How can you perform form validation

Short answer: synchronously in Angular? Use Async Validators returning Observable<ValidationErrors | null>. ✅ function uniqueUsernameValidator(service: UserService): syncValidatorFn { return (control: AbstractControl): Observable<ValidationErrors | null> => { return service.checkUsername(control.value).pipe( map(isTaken => (isTaken ? {… uniqueUsername: true } :…… null)) ); }; } // Usage: new FormControl('', null,…

Explain a bit more

uniqueUsernameValidator(this.userService)); synchronously in Angular? synchronously in Angular? Use Async Validators returning Observable<ValidationErrors | null>. ✅

Example code

function uniqueUsernameValidator(service: UserService): syncValidatorFn { 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)); synchronously in Angular? synchronously in Angular? Use Async Validators returning Observable<ValidationErrors | null>. ✅ Example: function uniqueUsernameValidator(service: UserService): syncValidatorFn { 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)); synchronously in Angular? Use Async Validators returning Observable<ValidationErrors | null>. ✅ Example: function uniqueUsernameValidator(service: UserService): syncValidatorFn { 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

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.