How can you perform form validation

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));