Use flow.js to show type signature for value under cursor whenever the cursor is idle for more than half a second (assumes espresso-mode.el)

flow-type-at-pos.el

(defun call-process-on-buffer-to-string (command)
  (with-output-to-string
      (call-process-region (point-min) (point-max) shell-file-name nil standard-output nil shell-command-switch command)))


(defun flow-type ()
  (interactive)
  (let* ((info (json-read-from-string
                (call-process-on-buffer-to-string
                   (format "flow type-at-pos --json  %d %d" (line-number-at-pos) (current-column)))))
         (type (cdr (assoc 'type info))))
    (princ info)
    (if (not (string-equal type "(unknown)"))
        (message type))))

(defun flow-pragma-exists ()
  (save-excursion
    (save-match-data
      (goto-char (point-min))
      (not (null (search-forward "@flow" 200 t))))))

(defun flow-espresso-mode-hook ()
  (when (flow-pragma-exists)
        (run-with-idle-timer 0.5 t 'flow-type)))

(add-hook 'espresso-mode-hook 'flow-espresso-mode-hook)