typescript 35 lines · 6 steps

How an async username validator works in Angular

A debounced, HTTP-backed reactive form validator that checks whether a username is already taken.

Explained by highlit
Angular Intermediate async-validation reactive-forms rxjs debouncing http
1import { Injectable } from '@angular/core';
2import { HttpClient } from '@angular/common/http';
3import {
4 AsyncValidator,
5 AbstractControl,
6 ValidationErrors,
7} from '@angular/forms';
8import { Observable, of, timer } from 'rxjs';
9import { catchError, map, switchMap } from 'rxjs/operators';
10 
11@Injectable({ providedIn: 'root' })
12export class UsernameAvailabilityValidator implements AsyncValidator {
13 constructor(private http: HttpClient) {}
14 
15 validate(
16 control: AbstractControl,
17 ): Observable<ValidationErrors | null> {
18 const username = (control.value ?? '').trim();
19 if (!username) {
20 return of(null);
21 }
22 
23 return timer(400).pipe(
24 switchMap(() =>
25 this.http.get<{ available: boolean }>('/api/users/availability', {
26 params: { username },
27 }),
28 ),
29 map(({ available }) =>
30 available ? null : { usernameTaken: { value: username } },
31 ),
32 catchError(() => of({ availabilityCheckFailed: true })),
33 );
34 }
35}
01 / 01
STEP 01

Walkthrough

Space play ←→ step click any line
Three takeaways
  1. 1Async validators return an Observable so form validity can depend on a server round-trip.
  2. 2A leading timer plus switchMap debounces input and cancels stale in-flight requests.
  3. 3catchError keeps a failed network call from silently blocking form submission.

Related explainers

typescript 
interface ParsedName {
  first: string;
  middle: string;
  last: string;

Parsing human names into structured parts

parsing string-manipulation normalization
Intermediate 9 steps
go 
package middleware
 
import (
	"compress/gzip"

How gzip HTTP middleware works in Go

middleware compression object-pooling
Intermediate 7 steps
typescript 
import { useCallback, useEffect, useRef, useState } from "react";
 
interface UseResendCooldownOptions {
  cooldownSeconds?: number;

A resend cooldown hook in React

custom-hooks timers state-management
Intermediate 7 steps
typescript 
import {
  Injectable,
  PipeTransform,
  ArgumentMetadata,

A custom validation pipe in NestJS

validation dto recursion
Intermediate 10 steps
rust 
use std::collections::HashMap;
 
#[derive(Debug)]
pub struct RequestHead {

Parsing an HTTP request head in Rust

parsing error-handling iterators
Intermediate 9 steps
typescript 
import {
  CallHandler,
  ExecutionContext,
  Injectable,

Recording HTTP metrics with a NestJS interceptor

interceptor observability prometheus
Intermediate 5 steps

Share this explainer

Here's the card — post it anywhere.

How an async username validator works in Angular — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code