Javascript Reference - JavaScript isNaN() Function

The isNaN() function determines whether a value is an illegal number (NaN).

This function returns true if the value is NaN, and false if not.

Browser Support


`isNaN`	Yes	Yes	Yes	Yes	Yes

Syntax

isNaN(value)

Parameter Values

Parameter	Description
value	Required. The value to be tested

Return Value

A Boolean. Returns true if the value is NaN, otherwise it returns false.

Example

The following code shows how to check if a string is a Not-A-Number with isNaN function.


console.log(isNaN("10"));
//false - can be converted to number 10
/*from  w  w  w.  ja va  2  s.  c om*/
var entry1 = "123.123";
var number = new Number();
number = Number(entry1);
if (isNaN(number)){
    console.log("You did not enter a valid number.");
}else{
    console.log(number.valueOf());
}

The code above generates the following result.

Example 2

The following code shows how to check whether a number is an illegal number.


<!DOCTYPE html>
<html>
<body>
<button onclick="myFunction()">test</button>
<!--  www  .java  2s.  c  o  m-->
<p id="demo"></p>

<script>
function myFunction() {
    var a = isNaN(123) + "<br>";
    var b = isNaN(-1.23) + "<br>";
    var c = isNaN(5-2) + "<br>";
    var d = isNaN(0) + "<br>";
    var e = isNaN("Hello") + "<br>";
    var f = isNaN("2014/12/12") + "<br>";

    var res = a + b + c + d + e + f;
    document.getElementById("demo").innerHTML = res;
}
</script>

</body>
</html>

The code above is rendered as follows: