Usage: (require 'flymake-json) Then, if you're using `json-mode': (add-hook 'json-mode 'flymake-json-load) or, if you use `js-mode' for json: (add-hook 'js-mode-hook 'flymake-json-maybe-load) otherwise: (add-hook 'find-file-hook 'flymake-json-maybe-load) Uses flymake-easy, from https://github.com/purcell/flymake-easy