<button onclick="this.style.display='none'">
  Click to Remove Me!
</button>

"use strict";
function myFunction() {
  return this;
}

function myFunction() {
  return this;
}

[object Window]:

var x = this;
x // [object Window]

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

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

The person object is the owner of the fullName method.

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.

  // window is the object that all global variables and functions are defined on, hence:
    window.showFullName (); // Peter Ally

    // "this" inside the showFullName () method that is defined inside the person object still refers to the person object, hence:
    person.showFullName (); // Penelope Barrymore

var person = {
    firstName   :"Penelope",
    lastName    :"Barrymore",
    showFullName:function () {
    // "this" on the line below refers to the person object, because the showFullName function will be invoked by person object.
    console.log (this.firstName + " " + this.lastName);
    }
    }

    showFullName (); // Peter Ally
    person.showFullName() // Penelope Barrymore

   var firstName = "Peter",
    lastName = "Ally";

    function showFullName () {
    // "this" inside this function will have the value of the global variables
    // because the showFullName () function is defined in the global scope, just like the firstName and lastName
    console.log (this.firstName + " " + this.lastName);
    }

var person = {
    firstName   :"Penelope",
    lastName    :"Barrymore",
    // Since the "this" keyword is used inside the showFullName method below, and the showFullName method is defined on the person object,
    // "this" will have the value of the person object because the person object will invoke showFullName ()
    showFullName:function () {
    console.log (this.firstName + " " + this.lastName);
    }

    }

    person.showFullName (); // Penelope Barrymore