Re: Determining the type (array, object, or scalar) of a JSON value

no argument against json_typeof, but why can you just peek the first
non-whitespace character?  json  is famously easy to parse (see here:
http://www.json.org/)

create or replace function json_typeof(_json json) returns text as
$$
  select case substring(ltrim($1::text), 1, 1)
    when '[' then 'array'
    when '{' then 'object'
  end;
$$ language sql immutable;

you could expand this mechanic fairly easy to cover all json types.
note exception handlers are very heavy for this type of operation.