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The JavaScript this Keyword

In this example, this refers to the person object.

Because fullName is a method of the person object.

// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  id: 5566,
  fullName : function() {
    return this.firstName + " " + this.lastName;
  }
};

// Display data from the object:
document.getElementById("demo").innerHTML = person.fullName();

i.e. this.firstName is the firstName property of this (the person object).