Build a Library - underscore is not necessary?

I’m curious how methods in Media class can work without underscores, when there is no reference for this.isCheckedOut and this.ratings.

Why is the instructor using underscores for constructors, getters, and setters but not methods?

class Media {
  constructor(title){
    this._title = title;
    this._isCheckedOut = false;
    this._ratings = [];
  }

  get title() {
    return this._title;
  }

  get isCheckedOut() {
    return this._isCheckedOut;
  }

  get ratings() {
    return this._ratings;
  }

  toggleCheckOutStatus(){
    this.isCheckedOut = !this.isCheckedOut
  }

  getAverageRating(){
     let sum = this.ratings.reduce((a,b) => a + b);
     return sum / this.ratings.length;
  }

  addRating(value){
    return this.ratings.push(value);
  }

  set isCheckedOut(value) {
    return this._isCheckedOut = value;
  }
}

class Book extends Media {
  constructor(author, title, pages) {
  super(title);
  this._author= author;
  this._pages= pages;
  }

  get author() {
    return this._author;
  }

  get pages() {
    return this._pages;
  }
}

const historyOfEverything = new Book('Bill Bryson', 'A Short History of Nearly Everything', 544)
historyOfEverything.toggleCheckOutStatus();
console.log(historyOfEverything.isCheckedOut)
historyOfEverything.addRating(4);
historyOfEverything.addRating(5);
historyOfEverything.addRating(5);
console.log(historyOfEverything.getAverageRating());

The syntax implies both a setter and a getter, which are both present in the code base.

  get isCheckedOut() {
    return this._isCheckedOut;
  }
  set isCheckedOut(value) {
    return this._isCheckedOut = value;
  }