change-reference-to-value

Apply: 56 — Change Reference to Value

Target state: An object treated as a sharable record (with setters) becomes a value object — immutable, equal by content, replaced rather than mutated.

Why apply it: Concurrency hazards disappear; the type system can mark fields readonly; the object can travel safely across boundaries.

Pitfall: Comparison semantics shift from identity to equality — every call site that depended on === or identity caches needs review.

// Avoid:
class Phone {
  constructor() { this.area = null; this.number = null; }
}
phone.area = '617';

// Prefer:
class Phone {
  constructor(area, number) { this._area = area; this._number = number; }
  area()         { return this._area; }
  number()       { return this._number; }
  withArea(area) { return new Phone(area, this._number); }
}

Removes smells: Mutable Data