Convert json to csv with jq

to_csv.jq

def to_csv:
  ( [.[] | keys | .[]] | unique ) as $keys
  | ( $keys | @csv )
  , ( .[]
    | . as $row
    | reduce ($keys | .[]) as $key ([]; [.[], ($row | .["\($key)"])])
    | @csv
    );

def uniform_array_to_csv:
    ( .[0] | keys_unsorted | @csv)
  , (.[] | [.[]] | @csv );

Converts an array of objects (or an object of objects) to an array of csv rows.

Objects do not have to be uniform, as missing keys result in null values in the output — i.e. skipped fields.

First, all the keys are extracted from the whole input, and used for a header row.

Then each object in the input array (or input top-level object) gets converted into a data row.
The reduce expression runs through each key, adding the corresponding value (or null) to the output.

If you have a large array of uniform objects, then do not use to_csv!
Just do the simpler thing — uniform_array_to_csv.

(This would benefit from some fuzz testing!)