composition vs inheritance explained by example

composition_vs_inheritance.js

// we have to represent human relation in oop style

class Human {
  constructor(firstName,lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }
  sleepNow() {
    console.log(`${this.firstName} ${this.lastName} is sleeping`)
  }
  
  eatNow() {
     console.log(`${this.firstName} ${this.lastName} is eating`); 
  }
  
}

class Young extends Human {
   constructor(firstName,lastName,age) {
      super(firstName,lastName);
     this.age = age;
   }
  
  startStudy() {
    console.log(`${this.firstName} is studing`)
  }
  
  goToSchool() {
    console.log(`${this.firstName} is going to school`)
  }
  
  
}

class Male extends Young {
  constructor(firstName,lastName,age,gender) {
    super(firstName,lastName,age);
    this.gender = gender;
  }
  
  // here all the method of young and human gets also Male does not need some of the method all method are made available to Male class
 
}

// The main issue with inheritance is that we predefine the whole architecture we have to define the whole chain of Human, Young,Male
// there properties and method this also make tight coupling suppose a method sleepNow has need some change like we want to perform
// prayer before sleep then it will effect the whole chain from where it being called this could be desired result but some time
// this could result in breakdown or unexpected behaviour


// How compose can solve this problem
getSleepAbility(human) {
  return Object.assign({},human,{toSleep:()=>{console.log('this provide sleep ability.')}});
}

function man(firstName,lastName,age,gender) {
   let man = {
      firstName,
     lastName,
     age,
     gender
   };
   return getSleepAbility(man);
}

// In FP programming we more focus on the what it has or what ability it has so i have created two function function man which get
// ability to sleep from getSleepAbility this way our code is future capable no tight coupling