Typescript - Mixins

TypeScript (and JavaScript) classes support strict single inheritance.

// So you cannot do:
class Tagged {
    constructor(private tag: string) {}
}

class Timestamped {
    constructor(private timestamp: string) {}
}

class User extends Tagged, Timestamped {} // TS1174: Classes can only extend a single class.

So you can use Mixins if you want to make use of this mechanism

1. Define Constructor type

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

2. A mixin that adds a property

function Timestamped<TBase extends Constructor>(Base: TBase) {
  return class extends Base {
    timestamp = Date.now();
  };
}

3. A mixin that adds a property and methods

function Activatable<TBase extends Constructor>(Base: TBase) {
  return class extends Base {
    isActivated = false;

    activate() {
      this.isActivated = true;
    }

    deactivate() {
      this.isActivated = false;
    }
  };
}

4. Use the mixin

class User {
  name = '';
}

// User that is Timestamped
const TimestampedUser = Timestamped(User);

// User that is Timestamped and Activatable
const TimestampedActivatableUser = Timestamped(Activatable(User));

const timestampedActivatableUserExample = new TimestampedActivatableUser();
console.log(timestampedActivatableUserExample.timestamp);
console.log(timestampedActivatableUserExample.isActivated);