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HTML / CSS / JS

// Exhaustiveness helper
function assertNever(value: never, message = "Unhandled case"): never {
    throw new Error(`${message}: ${JSON.stringify(value)}`);
}

enum TrafficLight { Red = "RED", Yellow = "YELLOW", Green = "GREEN" }

function getAction(light: TrafficLight): string {
    switch (light) {
        case TrafficLight.Red:    return "Stop";
        case TrafficLight.Yellow: return "Caution";
        case TrafficLight.Green:  return "Go";
        default:
            // If a new member is added without updating this switch,
            // TypeScript will error here because light is no longer never
            return assertNever(light);
    }
}

// Discriminated union exhaustiveness
type PaymentMethod =
    | { type: "card";   last4: string; brand: string }
    | { type: "paypal"; email: string }
    | { type: "crypto"; address: string; coin: string };

console.log(describePayment({ type: "card", last4: "4242", brand: "Visa" }));