt(my_string);

Here is the code.

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 ");

}