Handling Callbacks in Classes - node.js

Question

I'm building classes to find and quickly operate actions on mongodb documents. This is the UserCursor class. (Not talking about MongoDB's cursor)
exports { UserCursor };
class UserCursor {
private __id: object;
constructor(query: { _id?: object, otherId?: number }) {
let { _id, otherId } = query; // Shortens the vars' name
if (!_id && !otherId) return; // Checks if 1 identifier is provided
if (_id) { // If a _id is provided
Users.findOne({ _id }, (err, user) => {
this.__id = user._id;
});
} else if (otherId) { // If a otherId is provided
Users.findOne({ otherId }, (err, user) => {
console.log(1); // Debug, you'll see later
this.__id = user._id;
});
}
}
// Returns this.__id (which should have been initialized in the constructor)
get _id() {
console.log(2)
return this.__id;
}
}
When run, the console returns
2
1
I think you got the problem: the mongo callback in the constructor gets on after _id operates. How could I manage that, since the constructor gets activated each time the class is used?

Answer 1

It's not entirely clear to me, what exactly you want to happen and how you use this class but I assume you want to instantiate it and then be able to get _id instantaneously. If it's not the case, you may still get some useful info from my answer. Feel free to provide more details, I will update it.
So mongodb operations are asynchronous, if you do
const cursor = new UserCursor(...)
console.log(cursor._id)
(I assume you want this), first all operations in this thread will run, including the call to get _id(), then the callback code will. The problem with such asynchronous things is that now to use this _id you will have to make all of your code asynchronous as well.
So you will need to store a Promise that resolves with _id from mongodb and make a method getId that returns this promise like this:
private __id: Promise<object>
constructor(...) {
// ...
if(_id) {
this.__id = new Promise((resolve, reject) => {
Users.findOne({ _id }, (err, user) => {
if(err) return reject(err)
resolve(user._id)
});
})
} else {
// Same here
}
}
public getId() {
return this.__id;
}
Then use it:
const cursor = new UserCursor(...)
cursor.getId().then(_id => {
// Do smth with _id
})
Or
async function doStuff() {
const cursor = new UserCursor()
const _id = await cursor.getId()
}
doStuff()
If you now do it inside some function, you'll also have to make that function async
Also you could leave a getter like you have now, that will return a promise, but I find it less readable than getId():
cursor._id.then(_id => { ... })
const _id = await cursor._id