クラスフィールド、privateな#フィールド、staticメンバー

クラスを使う理由

パブリックフィールドとコンストラクター

// Public fields and a simple method
class Counter {
  // public field with default
  value = 0;

  constructor(start) {
    // use a default if start is not provided
    this.value = Number.isFinite(start) ? start : 0;
  }

  inc() {
    this.value = this.value + 1;
  }
}

const c1 = new Counter(2);
c1.inc();
console.log("value:", c1.value); // 3

プライベート#フィールド

// Private # fields hide internal state
class SafeCounter {
  // private field (only inside the class)
  #value = 0;

  constructor(start) {
    this.#value = Number.isFinite(start) ? start : 0;
  }

  inc() { this.#value = this.#value + 1; }
  getValue() { return this.#value; } // expose via method
}

const sc = new SafeCounter(5);
sc.inc();
console.log("safe value:", sc.getValue());

// External access fails (uncommenting the next line would throw):
// console.log(sc.#value);

安全に読み取るためのgetter

// Getters provide read-only views; simple guard logic
class User {
  #name = "Anonymous";
  constructor(name) {
    if (typeof name === "string" && name.trim().length > 0) {
      this.#name = name.trim();
    }
  }
  get name() {
    return this.#name;
  }
}

const u = new User(" Ayla ");
console.log("name:", u.name);

staticメンバー

// Static = belongs to the class, not to instances
class MathHelp {
  static PI = 3.14159;
  static circleArea(r) {
    if (!(typeof r === "number" && r >= 0)) return 0;
    return MathHelp.PI * r * r;
  }
}

console.log("PI:", MathHelp.PI);
console.log("area r=2:", MathHelp.circleArea(2));

// Instances do not have the static members:
const mh = new MathHelp();
console.log("instance has PI?", "PI" in mh); // false

初心者に適した習慣

プライベートフィールドの基礎クイズ

まとめ