Undefined in nullable fields

[ivogabe]

The spec marks optional fields with | null. I was wondering whether optional fields may also be undefined. The spec describes it this way:

The loc field represents the source location information of the node. If the node contains no information about the source location, the field is null; otherwise it is an object consisting of ..

This would suggest that undefined is not allowed for these fields. Though various implementations do not set optional fields, causing that those fields are undefined. For instance, Acorn and Esprima do not set loc when location tracking is not turned on, meaning that loc is undefined. When writing transforms it is also easier to write { type: "Identifier", name: "foo" } than { type: "Identifier", loc: null, name: "foo" }.

Is the latter the desired interpretation? If so, could the spec be updated to reflect that?