<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Документ без названия</title>
</head>
<body>
<form name="form">
<input name="string" type="text">
<input onclick="getStringLength()" type="button" value="Ответ">
<br>
<input onclick="window.close()" type="button" value="Закрыть">
</form>
<script type="text/javascript">
var form = document.forms.form;
function getStringLength() {
var k = form.elements.string.value.replace(/\s+/g, '').length;
alert(k);
};
</script>
</body>
</html>