type Easing = "ease-in" | "ease-out" | "ease-in-out"; class UIElement { animate(dx: number, dy: number, easing: Easing) { if (easing === "ease-in") { // ... } else if (easing === "ease-out") { } else if (easing === "ease-in-out") { } else { // error! should not pass null or undefined. } } } let button = new UIElement(); button.animate(0, 0, "ease-in"); button.animate(0, 0, "uneasy"); // error: "uneasy" is not allowed here