dialog boxes in JavaScript always return string. Use isNaN() to determine, like this:
<!DOCTYPE html>
<html>
<head>
<script>
var ask= prompt("Please insert number or string");
if (isNaN(ask)){
alert("You enter a string");
} else {
alert("You enter a Number");
}
</script>
</head>
<body>
</body>
</html>