[emacs] Add flycheck eslint hook for js-mode(s)

* emacs.d/emacs.org (Linting and Code Quality): Added.

emacs.d/emacs.org

@@ -717,6 +717,7 @@ side-by-side windows.
 #+END_SRC
 
 ** JavaScript
+*** Indentation
 =js-mode='s indentation strategy is abhorrent.
 
 #+BEGIN_SRC emacs-lisp
@@ -725,12 +726,44 @@ side-by-side windows.
               (setq indent-line-function 'indent-relative)))
 #+END_SRC
 
+*** Linting and Code Quality
+[[http://eslint.org/][ESLint]] is a linting and codestyle tool for ECMAScript.  Flycheck
+supports it.
+
+#+BEGIN_SRC emacs-lisp
+  (add-hook 'js-mode-hook 'flycheck-mode t)
+#+END_SRC
+
+This code is based on [[https://emacs.stackexchange.com/questions/21205/flycheck-with-file-relative-eslint-executable/21207#21207][an Emacs StackExchange answer]]:
+
+#+BEGIN_SRC emacs-lisp
+  (defun my-flycheck-find-eslint ()
+    (let* ((root (locate-dominating-file
+                  (or (buffer-file-name)
+                      default-directory)
+                  "node_modules"))
+           (esbin (and root
+                       (expand-file-name
+                        "node_modules/.bin/eslint"
+                        root))))
+      (and esbin
+           (file-executable-p esbin)
+           esbin)))
+
+  (defun my-flycheck-set-eslint ()
+    (setq-local flycheck-javascript-eslint-executable
+                (my-flycheck-find-eslint)))
+
+  (add-hook 'flycheck-mode-hook #'my-flycheck-set-eslint)
+#+END_SRC
+
 ** Web Mode
 #+BEGIN_SRC emacs-lisp
   (setq web-mode-markup-indent-offset 2
         web-mode-enable-auto-pairing  t
         web-mode-enable-auto-closing  t)
 #+END_SRC
+
 * Editing
 I'm told that Emacs is an operating system.  I'm also told that it is
 an editor.  I've been convinced of both.