compareMoney2.ts

enum MoneyType {
  Dollar,
  Yen
}

class Money {
  constructor(
    private readonly _value: number,
    private readonly _type: MoneyType
  ) {}

  equals(other: Money) {
    this._raiseIfTypeIsDifferent(other)
    return this._value === other._value
  }

  add(other: Money) {
    this._raiseIfTypeIsDifferent(other)
    return new Money(
      this._value + other._value,
      this._type
    )
  }

  private _raiseIfTypeIsDifferent(other: Money) {
    if (this._type !== other._type) {
      throw Error("Invalid type of Money")
    }
  }
}

// Value Object
const dollar = new Money(100, MoneyType.Dollar)
const yen = new Money(100, MoneyType.Yen)

// ランタイムエラーになる
dollar.add(yen)