Appending (pushing) and removing from a JSON array in PostgreSQL 9.5+

To add the value use the JSON array append opperator (||)

UPDATE jsontesting
SET jsondata = jsondata || '["newString"]'::jsonb
WHERE id = 7;

Removing the value looks like this

UPDATE jsontesting
SET jsondata = jsondata - "newString"
WHERE id = 7; 

Concatenating to a nested field looks like this

UPDATE jsontesting
SET jsondata = jsonb_set(
  jsondata::jsonb,
  array['nestedfield'],
  (jsondata->'nestedfield')::jsonb || '["newString"]'::jsonb) 
WHERE id = 7;

To add to Evan Carroll's answer, you may want to do the following to set the column to an empty array if it is NULL. The append operator (||) does nothing if the column is currently NULL.

UPDATE jsontesting SET jsondata = (
    CASE
        WHEN jsondata IS NULL THEN '[]'::JSONB
        ELSE jsondata
    END
) || '["newString"]'::JSONB WHERE id = 7;

I was facing similar issue to append to an existing json data in postgres with new key-value pair. I was able to fix this using append operator || as follows:

UPDATE jsontesting
SET jsondata = jsondata::jsonb || '{"add_new_data": true}'
WHERE id = 7;