How can you perform form validation asynchronously in Angular?

Q: How can you perform form validation asynchronously in Angular?

Short answer: Use Async Validators returning Observable<ValidationErrors | null>. ✅ Example code function uniqueUsernameValidator(service: UserService): AsyncValidatorFn { return (control: AbstractControl): Observable<ValidationErrors | null> => { return service.checkUsername(control.value).pipe( map(isTaken => (isTaken ? { uniqueUsername: true } : null)) ); }; } // Usage: new FormControl('', null, uniqueUsernameValidator(this.userService)); Real-world example (ShopNest) ShopNest admin can be built in Angular with modules/components, services for API calls, and reactive forms for product edit. Say this in the interview Define — one clear sentence (the short answer above). Example — relate it to a project like ShopNest or your real work. Trade-off — when you would not use it.

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Accepted Answer

Short answer: Use Async Validators returning Observable. ✅ Example code function uniqueUsernameValidator(service: UserService): AsyncValidatorFn { return (control: AbstractControl): Observable => { return service.checkUsername(control.value).pipe( map(isTaken => (isTaken ? { uniqueUsername: true } : null)) ); }; } // Usage: new FormControl('', null, uniqueUsernameValidator(this.userService)); Real-world example (ShopNest) ShopNest admin can be built in Angular with modules/components, services for API calls, and reactive forms for product edit. Say this in the interview Define — one clear sentence (the short answer above). Example — relate it to a project like ShopNest or your real work. Trade-off — when you would not use it.