Use a Mixin to Add Common Behavior Between Unrelated Objects

Behavior is shared through inheritance. However, there are cases when inheritance is not the best solution. Inheritance does not work well for unrelated objects like Bird and Airplane. They can both fly, but a Bird is not a type of Airplane and vice versa.

For unrelated objects, it's better to use mixins. A mixin allows other objects to use a collection of functions.

let flyMixin = function(obj) {
  obj.fly = function() {
    console.log("Flying, wooosh!");
  }
};

The flyMixin takes any object and gives it the fly method. Let's see an example.

let bird = {
  name: "Donald",
  numLegs: 2
};

let plane = {
  model: "777",
  numPassengers: 524
};

flyMixin(bird);
flyMixin(plane);

Here bird and plane are passed into flyMixin, which then assigns the fly function to each object. Now bird and plane can both fly:

bird.fly(); // prints "Flying, wooosh!"

plane.fly(); // prints "Flying, wooosh!"


Note: how the mixin allows for the same fly method to be reused by unrelated objects bird and plane.