(require 'company)

(require 'cl-lib)

(require 'web-mode)

(unless (fboundp 'web-mode-language-at-pos)

  (defun web-mode-language-at-pos (&optional pos)

    (plist-get (web-mode-point-context (or pos (point))) :language)))

(defvar company-dom--document-methods

  '("getElementById" "getElementsByClassName" "createElement" "createElementNS" "createEvent"))

(defvar company-dom--window-methods

  '("console" "document" "location"))

(defun company-dom--prefix ()

  (when (looking-back "\\<\\w+\\(?:\\.?\\w+\\>\\|\\.\\)")

    (match-string 0)))

(defun company-dom--candidates (prefix)

  (cond

   ((string-match "^window\\.\\(.*\\)" prefix)

    (mapcar

     (lambda (str) (concat "window." str))

     (all-completions (match-string 1) company-dom--window-methods)))

   ((string-match "^document\\.\\(.*\\)" prefix)

    (mapcar

     (lambda (str) (concat "document." str))

     (all-completions (match-string 1) company-dom--document-methods)))

   (t

    (all-completions prefix '("document" "window")))

   ))

(defun company-web (command &optional arg &rest ignored)

  (interactive (list 'interactive))

  (cl-case command

    (interactive (company-begin-backend 'company-web))

    (prefix

     (pcase (plist-get (web-mode-point-context (point)) :language)

       ("css" (company-css 'prefix))

       ("javascript" (company-dom--prefix))))

    (candidates

     (pcase (plist-get (web-mode-point-context (point)) :language)

       ("css" (company-css 'candidates arg))

       ("javascript" (company-dom--candidates arg))))

    (meta (format "This value is named %s" arg))))

(defun company-web-register ()

  (set (make-local-variable 'company-backends) '(company-web)))

(add-hook 'web-mode-hook #'company-web-register)

;; Debug company with M-x company-diag.