'this' in a method invocation and testing when a function is added to the object.

this.js

// You need log4javascript for the logging to work
// Or use the console, if log4javascript is not available
if (typeof log !== 'Object') {
    log = {
        debug: function (message) {
            console.log(message);
        }
    }
}


var object1 = {
    $get: function () {
        log.debug(this);
        this.f = function () {
            return 42;
        }
    }
}

// When calling $get() directly 'this' points to object1 itself where function f() is not yet added
object1.$get();
// But calling f() is possible, because $get() added it
object1.f()

// Creating a new instance with prototype inheritance
function constr() {};
constr.prototype = object1;
var instance = new constr();

// Now 'this' inside $get() will point to the new instance where function f() is already added
// instance.$get();
log.debug(instance.f());