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javascriptnanparseint

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.