View difference between Paste ID: S2BZ2QcG and FsZhKwY2

 function solveClasses() {
     class Pet {
         comments = [];
 
         constructor(owner, name) {
             this.owner = owner;
             this.name = name;
         }
 
         addComment(comment) {
             if (this.comments.includes(comment)) {
                 throw new Error('This comment is already added!');
             }
 
             this.comments.push(comment);
 
             return 'Comment is added.';
         }
 
         feed() {
             return `${this.name} is fed`
         }
 
         toString() {
             let result = `Here is ${this.owner}'s pet ${this.name}.`;
 
             if (this.comments.length > 0) {
                 result += '\n';
                 result += `Special requirements: ${this.comments.join(', ')}`;
             }
 
             return result;
         }
     }
 
     class Cat extends Pet {
         constructor(owner, name, insideHabits, scratching) {
             super(owner, name);
 
             this.insideHabits = insideHabits;
             this.scratching = scratching;
         }
 
         feed() {
             return super.feed() + ', happy and purring.';
         }
 
         toString() {
             let result = super.toString();
 
             result += '\n';
             result += 'Main information:';
             result += '\n';
             result += `${this.name} is a cat with ${this.insideHabits}`;
 
             if (this.scratching) {
                 result += ', but beware of scratches.';
             }
 
             return result;
         }
     }
 
     class Dog extends Pet {
         constructor(owner, name, runningNeeds, trainability) {
             super(owner, name);
 
             this.runningNeeds = runningNeeds;
             this.trainability = trainability;
         }
 
         feed() {
             return super.feed() + ', happy and wagging tail.';
         }
 
         toString() {
             let result = super.toString();
 
             result += '\n';
             result += 'Main information:';
             result += '\n';
             result += `${this.name} is a dog with need of ${this.runningNeeds}km running every day and ${this.trainability} trainability.`;
 
             return result;
         }
     }
 
     return {
         Pet,
         Cat,
         Dog
     }
 }
 
 let classes = solveClasses();
 // let pet = new classes.Pet('Ann', 'Merry');
 // console.log(pet.addComment('likes bananas'));
 // console.log(pet.addComment('likes sweets'));
 // console.log(pet.feed());
 // console.log(pet.toString());
 
 // let cat = new classes.Cat('Jim', 'Sherry', 'very good habits', true);
 // console.log(cat.addComment('likes to be brushed'));
 // console.log(cat.addComment('sleeps a lot'));
 // console.log(cat.feed());
 // console.log(cat.toString());
 
 let dog = new classes.Dog('Susan', 'Max', 5, 'good');
 console.log(dog.addComment('likes to be brushed'));
 console.log(dog.addComment('sleeps a lot'));
 console.log(dog.feed());
 console.log(dog.toString());