PostgreSQL compare two jsonb objects

UPDATED

CREATE OR REPLACE FUNCTION jsonb_diff_val(val1 JSONB,val2 JSONB)
RETURNS JSONB AS $$
DECLARE
  result JSONB;
  v RECORD;
BEGIN
   result = val1;
   FOR v IN SELECT * FROM jsonb_each(val2) LOOP
     IF result @> jsonb_build_object(v.key,v.value)
        THEN result = result - v.key;
     ELSIF result ? v.key THEN CONTINUE;
     ELSE
        result = result || jsonb_build_object(v.key,'null');
     END IF;
   END LOOP;
   RETURN result;
END;
$$ LANGUAGE plpgsql;

Query:

SELECT jsonb_diff_val(
    '{"a":"aaa", "b":"bbb", "c":"ccc"}'::jsonb,
    '{"a":"aaa", "b":"jjj", "d":"ddd"}'::jsonb
);
            jsonb_diff_val             
---------------------------------------
 {"b": "bbb", "c": "ccc", "d": "null"}
(1 row)