JSON to CSV

Turns an array of objects into CSV, using the union of every key as the header so a field present on only some rows still gets a column.

Taking the union rather than the first object’s keys is the difference between a correct export and a silently lossy one. If row one has three fields and row two has a fourth, using row one as the template drops that fourth field entirely and nobody notices until the data is needed. Nested objects and arrays are a genuine problem with no good answer — CSV is flat and JSON is not. Those are serialised as JSON text inside the cell, which at least preserves the data; flattening them into dotted columns first, with the JSON flattener, usually gives a more useful spreadsheet.