Immutable data objects with type safe .copy(), .map() and .equals() inspired by case class in Scala and data class in Kotlin

• Installation
• Usage
• Type safety
• API Reference
  • copy
  • map
  • equals
  • toString
• Can I use it with JavaScript?
• Prior art

Installation

yarn add copyable

TypeScript >= 2.8 required.

Usage

import { Copyable } from 'copyable';

class Person extends Copyable {
  constructor(readonly name: string, readonly age: number) {
    super();
  }
}

const person1 = new Person('John', 25);

const person2 = person1.copy({ name: 'James', age: 20 });
// -> Person(name="James", age=20)
person1.copy('age', 30);
// -> Person(name="John", age=30)

person1.map({
  name: name => 'Older ' + name,
  age: age => age + 20
});
// -> Person(name="Older John", age=45)
person1.map('age', age => age + 20);
// -> Person(name="John", age=45)

person1.equals(person2); // false
person1.equals(new Persion('John', 25)); // true

person1.toString();
/* -> 
  Person(
    name="John",
    age=25
  )
*/

Type safety

person1.copy({ age: '20' }); // comple error: Types of property 'age' are incompatible. Type 'string' is not assignable to type 'number'.

person1.copy({ notAPropertyOfPerson: 20 }); // comple error: Object literal may only specify known properties

person1.copy({ equals: () => false }); // Nope. Can't replace methods.

API Reference

copy

• copy(patch?: Patch<this>): this

  Returns a copy of the object.

  Accepts an object that contains new values for specified properties

  import { Copyable } from 'copyable';
  
  class Person extends Copyable {
    constructor(readonly name: string, readonly age: number) {
      super();
    }
  }
  
  const person1 = new Person('John', 25);
  person1.copy({ name: 'James', age: 20 });
  // -> Person(name="James", age=20)

  Parameters

  • (Optional) patch: Patch<this>

  Returns

  typeof this

• copy<K extends PatchKey<this>>(key: K, val: this[K]): this

  Returns a copy of the object.

  Accepts a key and a new value for it

  import { Copyable } from 'copyable';
  
  class Book extends Copyable {
    constructor(readonly title: string) {
      super();
    }
  }
  
  const book1 = new Book('TypeScript Deep Dive');
  book1.copy('title', 'TypeScript handbook');
  // -> Book(title="TypeScript handbook")

  Parameters

  • key: K extends PatchKey<this>
  • value: this[key]

  Returns

  typeof this

map

• map(patchMap: MapPatch<this>): this

  Maps the object. Allows to copy the object based on its current values.

  Accept an object that contains mapper for specified properties.

  const person1 = new Person('John', 25);
  // John got older
  person1.map({
    name: name => 'Older ' + name,
    age: age => age + 20
  });
  // -> Person(name="Older John", age=45)

  Parameters

  • (Optional) patchMap: MapPatch<this>

  Returns

  typeof this

• map<K extends PatchKey<this>>(key: K, mapper: Mapper<this[K]>): this

  Maps the object. Allows to copy the object based on its current values.

  Accept a key and a mapper for it.

  import { Copyable } from 'copyable';
  
  class Book extends Copyable {
    constructor(readonly title: string) {
      super();
    }
  }
  
  const book1 = new Book('TypeScript Deep Dive');
  book1.map('title', title => title + ' Part 2');
  // -> Book(title="TypeScript Deep Dive Part 2")

Parameters

• key: K extends PatchKey<this>
• mapper: Mapper<this[K]>

Returns

typeof this

equals

equals(other: this | null | undefined): boolean

Checks for structural equality

Note that the comparision is shallow!

const book1 = new Book('TypeScript Deep Dive');
const book2 = new Book('TypeScript handbook');

book1.equals(book2); // false
book1.equals(new Book('TypeScript Deep Dive')); // true

Returns

boolean

toString

toString(): string

Returns a string representation of the object that is a bit nicer than the default one ('[object Object]')

const person = new Person('John', 25);
person.toString();
// Person(
//   name="John",
//   age=25
// )

Returns

string

Can I use it with JavaScript?

Yes! You won't have any type checking, of course, so you risk shooting yourself in a foot.

Prior art

Inspired by Case Classes in Scala and Data Classes in Kotlin.

Implementation influenced by ts-copyable and dataclass.