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

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>

Demo of isNan

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 ");

}

Converting String to Number by parseInt
JavaScript Math Reference

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tcx	11-10-2009
yes, agree, this is very useful topic it helps me in project, good concept not finding in other sites

Dhaval Shingala	25-02-2010
The simplest Way is Why u check entered input is Number or not just check on Keypress Event is number or not The Function for checking Is number is function isNumberKey(B,C){if(!C){C=0}var A=(B.which)?B.which:event.keyCode;if(A>31

Rakesh Joshi	16-04-2010
I observed that isNaN has a flaw. If we enter a value with alphabets at the start, it will detect. But if it starts with a number and continues with alphabets or ends with alphabets, then isNaN will return false, meaning to say it is a number, which is not the desired output. Is there any other alternative?

	01-09-2010
write a javascript code that will pop-up when an input of not a number is enter and return true when number data is enter. but without using in inbuild function such as isNan.

Ray	30-08-2012
Use Integer.parseInt("value") instead and put that in a try catch block.

Me2	07-03-2009
Cool

angelito	09-06-2012
This is okay..

san	11-02-2014
@Ray, this is JavaScript not Java.

kamal	07-08-2014
thanx alot very useful

Checking number using isNaN function

Empty string and isNaN

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