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Backend question · Object-oriented programming › Value objects, equality & invariants

What does this print?

class Money {
  constructor(amount, currency) {
    this.amount = amount;
    this.currency = currency;
  }
}
const prices = new Set([new Money(5, "EUR"), new Money(5, "EUR")]);
console.log(prices.size, new Money(5, "EUR") === new Money(5, "EUR"));
2 false

It prints 2 false. For objects, === and Set compare identity: "is this the same object in memory?". They do not compare the values inside. Two new calls make two different objects.

"1 false" is tempting if you expect Set to be smarter than ===. It is not: Set and Map keys use the same identity rule. For value objects, write an equals(other) method. To use them as keys, use a string key such as "5:EUR".