javascript - Express.js 无法读取未定义的属性 'req'

Question

我的 Express.js 应用一直在抛出 Cannot read property 'req' of undefined。本质上，它监听 GET 请求，获取 content 查询，然后用表格回复。以下是出现问题的部分。

index.js

var panels = require('./modules/panels.js');
app.get('/panel', function (req, res) {
    var user;
    if (user = req.session.user) {
        panels.getContents(req.query.content, user.innId, res.send);
    } else {
        res.sendStatus(401);
    }
});

modules/panels.js

exports.getContents = function(panelName, innId, callback) {
    var response = "";
    switch (panelName) {
        case 'tenants':
            con.query(queryString, queryParams, function(err, rows) {
                if (err) {
                    handle(err);
                } else {
                    if (rows.length == 0) {
                        var tenants = 0;
                        var debtors = 0;
                    } else {
                        var tenants = rows[0].tenants;
                        var debtors = rows[0].length;
                    }
                    response = convertToHTMLTable(rows);
                }
                callback(response); /*THE ERROR POINTS HERE */
            });
            break;

        /* other cases here */

        default:
            response += "Invalid Request";
            callback(response);
    }
}

我做错了什么？我的猜测是我不应该将 res.send 作为回调传递。那么，我该如何解决呢？

Answer 1

我也遇到过这个错误，在仔细阅读错误消息并盯着我的代码看了太久之后，它点击了。

我(和上面的提问者)的代码看起来像这样:

  somethingAsync
  .then(res.send) // <-- storing send() divorced from parent object "res"
}

问题是当稍后调用 res.send 时，它脱离了它的动态作用域——这意味着在 send 方法中调用 this，通常指的是 res，现在指的是 global。显然，res.send 包含对 this.req 的引用，给出了错误消息。

等效场景:

res = {
    send: function() { 
        console.log(this.originalMessage.req);
    },
    originalMessage: { req: "hello SO" },
};

res.send();
// # "hello SO"
const x = {};
x.send = res.send;
x.send();
// # Uncaught TypeError: Cannot read property 'req' of undefined

或者，换句话说:

new Promise(_ => _())
.then(_ => console.log(this.msg.req));
// # Uncaught (in promise) TypeError: Cannot read property 'req' of undefined

两种解决方案:

handler(req, res) {
  somethingAsync
  .then(() => res.send())
   // ^ send will be called as a method of res, preserving dynamic scope
   // that is, we're storing a function that, when called, calls
   // "res.send()", instead of storing the "send" method of "res" by itself.
}

理论上更惯用的方法是

handler(req, res) {
  somethingAsync
  .then(res.send.bind(res))
  //^ res is explicitly bound to the saved function as its dynamic scope
}

动态作用域的有害小陷阱，那个。似乎 express 应该放弃那里的动态范围，或者至少抛出一个更好的错误......