freezedts

Immutable class generation for TypeScript — deep copying, value equality, and runtime immutability via decorators and code generation.

A TypeScript port of Dart's freezed package by Remi Rousselet.

Table of Contents

• Motivation
• Installation
• Running the Generator
• Creating Classes
  • Basic Usage
  • Field Configuration
  • Nested Freezed Types
  • Collections
• Deep Copy
• Deep Equality
• Configuration
  • Per-Class Configuration
  • Project-Wide Configuration
  • Resolution Order
• Building the Library

Motivation

TypeScript has no native support for immutable classes with named parameters. Achieving truly immutable data classes requires significant boilerplate:

• readonly on every property
• Object.freeze() in every constructor
• Manual copyWith methods for creating modified copies
• Manual equals methods for structural comparison
• Recursive freezing of nested collections

freezedts eliminates this boilerplate. You write a class with a decorator, and the generator produces an abstract base class that handles immutability, deep copying, value equality, toString(), and collection freezing.

// You write this:
@freezed()
class Person extends $Person {
  constructor(params: { firstName: string; lastName: string; age: number }) {
    super(params);
  }
}

The generator produces $Person with:

• readonly properties
• Object.freeze(this) in the constructor
• person.with({ age: 31 }) for copying
• person.equals(other) for deep structural comparison
• person.toString() → "Person(firstName: John, lastName: Smith, age: 30)"
• Recursive freezing of arrays, Maps, and Sets

Installation

npm install freezedts
npm install -D freezedts-cli

freezedts is the runtime — add it as a production dependency. It has zero transitive dependencies.

freezedts-cli is the code generator — add it as a dev dependency. It depends on ts-morph for AST parsing.

Requirements: TypeScript 6, ESM, TC39 stage 3 decorators.

Running the Generator

# Generate .freezed.ts files for all source files in the current directory
npx freezedts

# Generate for a specific directory
npx freezedts src

# Watch mode — regenerate on file changes
npx freezedts --watch
npx freezedts -w src

# Use a custom config file
npx freezedts --config path/to/freezedts.config.json
npx freezedts -c custom.json -w src

# Force generating all freezed files
npx freezedts --force

# Show help
npx freezedts --help
npx freezedts -h

The generator scans for .ts files containing @freezed() classes and produces .freezed.ts files alongside them.

Only changed files are regenerated (mtime-based).

Creating Classes

Basic Usage

1. Create your source file (e.g., person.ts):

import { freezed } from 'freezedts';
import { $Person } from './person.freezed.ts';

@freezed()
class Person extends $Person {
  constructor(params: { firstName: string; lastName: string; age: number }) {
    super(params);
  }
}

2. Run the generator:

npx freezedts

3. The generated person.freezed.ts provides an abstract base class $Person with readonly properties, Object.freeze(this), with(), equals(), and toString().

Where a source file contains multiple @freezed classes, they will be generated to the same file.

import { freezed } from 'freezedts';
import { $Person, $Address } from './person.freezed.ts';

@freezed()
class Person extends $Person {
  ...
}

@freezed()
class Address extends $Address {
  ...
}

Field Configuration

Configure defaults and validation in the @freezed() decorator's fields option:

@freezed({
  fields: {
    port: {
      default: 3000,
      assert: (v: number) => v > 0 && v < 65536,
      message: 'port out of range',
    },
    host: { default: 'localhost' },
  },
})
class ServerConfig extends $ServerConfig {
  constructor(params: { name: string; host?: string; port?: number }) {
    super(params);
  }
}

const config = new ServerConfig({ name: 'api' });
config.host; // 'localhost'
config.port; // 3000

new ServerConfig({ name: 'api', port: -1 }); // throws: "port out of range"

Field config options:

Option	Type	Description
default	unknown	Default value when the parameter is undefined
assert	(value) => boolean	Validation function run at construction time
message	string | undefined	An optional error message when the assertion fails. If omitted, a basic error message will be generated.

Collections

Arrays, Maps, and Sets are recursively frozen at construction time. Mutation attempts throw at runtime:

@freezed()
class Team extends $Team {
  constructor(params: { name: string; members: string[]; scores: number[] }) {
    super(params);
  }
}

const team = new Team({ name: 'Alpha', members: ['Alice', 'Bob'], scores: [10, 20] });
team.members.push('Charlie'); // throws TypeError
team.members[0] = 'Zara';    // throws TypeError

Deep Copy

The with() method creates a new frozen instance with selective property overrides:

const alice = new Person({ firstName: 'Alice', lastName: 'Smith', age: 30 });
const bob = alice.with({ firstName: 'Bob' });
// bob → Person(firstName: Bob, lastName: Smith, age: 30)
// alice is unchanged

For nested @freezed types, with supports proxy-chained deep copies:

@freezed()
class Assistant extends $Assistant {
  constructor(params: { name: string }) { super(params); }
}

@freezed()
class Director extends $Director {
  constructor(params: { name: string; assistant: Assistant }) { super(params); }
}

@freezed()
class Company extends $Company {
  constructor(params: { name: string; director: Director }) { super(params); }
}

const co = new Company({
  name: 'Acme',
  director: new Director({
    name: 'Jane',
    assistant: new Assistant({ name: 'John' }),
  }),
});

// Shallow copy
co.with({ name: 'NewCo' });

// Deep copy — update a nested freezed property
co.with.director({ name: 'Larry' });
co.with.director.assistant({ name: 'Sue' });

Deep Equality

The equals() method performs structural comparison:

const a = new Person({ firstName: 'Alice', lastName: 'Smith', age: 30 });
const b = new Person({ firstName: 'Alice', lastName: 'Smith', age: 30 });

a === b;       // false (different instances)
a.equals(b);   // true (same structure)

Configure equality mode per class:

@freezed({ equality: 'deep' })    // default — recursive structural comparison
class DeepPerson extends $DeepPerson { ... }

@freezed({ equality: 'shallow' }) // === for primitives, .equals() for nested freezed types
class ShallowPerson extends $ShallowPerson { ... }

Configuration

Per-Class Configuration

Disable generation of specific methods via the @freezed() decorator:

@freezed({ copyWith: false })   // skip with() generation
@freezed({ equal: false })      // skip equals() generation
@freezed({ toString: false })   // skip toString() generation
@freezed({ copyWith: false, equal: false, toString: false })  // only immutability

Project-Wide Configuration

Create a freezedts.config.json in your project root:

{
  "freezed": {
    "options": {
      "format": true,
      "copyWith": false,
      "equal": false,
      "toString": false
    }
  }
}

All options default to true (enabled) except format which defaults to false.

Resolution Order

Per-class @freezed() options override project-wide freezedts.config.json defaults. If neither specifies a value, the built-in default applies (all features enabled).

per-class @freezed()  →  freezedts.config.json  →  built-in defaults
    (highest priority)                              (lowest priority)

Building the Library

# Install dependencies (sets up workspace symlinks)
npm install

# Build both packages
npm run build

# Run tests
npm test

# Run the generator on this project's source files
npm run generate

# Watch mode for tests
npm run test:watch

The project is structured as an npm workspace with two packages:

• freezedts — runtime library (zero dependencies)
• freezedts-cli — code generator (depends on ts-morph)

Other tools:

• TypeScript 6 with ES2022 target and Node16 module resolution
• bun:test for testing