(defun my/pretty-js-symbols ()
  (push '("===" . ?≡) prettify-symbols-alist)
  (push '("function" . ?𝑓) prettify-symbols-alist))

(defun my/pretty-r-symbols ()
  (push '("%>%" . ?⤚) prettify-symbols-alist)
  (push '("%$%" . ?⤜) prettify-symbols-alist)
  (push '("==" . ?≡) prettify-symbols-alist)
  (push '("function" . ?𝑓) prettify-symbols-alist))

(when (boundp 'global-prettify-symbols-mode)
  (add-hook 'js2-mode-hook 'my/pretty-js-symbols)
  (add-hook 'ess-mode-hook 'my/pretty-r-symbols)
  (add-hook 'inferior-ess-mode-hook 'my/pretty-r-symbols)
  (global-prettify-symbols-mode +1))

(global-prettify-symbols-mode)