*scratch*.el

(defun prettify-json ()
  "NOTE: requires ruby in $PATH. Replace a valid json region with
pretty printed json By default uses the jj method to print,
uses pp if there is a prefix argument"
  (interactive)
  (let* ((print-method (if current-prefix-arg "pp" "jj"))
         (cmd (format
               "ruby -e 'require \"json\"; require \"pp\"; %s JSON.parse(gets)'" print-method)))
    (shell-command-on-region
     (region-beginning)
     (region-end)
     cmd nil t)))

Can also:

(defun prettify-json (start end)
  "NOTE: requires ruby in $PATH. Replace a valid json region with
pretty printed json By default uses the jj method to print,
uses pp if there is a prefix argument"
  (interactive "r")
  (let* ((print-method (if current-prefix-arg "pp" "jj"))
         (cmd (format
               "ruby -e 'require \"json\"; require \"pp\"; %s JSON.parse(gets)'" print-method)))
    (shell-command-on-region start end cmd nil t)))