<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <script type="text/javascript">
            function prompter(defaultval) {
                var reply = prompt("js prompt", defaultval);
                console.log("Prompt reply: " + reply);
            }
        </script>
    </head>
    <body>
        <input type="button" onclick="prompter('')" value="Show prompt" id="button">
        <input type="button" onclick="prompter('default')" value="Show prompt with default value" id="button-default">
    </body>
</html>