In the inheritance section we looked at one way to extend a class functionality, there is second way using delegation to extend functionality. With delegation, one object will contain a reference to a different object that it will hand off a request to perform the functionality.

The code below shows how to use delegation with the Bird class and Penguin class. The Penguin class has a reference to the Bird class and it delegates the call made to it's walk method over to Bird's walk method.

// ES6
class Bird {
  constructor(weight, height) {
    this.weight = weight;
    this.height = height;
  walk() {

class Penguin {
  constructor(bird) {
    this.bird = bird;
  walk() {
  swim() {

const bird = new Bird(...);
const penguin = new Penguin(bird);
penguin.walk(); //walk!
penguin.swim(); //swim!

A good discussion on 'behaviour delegation' can be found here.

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