sequential-execution-plain-callback-in-Express-routes.md

A specific Use Case -

Here in the below backend Express Route - I have to capture visitor data (after OTP was sent and OTP Mongodb schema updated with that latest OTP) when a visitor to the site downloads data.

But I need to first pick the latest OTP (that was generated and saved in mongodb) from the already saved database before comparing with the otp input by the user when prompted.

router.route("/visitor/:id").put((req, res, next) => {
  let visitorData = req.body;
  let visitorEmail = req.body.company_email;
  let latestOtp = [];

  // Wrapper function that will wrap the database call (the find() query) in a function and pass it a callback function that gets executed after the database query has finished.
  // Also always make sure the callback is indeed a function, without this check, if the findLatestOTP() is called either without the callback function as a parameter OR in place of a function a non-function is passed, our code will result in a runtime error.
  function findLatestOTP(mongoCollection, callback) {
    mongoCollection
      .find({ visitor_email: visitorEmail })
      .limit(1)
      .sort({ createdAt: -1 })
      .exec((err, record) => {
        if (err) {
          console.log(err);
        } else {
          latestOtp.push(record[0].generated_otp);
          if (typeof callback === "function") {
            callback();
          }
        }
      });
  }

  findLatestOTP(OTP, function() {
    if (req.body.otpReceivedByVisitor !== latestOtp[0]) {
      return res
        .status(401)
        .send({ success: false, msg: "Incorrect Code was input" });
    } else {
      DOCUMENTMODAL.findById(req.params.id, (err, record) => {
        if (err) {
          console.log(err);
        }
        record.visitor.push(visitorData);
        record.save((err, updatedRecord) => {
          if (err) {
            return next(err);
          }
          res.status(200).send(updatedRecord);
        });
      });
    }
  });
});

Some related Explanation -

JavaScript is single threaded, that means only one statement is executed at a time. As the JS engine processes our script line by line, it uses this single Call-Stack to keep track of codes that are supposed to run in their respective order.

Like what a stack does, a data structure which records lines of executable instructions and executes them in LIFO manner. So say if the engine steps into a function foo(){ it PUSH-es foo() into the stack and when the execution of foo()return; } is over foo() is POP-ped out of the call-stack.