The isNan() method is used in JavaScript to check whether there’s the existence of an undefined value or can say checks for a NaN object.
Example
To check whether a JavaScript date is valid or not, you can try to run the following code −
<!DOCTYPE html>
<html>
<body>
<script>
var date1, date2;
date1 = new Date("2018/1/1");
if( ! isNaN ( date1.getMonth() )) {
document.write("Valid date1: "+date1);
} else {
document.write("<br>Invalid date1");
}
date2 = new Date("20181/1");
if( ! isNaN ( date2.getMonth() )) {
document.write("<br> Valid date2: "+date2);
} else {
document.write("<br>Invalid date2");
}
</script>
</body>
</html>The isNaN(null) == false is semantically correct. This is because null is not NaN.