サンプルコードで身につける: リテラル型

type LightSwitch = "on" | "off";

let kitchenLight: LightSwitch = "off";
console.log(kitchenLight);

kitchenLight = "on";
console.log(kitchenLight);

// kitchenLight = "half"; // 候補にない値はエラー

type DiceValue = 1 | 2 | 3 | 4 | 5 | 6;

const firstRoll: DiceValue = 3;
const secondRoll: DiceValue = 6;

console.log("出た目の合計: " + (firstRoll + secondRoll));

function isHighRoll(roll: DiceValue): boolean {
  return roll >= 4;
}
console.log(isHighRoll(firstRoll));
console.log(isHighRoll(secondRoll));
// const badRoll: DiceValue = 7; // 7 は候補にないのでエラー

function buildButton(buttonLabel: string, size: "small" | "medium" | "large"): string {
  if (size === "small") {
    return "[" + buttonLabel + "]";
  }
  if (size === "medium") {
    return "[ " + buttonLabel + " ]";
  }
  return "[  " + buttonLabel + "  ]";
}

console.log(buildButton("送信", "small"));
console.log(buildButton("送信", "medium"));
console.log(buildButton("送信", "large"));
// buildButton("送信", "big"); // 候補にない文字列はエラー

type CoffeeSize = "S" | "M" | "L";

const sizePrices = { S: 300, M: 350, L: 400 };

function coffeePrice(size: CoffeeSize): number {
  // size は3択に限定されているので、対応する価格が必ず見つかる
  return sizePrices[size];
}

console.log(coffeePrice("S"));
console.log(coffeePrice("L"));
// coffeePrice("XL"); // 料金表にないサイズはエラー

type OrderStep = "ordered" | "paid" | "shipped" | "delivered";

function nextStep(current: OrderStep): OrderStep {
  if (current === "ordered") return "paid";
  if (current === "paid") return "shipped";
  return "delivered";
}

let currentStep: OrderStep = "ordered";
console.log(currentStep);

currentStep = nextStep(currentStep);
console.log(currentStep);

currentStep = nextStep(currentStep);
console.log(currentStep);