Along with traditional OO hierarchies, another popular way of building up classes from reusable components is to build them by combining simpler partial classes. You may be familiar with the idea of mixins or traits for languages like Scala, and the pattern has also reached some popularity in the JavaScript community.

How Does A Mixin Work?

The pattern relies on using generics with class inheritance to extend a base class. TypeScript's best mixin support is done via the class expression pattern. You can read more about how this pattern works in JavaScript here.

To get started, we'll need a class which will have the mixins applied on top of:

class Sprite {
  name = "";
  x = 0;
  y = 0;

  constructor(name: string) {
    this.name = name;
  }
}

Then you need a type and a factory function which returns a class expression extending the base class.

// To get started, we need a type which we'll use to extend
// other classes from. The main responsibility is to declare
// that the type being passed in is a class.

type Constructor = new (...args: any[]) => {};

// This mixin adds a scale property, with getters and setters
// for changing it with an encapsulated private property:

function Scale<TBase extends Constructor>(Base: TBase) {
  return class Scaling extends Base {
    // Mixins may not declare private/protected properties
    // however, you can use ES2020 private fields
    _scale = 1;

    setScale(scale: number) {
      this._scale = scale;
    }

    get scale(): number {
      return this._scale;
    }
  };
}

With these all set up, then you can create a class which represents the base class with mixins applied:

// Compose a new class from the Sprite class,
// with the Mixin Scale applier:
const EightBitSprite = Scale(Sprite);

const flappySprite = new EightBitSprite("Bird");
flappySprite.setScale(0.8);
console.log(flappySprite.scale);

Constrained Mixins

In the above form, the mixin's have no underlying knowledge of the class which can make it hard to create the design you want.

To model this, we modify the original constructor type to accept a generic argument.

// This was our previous constructor:
type Constructor = new (...args: any[]) => {};
// Now we use a generic version which can apply a constraint on
// the class which this mixin is applied to
type GConstructor<T = {}> = new (...args: any[]) => T;

This allows for creating classes which only work with constrained base classes:

type Positionable = GConstructor<{ setPos: (x: number, y: number) => void }>;
type Spritable = GConstructor<Sprite>;
type Loggable = GConstructor<{ print: () => void }>;

Then you can create mixins which only work when you have a particular base to build on:

function Jumpable<TBase extends Positionable>(Base: TBase) {
  return class Jumpable extends Base {
    jump() {
      // This mixin will only work if it is passed a base
      // class which has setPos defined because of the
      // Positionable constraint.
      this.setPos(0, 20);
    }
  };
}

Alternative Pattern

Previous versions of this document recommended a way to write mixins where you created both the runtime and type hierarchies separately, then merged them at the end:

// Each mixin is a traditional ES class
class Jumpable {
  jump() {}
}

class Duckable {
  duck() {}
}

// Including the base
class Sprite {
  x = 0;
  y = 0;
}

// Then you create an interface which merges
// the expected mixins with the same name as your base
interface Sprite extends Jumpable, Duckable {}
// Apply the mixins into the base class via
// the JS at runtime
applyMixins(Sprite, [Jumpable, Duckable]);

let player = new Sprite();
player.jump();
console.log(player.x, player.y);

// This can live anywhere in your codebase:
function applyMixins(derivedCtor: any, constructors: any[]) {
  constructors.forEach((baseCtor) => {
    Object.getOwnPropertyNames(baseCtor.prototype).forEach((name) => {
      Object.defineProperty(
        derivedCtor.prototype,
        name,
        Object.getOwnPropertyDescriptor(baseCtor.prototype, name) ||
          Object.create(null)
      );
    });
  });
}

This pattern relies less on the compiler, and more on your codebase to ensure both runtime and type-system are correctly kept in sync.

Constraints

The mixin pattern is supported natively inside the TypeScript compiler by code flow analysis. There are a few cases where you can hit the edges of the native support.

Decorators and Mixins #4881

You cannot use decorators to provide mixins via code flow analysis:

// A decorator function which replicates the mixin pattern:
const Pausable = (target: typeof Player) => {
  return class Pausable extends target {
    shouldFreeze = false;
  };
};

@Pausable
class Player {
  x = 0;
  y = 0;
}

// The Player class does not have the decorator's type merged:
const player = new Player();
player.shouldFreeze;
Property 'shouldFreeze' does not exist on type 'Player'.

// The runtime aspect could be manually replicated via
// type composition or interface merging.
type FreezablePlayer = Player & { shouldFreeze: boolean };

const playerTwo = new Player() as unknown as FreezablePlayer;
playerTwo.shouldFreeze;

Static Property Mixins #17829

More of a gotcha than a constraint. The class expression pattern creates singletons, so they can't be mapped at the type system to support different variable types.

You can work around this by using functions to return your classes which differ based on a generic:

function base<T>() {
  class Base {
    static prop: T;
  }
  return Base;
}

function derived<T>() {
  class Derived extends base<T>() {
    static anotherProp: T;
  }
  return Derived;
}

class Spec extends derived<string>() {}

Spec.prop; // string
Spec.anotherProp; // string