Composition

In JavaScript, composition refers to a design principle where objects are composed of other objects or behaviors, rather than relying on inheritance.

It promotes code reuse, modularity, and flexibility. Instead of creating complex class hierarchies through inheritance, you build objects by assembling them from smaller, more specialized pieces.

// Let's say we have a 'Person' object with basic properties and methods
class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }

  greet() {
    console.log(`Hello, my name is ${this.name} and I am ${this.age} years old.`);
  }
}

// Now, instead of creating a subclass, we'll create smaller, reusable objects
// that we can compose to create more complex objects.

// Define a 'Job' object
const Job = {
  position: 'Developer',
  salary: '$60,000',
  describeJob() {
    console.log(`I work as a ${this.position} and earn ${this.salary} per year.`);
  }
};

// Define a 'Hobby' object
const Hobby = {
  hobby: 'Painting',
  describeHobby() {
    console.log(`In my free time, I enjoy ${this.hobby}.`);
  }
};

// Now, let's compose a person with a job and a hobby
function createPersonWithJobAndHobby(name, age, job, hobby) {
  const person = new Person(name, age);
  Object.assign(person, job); // Copy properties from job to person
  Object.assign(person, hobby); // Copy properties from hobby to person
  return person;
}

// Create a person with a job and a hobby
const john = createPersonWithJobAndHobby('John', 30, Job, Hobby);

john.greet(); // Output: Hello, my name is John and I am 30 years old.
john.describeJob(); // Output: I work as a Developer and earn $60,000 per year.
john.describeHobby(); // Output: In my free time, I enjoy Painting.

In this example:

• We have a Person class representing a basic object with properties and methods.

• We define smaller objects Job and Hobby, each encapsulating specific behaviors or properties.

• We use a function createPersonWithJobAndHobby to compose a Person object with a Job and a Hobby.

• Finally, we create a person named John with a job and a hobby and demonstrate how the composed object can access all properties and methods from its constituent parts.