How to debounce async validator in Angular 4 with RxJS observable?

Question

I'm using custom async validator with Angular 4 reactive forms to check if E-Mail address is already taken by calling a backend.

However, Angular calls the validator, which makes request to the server for every entered character. This creates an unnecessary stress on the server.

Is it possible to elegantly debounce async calls using RxJS observable?

import {Observable} from 'rxjs/Observable';

import {AbstractControl, ValidationErrors} from '@angular/forms';
import {Injectable} from '@angular/core';

import {UsersRepository} from '../repositories/users.repository';


@Injectable()
export class DuplicateEmailValidator {

  constructor (private usersRepository: UsersRepository) {
  }

  validate (control: AbstractControl): Observable<ValidationErrors> {
    const email = control.value;
    return this.usersRepository
      .emailExists(email)
      .map(result => (result ? { duplicateEmail: true } : null))
    ;
  }

}

Answer 1

While @Slava's answer is right. It is easier with Observable :

return (control: AbstractControl): Observable<ValidationErrors> => {
      return Observable.timer(this.debounceTime).switchMap(()=>{
        return this.usersRepository
            .emailExists(control.value)
            .map(result => (result ? { duplicateEmail: true } : null));
      });
}

updated with modern RxJS:

return (control: AbstractControl): Observable<ValidationErrors> => {
    return timer(this.debounceTime).pipe(
        switchMap(()=>this.usersRepository.emailExists(control.value)),
        map(result => (result ? { duplicateEmail: true } : null))
    );
}

Notes:

• Angular will automatically unsubscribe the returned Observable
• timer() with one argument will only emit one item
• since timer emits only one value it does not matter if we use switchMap or flatMap
• you should consider to use catchError in case that the server call fails
• angular docs: async-validation

Answer 2

UPDATE RxJS 6.0.0:

import {of, timer} from 'rxjs';
import {map, switchMap} from 'rxjs/operators';


return (control: AbstractControl): Observable<ValidationErrors> => {
  return timer(500).pipe(
    switchMap(() => {
      if (!control.value) {
        return of(null)
      }
                      
      return this.usersRepository.emailExists(control.value).pipe(
        map(result => (result ? { duplicateEmail: true } : null))
      );
    })
  )
}

---

*RxJS 5.5.0

For everyone who is using RxJS ^5.5.0 for better tree shaking and pipeable operators

import {of} from 'rxjs/observable/of';
import {map, switchMap} from 'rxjs/operators';
import {TimerObservable} from 'rxjs/observable/TimerObservable';


return (control: AbstractControl): Observable<ValidationErrors> => {
  return TimerObservable(500).pipe(
    switchMap(() => {
      if (!control.value) {
        return of(null)
      }
                      
      return this.usersRepository.emailExists(control.value).pipe(
        map(result => (result ? { duplicateEmail: true } : null))
      );
    })
  )
}

Answer 3

After studying some offered solutions with Observables I found them too complex and decided to use a solution with promises and timeouts. Although blunt, this solution is much simpler to comprehend:

import 'rxjs/add/operator/toPromise';

import {AbstractControl, ValidationErrors} from '@angular/forms';
import {Injectable} from '@angular/core';

import {UsersRepository} from '../repositories/users.repository';


@Injectable()
export class DuplicateEmailValidatorFactory {

  debounceTime = 500;


  constructor (private usersRepository: UsersRepository) {
  }

  create () {

    let timer;

    return (control: AbstractControl): Promise<ValidationErrors> => {

      const email = control.value;

      if (timer) {
        clearTimeout(timer);
      }

      return new Promise(resolve => {
        timer = setTimeout(() => {
          return this.usersRepository
            .emailExists(email)
            .map(result => (result ? { duplicateEmail: true } : null))
            .toPromise()
            .then(resolve)
          ;
        }, this.debounceTime);
      });

    }

  }

}

Here, I'm converting existing observable to promise using toPromise() operator of RxJS. Factory function is used because we need a separate timer for each control.

---

Please consider this a workaround. Other solutions, which actually use RxJS, are most welcome!