ES5 #15.4.5.2 says:

"The length property of this Array object is a data property whose value is always numerically greater than the name of every deletable property whose name is an array index."

This means that when setting length to a lower value, deletable array elements with ids higher or equal than new length must be deleted, while non-deletable elements must cause the new length to be one greater than the biggest non-deletable id.

Both is currently not done for elements defined via Object.defineProperty with non-standard attributes (stored as generic property instead of in array data).