How to rewrite this code using rxjs observables instead async/await? - rxjs

Question

I need that createUser function returns Observable<UserEntity> but in this function I also have to make 2 queries to DB and check if this user exists. The code below uses async/await and looks pretty good and clean. But the problem is that I use rxjs everywhere in this project and would like to write it somehow using rxjs. Can it be as clean as now but with Observables?
async create(user: CreateUserDTO): Promise<UserEntity> {
const userByEmail = await this.getUserByEmail();
const userByLogin = await this.getUserByLogin();
if (userByLogin || userByEmail)
// retrun error here
return await this.createUser(user);
}

Answer 1

I am using RxJs 6.5
forkJoin will emit result when both async functions getUserByEmail & getUserByLogin complete their execution
If getUserByEmail & getUserByLogin returns Promise,for that using from to convert a promise into an observable
mergeMap to subscribe the inner observable.In our case createUser returns observable
create(user: CreateUserDTO): Observable < UserEntity > {
//If getUserByEmail & getUserByLogin returs Promise
const getUserByEmail$ = from(this.getUserByEmail());
const getUserByLogin$ = from(this.getUserByLogin());
//If Both returns Observable
//const getUserByEmail$ = this.getUserByEmail();
//const getUserByLogin$ = this.getUserByLogin();
return forkJoin({
userByEmail: this.getUserByEmail(),
userByLogin: this.getUserByLogin(),
}).pipe(
tap((res) => {
if (res.userByEmail || res.userByLogin) {
throw 'User exists!';
}
}),
mergeMap(() => {
return from(this.createUser(user));
//If createUser returns Observable,then
//return this.createUser(user);
})
);
}

Answer 2

Assuming that this.getUserByEmail(), this.getUserByLogin() and this.createUser(user) return Promises, the code could look like this
create(user: CreateUserDTO): Observable<UserEntity> {
// with the rxjs from function we turn a Promise into an Observable
const userByEmail$ = from(this.getUserByEmail());
const userByLogin$ = from(this.getUserByLogin());
// with forkjoin we create an Observable which notifies when all the
// Observables which have been passed in as parameters notify
return forkJoin([userByEmail$, userByLogin$]).pipe(
// with concatMap you wait for the upstream Observable (i.e. the
// Observable created by forkJoin) to notify and complete, and then
// you return the next Observable in the chain, which is, in this case,
// the Observable which (when subscribed) creates the user
concatMap(([userByLogin, userByEmail]) =>
if (userByLogin || userByEmail) {
// throw error here
}
return from(this.createUser(user))
})
)
}
Otherwise, if this.getUserByEmail(), this.getUserByLogin() and this.createUser(user) return Observables you do not need to use the from rxjs function and the code would be slightly simpler, like this
create(user: CreateUserDTO): Observable<UserEntity> {
return forkJoin([this.getUserByEmail(), this.getUserByLogin()]).pipe(
concatMap(([userByLogin, userByEmail]) =>
if (userByLogin || userByEmail) {
// throw error here
}
return from(this.createUser(user))
})
)
}