Telegram Integration Using Firebase - Sending POST Requests

Currently, I am in the process of creating a Firebase cloud function (using Express) that performs the following tasks: - Checking if a user exists in the database. - Sending a message to the Telegram API based on whether the user exists or not.

The problem arises when attempting to execute the function. While Firebase logs successfully display the console.log indicating whether the user exists, the message fails to send to Telegram. The error log states:

[2020-02-15T10:41:34.568Z] @firebase/database: FIREBASE WARNING: Exception was thrown by use callback. Error: Can't set headers after they are sent. at validateHeader (_http_outgoing.js:491:11) at ServerResponse.setHeader (_http_outgoing.js:498:3) at ServerResponse.header (/srv/node_modules/express/lib/response.js:771:10) at ServerResponse.send (/srv/node_modules/express/lib/response.js:170:12) at ServerResponse.json (/srv/node_modules/express/lib/response.js:267:15) at ServerResponse.send (/srv/node_modules/express/lib/response.js:158:21) at firebase.database.ref.child.once.snapshot (/srv/index.js:59:40) at onceCallback (/srv/node_modules/@firebase/database/dist/index.node.cjs.js:4933:51) at /srv/node_modules/@firebase/database/dist/index.node.cjs.js:4549:22 at exceptionGuard (/srv/node_modules/@firebase/database/dist/index.node.cjs.js:698:9)

I would greatly appreciate any assistance provided. Thank you!

 app.post("/", async (req, res) => {

    const isTelegramMessage =
        req.body &&
        req.body.message &&
        req.body.message.chat &&
        req.body.message.chat.id &&
        req.body.message.from &&
        req.body.message.from.first_name &&
        req.body.update_id;
    const user_id = req.body.message.from.id
 firebase.initializeApp(firebaseConfig);
    let myUser;
    const chat_id = req.body.message.chat.id;
    const {
        first_name
    } = req.body.message.from;
    // Check User Exists
    firebase
        .database()
        .ref("/telegramUsers")
        .child(req.body.message.from.id)
        .once("value", snapshot => {
            if (snapshot.exists()) {
                myUser = true;
                console.log("exists!", myUser);
                return res.status(200).send({
                    method: "sendMessage",
                    chat_id,
                    text: `Welcome Back ${first_name}`
                });
            } else {
                myUser = false;
                console.log("does not exist!");
                return res.status(200).send({
                    method: "sendMessage",
                    chat_id,
                    text: `Hello ${first_name}`
                });
            }
        });

    return res.status(200).send({
        status: "not a telegram message"
    });
});
javascriptfirebaseexpressfirebase-realtime-databasetelegram

Answer №1

It has been pointed out by others that you are repeating the process of returning and writing a response to the caller twice. When send is used to begin writing the body of the HTTP response, calling status (or even send) after already doing so on res will not work.

In code, this issue would manifest in something similar to the following:

 app.post("/", async (req, res) => {

    const isTelegramMessage =
        req.body &&
        req.body.message &&
        req.body.message.chat &&
        req.body.message.chat.id &&
        req.body.message.from &&
        req.body.message.from.first_name &&
        req.body.update_id;
    const user_id = req.body.message.from.id
    firebase.initializeApp(firebaseConfig);
    let myUser;
    const chat_id = req.body.message.chat.id;
    const {
        first_name
    } = req.body.message.from;
    // Check User Exists
    if (isTelegramMessage) {
      return firebase
        .database()
        .ref("/telegramUsers")
        .child(req.body.message.from.id)
        .once("value")
        .then(snapshot => {
            if (snapshot.exists()) {
                myUser = true;
                console.log("exists!", myUser);
                return res.status(200).send({
                    method: "sendMessage",
                    chat_id,
                    text: `Welcome Back ${first_name}`
                });
            } else {
                myUser = false;
                console.log("does not exist!");
                return res.status(200).send({
                    method: "sendMessage",
                    chat_id,
                    text: `Hello ${first_name}`
                });
            }
          });
    } else {
      return res.status(200).send({
        status: "not a telegram message"
      });
   }
});

The modifications made are:

  • Check for user existence is now only done if isTelegramMessage is true.
  • The result from the database read operation is now returned.
  • Usage of once().then(), ensuring the proper sequence for return res.status().send().... This safeguards against premature termination of Cloud Functions before both database load and response sending are completed.

Although the last two points may not be essential for HTTPS triggered Cloud Functions (since they terminate upon response), their inclusion is recommended to facilitate potential code transfers to other event-triggered Cloud Function types.

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