TypeError：无法读取未定义的属性“json”

Question

Hello fellow developers. I have a little problem here. Just started learning Node.js a couple of days ago. I build an API where i query some data from a MongoDB database. The query itself looks like this:

exports.getIndex = async (req, res) => {
    const marker = await Marker.find((data) => data);

    try {
        res.json(marker);
        console.log(marker)
    } catch (error) {
        console.log(error);
    }
};

I fire the query when i connect with a socket from my frontend. That function looks like this:

io.on("connection", (socket) => {
  adminController.getIndex();
  console.log("a user connected");
..etc
};

However, when my socket connects to the server i get:

TypeError: Cannot read property 'json' of undefined at Object.exports.getIndex (.../backend/controllers/admin.js:8:13)

Anybody had a similar problem? Any help would be much appreciated. Thanks. :)

Answer 1

So, as pointed out in the comment, you're using a function with 2 parameters: req and res .
When calling the function adminController.getIndex() , you didn't passed any parameters where your function expect to get 2.
req and res are a naming convention for request and response in a http server (defined in the built in module http server and also in "Express" based http servers).
As you're trying to trigger an API endpoint when you receive a connection from you socket.io connection, it appears that it should trigger an endpoint of your http server.

Here's a way to do it:

http.createServer(function (req, res) {
    // Put your socket.io connection event here
    // Pass req and res as parameters to your .getIndex(req, res) function
}).listen(9000);

Of course, there's tons of other ways to do that:D. This is just an example. Another way would be to differentiate your http server from the socket.io one. And in socket.io, create a http request with axios or request-promise and gather data from the result of the http request:).

http server:

http.createServer(function (req, res) {
    // Get data from DB and send it (your function)
}).listen(9000);

axios server:

io.on("connection", async (socket) => {
  const response = await axios.get('localhost:9000/')
  // Deal with the response body sent by your server (res.json(data)) with the 
  // variable response.data
  console.log("a user connected");
..etc
};

Cheers !