[SOLVED] why does isNaN always return true

why does isNaN always return true

Having some trouble trying to validate strings that are numbers in JS. isNaN in particular seems to be misbehaving.

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>No Quirks!</title>
</head>
     <body>
        <h1>JavaScript Test</h1>

        <button onclick="myNanIsTrue()">Is it a number?</button>

        <p id="writeToHTML"></p>

        <script type="text/javascript">
        function myNanIsTrue() {
             var g = parseInt("40000000") + "<br />";
             var n = g;
             if (isNaN(g)){
                  g="moo!";
             }
             n += g + "<br />";
             document.getElementById("writeToHTML").innerHTML = n;
        }
        </script>
     </body>
</html>

Tried a couple of permutations of this but it just doesn't seem to be working. Can anyone see why this isn't working?

Solution

You've written:

var g = parseInt("40000000") + "<br />";

Therefore g is a string: '40000000<br />'.

parseInt("40000000") returns an integer, but then you append a string to it. Javascript implicitly converts the numer to a string, then concatenates them.

String's not a number, so isNaN (correctly) returns true.