Retrieves the last item from the 'thisPointerStack' and invokes the provided callback with it.
@param {Function} callback The callback to be invoked with the current 'this' value.
@return The result of the invocation of the callback.
function getThisValueAndInvoke (callback) {
            return callback(currentThis !== undefined && currentThis !== null ?
                (typeof currentThis === 'object' ? currentThis : currentThis.valueOf()) : currentThis
            );
        }