Confused about promises. Do I need to return here?

Question

So I've recently started to learn Promises (Bluebird) and now I'm trying to use them as much as possible, but I'm a bit confused if I need to return promises in this case.

Here I have a Passport LocalStrategy that I made:

passport.use(new LocalStrategy(function(username, password, done) {
    users.get(username) // A
        .then(function(user) {
            if (!user) {
                return done(null, false, { message: 'Incorrect username.' });
            }
            bcrypt.compare(password, user.password).then(function(result) { // B
                if (result) {
                    return done(null, user);
                }
                return done(null, false, { message: 'Incorrect password.' });
            });
        })
        .catch(function(err) {
            return done(err);
        });
}));

users.get(username) on line A uses the pg-promise library to return a promise that will resolve into a user if one is found in the database and to false if the user was not found.

bcrypt.compare on line B uses bcrypt to check if the password and hash match. It returns a promise that will resolve to true or false.

The code works perfectly, I'm just confused if line A and B should return like so

return users.get(username) // A

return bcrypt.compare(password, user.password).then(function(result) { // B

The code works with and without returning the promises.

Is Passport/Node just waiting until it sees return done? Does this mean that this function is synchronous even though everything inside it is async? Normally you would return a promise and then use .then() on it but since LocalStrategy is not using .then() or .catch() I don't have to return anything? Any input is greatly appreciated. Thanks.

Answer 1

Passport does not support Promise that is why you must call done in a callback. You could return users.get(username)but the return value (promise) is never used. Do not forget that you can chain promises like the following :

users.get(username)
    .then(function(user) {
        if (!user) {
            return done(null, false, { message: 'Incorrect username.' });
        }
        return bcrypt.compare(password, user.password);
   })
   .then(function(result) { // B
        if (result) {
            return done(null, user);
        }
        return done(null, false, { message: 'Incorrect password.' });
    })
    .catch(function(err) {
        return done(err);
    });