这是代码：

$("#sign-in-btn").click(function () {
    var $username = $("#id_username").val();
    var $password = $("#id_password").val();
    var userID;
    $.getJSON("testdata.json", function (data) {
        var user = data.user;
        for (i = 0; i < user.length; i++) {
            if ($username == user[i].username && $password == user[i].password) {
                userID = user[i].id;
                setCookie("userID", userID, 7);
                return;
            }
        }
    });
    if (userID) {
        return true;
    } else {
        return false;
    }
});

请问是怎么回事？