MooTools is simply setting a value to a property that did not exist before the assignment. It isn't using Object.defineProperty() to create the property. You ca

MooTools is simply setting a value to a property that did not exist before the assignment. It isn't using Object.defineProperty() to create the property. You cannot call Object.defineProperty() twice for the same property (it throws an error). If assigning a value to a property that was previously defined as non-enumerable were to make it enumerable, then the very act of defining a property would have no value, except in the case where you are making a read-only property (ie using the "writable: false" option for Object.defineProperty).