Immutability

Benefits

• Thread safety: an immutable object can be safely read from multiple threads simultaneously
• Easier reasoning: a value cannot change "somewhere else"; state is simpler to track
• Undo/Redo: the old state is always available
• Pure functions: no side effects — easier to test

Example (Value Object)

class Money {
  public function __construct(
    public readonly int $amount,
    public readonly string $currency
  ) {}

  public function add(Money $other): self {
    return new self($this->amount + $other->amount, $this->currency);
  }
}

In JavaScript

const, Object.freeze(), spread operator {...obj, key: newVal}. Libraries: Immer, Immutable.js.