// global code
this.foo = 1;        // creating a global property
foo                  // accessing the global property via a direct reference

// global code
var foo = 1;         // creating a global variable
this.foo             // accessing the global variable via a property reference

foo // => undefined
bar // throws ReferenceError

var foo = 1;
this.bar = 1;

// the names "foo" and "bar" are bound to the global environment
var foo = 1;
this.bar = 1;

// the binding "bar" can be removed from the global environment subsequently
delete this.bar;

// the binding "foo" cannot be removed subsequently

Object.defineProperty( this, 'bar', { value: 1 }); // non-configurable by default

// "this" refers to the global object. But global object is also acting as the
// variable object! Because of that, the following code works:

var foo = 1;
alert(this.foo);   // 1

(function() {

    // "this" still refers to the global object! But the *variable* object has
    // changed because we're now in the execution context of a function, thus
    // the behavior changes:

    var bar = 2;
    alert(this.foo);  // 1
    alert(this.bar);  // undefined

})();