Agfdhyk

I need to fetch id of user from collection 'users' by calling a function and return it's value.

fetchId = (name) => {

        User.findOne({name: name}, (err, user) => {

            return user._id;

        });

    };

But this implementation returns null. What is the way to fix it?

following your example, if you don't want to use promises, you can simply pass a callback from the caller and invoke the callback when you have the result since the call to mongo is asynchronous.

fetchId = (name, clb) => {

  User.findOne({name: name}, (err, user) => {

    clb(user._id);

  });

};



fetchId("John", id => console.log(id));

Otherwise you can use the promise based mechanism omitting the first callback and return the promise to the caller.

fetchId = name => {

  return User.findOne({name: name}).then(user => user.id);

}; 





fetchId("John")

 .then(id => console.log(id));

A third way is a variation of suggestion #2 in @Karim's (perfectly good) answer. If the OP wants to code it as if results are being assigned from async code, an improvement would be to declare fetchId as async and await its result...

fetchId = async (name) => {

    return User.findOne({name: name}).then(user => user.id);

};



let someId = await fetchId("John");

console.log(id)

edit

For any method on an object -- including a property getter -- that works asynchronously, the calling code needs to be aware and act accordingly.

This tends to spread upward in your system to anything that depends on caller's callers, and so on. We can't avoid this, and there's no syntactic fix (syntax is what the compiler sees). It's physics: things that take longer, just take longer. We can use syntax to partially conceal the complexity, but we're stuck with the extra complexity.

Applying this to your question, say we have an object representing a User which is stored remotely by mongo. The simplest approach is to think of the in-memory user object as unready until an async fetch (findOne) operation has completed.

Under this approach, the caller has just one extra thing to remember: tell the unready user to get ready before using it. The code below employs async/await style syntax which is the the most modern and does the most to conceal -- but not eliminate :-( -- the async complexity...

class MyMongoUser {

    // after new, this in-memory user is not ready

    constructor(name) {

        this.name = name;

        this.mongoUser = null;  // optional, see how we're not ready?

    }



    // callers must understand: before using, tell it to get ready!

    async getReady() {

        this.mongoUser = await myAsyncMongoGetter();

        // if there are other properties that are computed asynchronously, do those here, too

    }



    async myAsyncMongoGetter() {

        // call mongo

        const self = this;

        return User.findOne({name: self.name}).then(result => {

            // grab the whole remote object. see below

            self.mongoUser = result;

        });

    }



    // the remaining methods can be synchronous, but callers must

    // understand that these won't work until the object is ready

    mongoId() {

        return (this.mongoUser)? this.mongoUser._id : null;

    }



    posts() {

        return [ { creator_id: this.mongoId() } ];

    }

}

Notice, instead of just grabbing the mongo _id from the user, we put away the whole mongo object. Unless this is a huge memory hog, we might as well have it hanging around so we can get any of the remotely stored properties.

Here's what the caller looks like...

let joe = new MyMongoUser('joe');

console.log(joe.posts()) // isn't ready, so this logs [ { creator_id: null } ];

await joe.getReady();

console.log(joe.posts()) // logs [ { creator_id: 'the mongo id' } ];