onchange_ex2.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

<form onchange="do_anything()">

    <input id="mySelect" type="checkbox" value="VN">Vietnam<br>
    <input id="mySelect" type="checkbox" value="US">US


</form>

<script>
    function do_anything() {
        var selectValue = document.getElementById("mySelect").value;
        console.log(selectValue);
    }

</script>

</body>
</html>