The this keyword

You must use the keyword this inside a class to define/access properties (except for defining methods).

class Publication {
  setTitle(title) {
    this.title = title;
  }

  setAuthor(author) {
    this.author = author;
  }

  getTitle() {
    return this.title;
  }

  getAuthor() {
    return this.author;
  }

  print() {
    console.log(this.getTitle() + " by " + this.getAuthor());
  }
}

const publication = new Publication();
publication.setTitle("CS280 Notes");
publication.setAuthor("Ali Madooei");
publication.print();

Unlike Java/C++, you don't declare fields. And, you are not bounded to declare the attributes inside the constructor. However, for improved readability, I suggest that you always initialize your class attribute inside the constructor.

The JavaScript class syntax is more similar to php/python's class definition. (Python classes, for instance, are constructed through a special method called __init__() which uses the self parameter-instead of this- to reference to the current instance of the class)