They could have used a Map (introduced in ES6 [1]) which maintains key insertion order. However, I'm not sure whether either the object or map interface would be significantly better or worse than the builder-style the author picked. It just doesn't seem to matter, IMO.

The interface that would be most flexible would probably be a list of alternating key values, e.g. ['fieldname', datastruct(), 'fieldname'] since then you could do all the list things to it.

[1] https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...