R3: Custom Range Validation

function ratingRange(min: number, max: number): ValidatorFn{
    return (c: AbstractControl): {[key : string]: boolean} | null => {
        if(c.value != undefined && (isNaN(c.value) || c.value < min || c.value > max)){
            return {'range': true}
        }
        return null;
    }
}