Understanding 'this' in Javascript.

this.js

// Summary from http://www.sitepoint.com/what-is-this-in-javascript/



//// GLOBAL SCOPE

// If there's no current object, 'this' refers to the global object
window.WhoAmI = "I'm the window object"

console.log(window.WhoAmI) // "I'm the window object"
console.log(this.WhoAmI)   // "I'm the window object"



//// CALLING A FUNCTION

// 'This' remains the global object. Think of these functions as simply methods on the global object.
window.WhoAmI = "I'm the window object"
function testThis() {
  console.log(this.WhoAmI); 
}

testThis(); // "I'm the window object"



//// CALLING OBJECT METHODS

// When calling an object constructor or any of its methods, 'this' refers to the instance of the object.
window.WhoAmI = "I'm the window object"
function Test() {
  this.WhoAmI = "I'm the test object";  
  this.CheckThis1 = function() {
    console.log(this.WhoAmI);
  };
}

Test.prototype.CheckThis2 = function() {
  console.log(this.WhoAmI);
};

var t = new Test();        // Note the 'new' keyword
console.log(t.WhoAmI);     // "I'm the test object"
console.log(t.CheckThis1); // "I'm the test object"
console.log(t.CheckThis2); // "I'm the test object"



//// USING CALL OR APPLY

// These methods run javascript functions as if they were methods of another object.

function setType(type) {
  this.WhoAmI = "I'm the " + type + " object";
}

var A = {};
setType.call(A, "A");
console.log(A.WhoAmI); // "I'm the A object";

var B = {};
setType.apply(B, ["B"]);
console.log(B.WhoAmI); // "I'm the B object";

/* 
The only difference between 'call' and 'apply' is that 'call' expects a discrete number of parameters
while 'apply' expects an array of parameters
*/