@julienw , I have tested this for many times. I am not sure why document's propertyDescriptor can only be fetched in setup. After some investigating, I found in another test, there is the same problem with this (?) You can check it here : https://github.com/mozilla-b2g/gaia/blob/master/apps/communications/contacts/test/unit/views/details_test.js#L126

Because we will always get `undefined` in this case, L126 use `if` to make sure anything won't go wrong. But I think there must be some loading priority problems in mocha especially in `suiteSetup` and `setup`. (that's why I can get the properties in `setup` but not in `suiteSetup`)

Any ideas (?!)