Javascript

this refer to the owner object

this in a method refers to the owner of the method
this alone refers to the global object window
this in a function refers to the global object window

// in a method
var person = {firstName: "John", lastName : "Doe",
  		fullName : function() { 
    			return this.firstName + " " + this.lastName; // refer to person
  			}
  		};

console.log(person.fullName());

// alone
var x = this; // refer to window

console.log(x);

// in a function
function f()
{
  	return this; // refer to window
}

console.log(f());

this in a function is undefined

"use strict";

// in a function
function f()
{
  	return this; // undefined
}

  		console.log(f());

Refers to the HTML element that received the event

<button onclick="this.style.color='red'">Click</button>