JavaScript FlatMap Empty Results Audio

Produce a 45-second JavaScript review from [actual flatMap trace]. Explain why returning an empty array removes an item, while an omitted return contributes undefined. Read one example of each and state [intended retention rule]. Avoid describing flatMap as recursively flattening every nesting level.

Use only the supplied facts and approved wording. If a necessary script or timing reference is missing, ask for it first. Produce the audio when supported; otherwise return the complete spoken script with pauses and a short production cue sheet, clearly labeled as a script. Use original voices and keep speech intelligible without music. Never claim an audio file exists unless it was generated.