https://www.w3schools.com/js/js_this.asp

What is this?

The JavaScript this keyword refers to the object it belongs to.

It has different values depending on where it is used:

• In a method, this refers to the owner object.
• Alone, this refers to the global object.
• In a function, this refers to the global object.
• In a function, in strict mode, this is undefined.
• In an event, this refers to the element that received the event.
• Methods like call(), and apply() can refer this to any object.

this in a Method

In an object method, this refers to the "owner" of the method.

In the example on the top of this page, this refers to the person object.

The person object is the owner of the fullName method.

fullName : function() {
  return this.firstName + " " + this.lastName;
}