userExistsValidator

import {AbstractControl, AsyncValidatorFn} from '@angular/forms';

export function userExistsValidator(user: UserService):AsyncValidatorFn  {
    return (control: AbstractControl) => {
        return user.findUserByEmail(control.value)
            .pipe(
                map(user => user ? {userExists:true} : null)
            );
    }
}