import {AsyncValidatorFn, AbstractControl } from '@angular/forms';
export function userNameShouldBeUnique(): AsyncValidatorFn {
    return (control: AbstractControl): { [key: string]: any } => {
        return new Promise(resolve => {
            setTimeout(() => {
                if (control.value == 'mosh')
                    resolve({ shouldBeUnique: true });
                else
                    resolve(null);
            }, 1000);
        });
    }
}

this.myForm = this.fb.group({
            username: [
                '',
                Validators.compose([Validators.required, forbiddenNameValidator(/bob/)]),
                Validators.composeAsync([userNameShouldBeUnique])
            ],
            password: ['', Validators.required]
        });