Someone

<html>
    <head>
        <script type="text/javascript">

            function changeText(element_id, text) {
                var el = document.getElementById(element_id);

                if (el !== null) {
                    el.value = text;
                    return true;
                } else {
                    return false;
                }
            }
        </script>
    </head>

    <body>
        <p><input type="text" name="fooName" id="fooInput" /></p>
        <p><button onclick="changeText('fooInput', 'testing1243');" >Blah</button></p>
    </body>

</html>