ddd-value-object-immutability

Report a value-object class whose instance fields can be reassigned. A value object is defined by its value; if it can change after construction, two holders of "the same" value drift apart.

This rule is part of eslint-plugin-modularity.

Quick SummaryCopy Anchor Link

Rule DetailsCopy Anchor Link

A class is a value object when its name ends with one of valueObjectPatterns (default Value, VO, ValueObject). The rule reports such a class when it declares instance state without readonly:

• a class field (amount: number;);
• a constructor parameter property (constructor(public amount: number)), which declares the same field.

Static fields are class state, not the value of an instance, and are not checked.

It does not report a value object that:

• declares only readonly fields and readonly parameter properties;
• calls Object.freeze(this) in its constructor (while checkObjectFreeze is true). Freezing a nested value, Object.freeze(this.items), leaves the instance's own fields writable and does not count.

The check is shallow: a readonly items: string[] field passes, though the array itself stays mutable.

ConfigurationCopy Anchor Link

ExamplesCopy Anchor Link

❌ IncorrectCopy Anchor Link

class MoneyValueObject {
  amount: number;
  constructor(amount: number, public currency: string) {
    this.amount = amount;
  }
}

✅ CorrectCopy Anchor Link

class MoneyValueObject {
  readonly amount: number;
  constructor(amount: number, private readonly currency: string) {
    this.amount = amount;
  }
}

class PriceVO {
  amount = 0;
  constructor() {
    Object.freeze(this);
  }
}

Configuration ExamplesCopy Anchor Link

// eslint.config.mjs
export default [
  {
    rules: {
      'modularity/ddd-value-object-immutability': ['error', { valueObjectPatterns: ['Money', 'VO'] }],
    },
  },
];

Related RulesCopy Anchor Link

• ddd-anemic-domain-model - Entities should carry behaviour

Use this rule

npm install -D oxlint-plugin-modularity

{  "jsPlugins": ["oxlint-plugin-modularity"],  "rules": {    "modularity/ddd-value-object-immutability": "error"  }}

import { defineConfig } from 'vite-plus';export default defineConfig({  lint: {    jsPlugins: ['oxlint-plugin-modularity'],    rules: {      'modularity/ddd-value-object-immutability': 'error',    },  },});