Understanding 'this' in Javascript

this.js

// A useful way of thinking about how the value of 'this' is bound in Javascript 
// functions is to imagine that Function.prototype.call is being used 
// implicitly at the call site to set the 'context' (the 'this' value). eg.

// assuming in each case
var o = {}
var o2 = {}
var fn = function(){}

// 1. bare function call
fn() // this == undefined in strict mode, global in non strict mode.

// 2. calling with Function.prototype.call
fn.call(o) // this == o

// 3. 'method' call (calling an object's function property)
o2.fn = fn
o2.fn() // this == o2
// equivalent to
fn.call(o2)

// 4. calling object's function property with Function.prototype.call
o2.fn = fn
o2.fn.call(o) // this == o

// 5. new
var o3 = new fn() // this = o3
// equivalent to
var o3 = Object.create(fn.prototype)
fn.call(o3)

// 6. calling function bound with Function.prototype.bind
var fn2 = fn.bind(o)
fn2() // this == o
// equivalent to
var fn2 = function(){ fn.call(o) }
fn2()

// 7. calling object function property which is a bound function
o2.fn = fn.bind(o)
o2.fn() // this == o
// equivalent to
o2.fn = function(){ fn.call(o) }
o2.fn()