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 ");
}

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    Me2

    07-03-2009

    Cool
    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.
    angelito

    09-06-2012

    This is okay..
    Ray

    30-08-2012

    Use Integer.parseInt("value") instead and put that in a try catch block.
    san

    11-02-2014

    @Ray, this is JavaScript not Java.
    kamal

    07-08-2014

    thanx alot very useful

    Post your comments , suggestion , error , requirements etc here .




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