passing variable to next middleware in req in express.js

Question

I am trying to pass variable to the next middlewares through the req object. getting some data from database and passing that data to request for next middlewares to use.

User.findone({ _id: someId })
    .then(user => { req.user = user })
    .catch(err => {  })

After that then and catch block i am using next() . Therefore for the next middlewares i am getting req.user undefined. but if i pass the next() function in the then block after req.user = user like .then(user=> {req.user = user; next()}) than i am getting req.user a valid user object to use for the next middlewares. what is the reason for this behaviour??

Answer 1

That's because the User.findOne function is asynchronous. The result of that function is only known in the then block.

const middleware = (req, res, next) => {
    User.findOne({ _id: someId })
        .then(user => {
            req.user = user;
        })
        .catch(err => { });
    next(); // If you put next() here, the request will go straight to the next middleware without waiting for User.findOne() to complete.
};

const middleware = (req, res, next) => {
    User.findOne({ _id: someId })
        .then(user => {
            req.user = user;
            next(); // Putting next() here is correct
        })
        .catch(err => {
            next(err); // Remember to handle error to avoid hanging the request
        });
};

Answer 2

then... is called after the User.findone promise resolves. Thus, if you put next() outside of then , it will be called before then .

You could read more details at promise-basics

Alternatively try to use async-await as it looks more straightforward.