ddd-ts-kit/domain-value-object-immutability

Architecture id: domain/value-object-immutability (added by this plugin). Default severity: error in recommended, error in strict.

Applies to: domain/value-objects/ of a context, tests excluded.

Why

A value object is its value. If it can change, two holders of the same instance disagree about what they hold, and equality by value stops meaning anything.

Reported

• a property or constructor parameter property without readonly (private and #private included)
• a set accessor
• outside the constructor, an assignment or an increment that starts at this (this.props.cents += 1)
• outside the constructor, an array mutator called on something of this (push, pop, shift, unshift, splice, sort, reverse, fill, copyWithin)

Incorrect:

export class Amount {
  constructor(private cents: number) {}
  add(other: Amount): void {
    this.cents += other.cents;
  }
}

Correct:

export class Amount extends ValueObject<AmountProps> {
  add(other: Amount): Amount {
    return new Amount({ cents: this.props.cents + other.props.cents });
  }
}

Options

None.

rules."domain/value-object-immutability" in ddd-ts-kit.config.json set to off silences the rule, and // ddd-ts-kit-disable-next-line domain/value-object-immutability -- <reason> silences one line, exactly as for ddd-ts-kit check.

Limits

• Deep immutability of what a property holds (readonly items: string[]) is not checked: prefer readonly string[].
• Map, Set and object mutation through method calls is not seen.

With the CLI checker

New with this plugin: the CLI checker has no equivalent, because the shape of a class needs a syntax tree.