在服务器到客户端之间转换数据

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我在服务器（node.js）到客户端（常规js）之间转换数据时遇到问题。 问题在于，当我查看F12时，数据转换得不好。

客户：

 //send user name if exist
    var xhr = new XMLHttpRequest();
    xhr.open("POST", "http://localhost:3000/start", true);
    xhr.setRequestHeader('Content-Type', 'application/json');
    xhr.send(JSON.stringify({
        userName: newUserName,
        password: newPassword
    }));

    //getting info if the user exist or not
    var xhttp = new XMLHttpRequest();
    xhttp.onreadystatechange = function () {
        if (this.readyState == 4 && this.status == 200) {
            if (xhttp.responseText !== null) { //exist
                document.getElementById("exist").style.display = "block";
                setTimeout(function () {
                    document.getElementById('exist').style.display = "none";
                }, 3000);
                console.log("exist");
            } else { //not exist
                console.log("not exist");
            }
        }
    };
    xhttp.open("POST", "http://localhost:3000/start");
    xhttp.send();

服务器：

app.post('/start', function (req, res) {
const userName = req.body.userName;
const userPassword = req.body.password;

//build new user if not exist
User.findOne({ userName: userName }, function (foundUser, err) {
    if (foundUser === null) { //the user doesnt exist yet
        const newUser = new User({
            userName: userName,
            userPassword: userPassword,
        });
        newUser.save();
        // res.status(200).send(false); //not exist
        console.log("new user");
    } else {
        console.log("exist");
        // res.status(200).send(true); // exist
    }
    res.status(200).send(foundUser);
    console.log(foundUser);
});
});
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