Promises

/* if we're doing function with promises, we must return it */

function findUser (login, password) {
    return db.models.user.find({ // here we MUST return promise
        login: login,
        password: password
    });
}

findUser('example', 'examplepw').then(function (user) {
    // Do something with user.
});

/* So if 'return' missed */

function findUserWrong (login, password) {
    db.models.user.find({
        login: login,
        password: password
    });
} // if not 'return' specified, function returns 'undefined'

findUserWrong('example', 'examplepw').then(function (user) { // function throws an error, because 'undefined' haven't any functions
    // Do something with user.
});

// even if we will write in async context
let user = await findUserWrong('example', 'examplepw'); // user will be undefined, because function returns 'undefined'

/* Complete example with ES6 */

function findUser (login, password) {
    return db.models.user.find({
        login,
        password
    });
}

findUser(login, password)
    .then(user => {
        // Do something...
    });