When I add a property

{ x: [ null ] }

to a document, everything works well.
But when I add a property

{ x: [ undefined ] }

to a document (usually because of buggy javascript code, but anyway), the following happens :
If requested from the mongo shell, the document shows as

{ x: [ null ] }

.
If requested from the node driver, the document shows in javascript as

{ x: [] }

.
Replacing undefined by null in the shell is dubious (but not the object of this report), but showing an empty array in node is a bug. Moreover, if I try to query documents with { "x.0": {$exists: true} }, it returns the document (with x an empty array...), and with { "x.0": {$exists: false} } it doesn't return anything.