Understanding JavaScript’s this keyword

(In Portugese)

The JavaScript this keyword is ubiquitous yet misconceptions abound.

What you need to know
Every execution context has an associated ThisBinding whose lifespan is equal to that of the execution context and whose value is constant. There are three types of execution context: global, function and evaluation. Here’s a tabular summary followed by a little more detail, and some examples:

Execution Context	Syntax of function call	Value of this
Global	n/a	global object (e.g. window)
Function	Method call: myObject.foo();	myObject
Function	Baseless function call: foo();	global object (e.g. window) (undefined in strict mode)
Function	Using call: foo.call(context, myArg);	context
Function	Using apply: foo.apply(context, [myArgs]);	context
Function	Constructor with new: var newFoo = new Foo();	the new instance (e.g. newFoo)
Evaluation	n/a	value of this in parent context

1. Global context
this is bound to the global object (window in a browser)

alert(this); //window