The this keyword
You must use the keyword this inside a class to define/access properties (except for defining methods).
In JavaScript, unlike in Java/C++, you don't declare fields.
Moreover, 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 refer to the current instance of the class)