Untitled

<!DOCTYPE html>
<html lang="pl">
<head>
    <meta charset="UTF-8">
    <title>praca</title>
</head>
<body>
        <form>
        <input type="text" id="ilgos">ilość gości</input>
        <br>
        <input type="checkbox" id="popr"></input>
        <button onclick="Function()">Przycisk</button>
        </form>

    <script>
        var checkbox = document.getElementById("popr");
        function Function(){
            if (checkbox.check == true){
                console.log(document.getElementById("ilgos").value);
            };
        };
    </script>

</body>
</html>