New buffers of the same name don't inherit default major mode?

Question

I'm using the following to create a new buffer "untitled" with the keyboard map C-n. This works fine for a new buffer but additional buffers of the same name where <num> gets applied seem to inherit the Fundamental major mode rather than Text. Why is this?

(setq major-mode 'text-mode)

(defun new-buffer-by-name (arg)
   "Creates a new buffer by name"
   (generate-new-buffer arg)
   (set-buffer-major-mode (get-buffer arg))
   (switch-to-buffer arg))

(global-set-key (kbd "C-n")
   (lambda () (interactive)
      (new-buffer-by-name "untitled")))

Answer 1

Looking at C-h v major-mode, you'll see:

Automatically becomes buffer-local when set.

To set the default value (and not only a buffer-local value), use (setq-default major-mode 'text-mode) instead.

Moreover, you create a new buffer with generate-new-buffer but don't actually run set-buffer-major-mode on it.

Lastly, I suggest to make new-buffer-by-name itself an interactive function.

The code then becomes :

(setq-default major-mode 'text-mode)

(defun new-buffer-by-name (bufname)
  "Creates a new buffer by name"
  (interactive (list "untitled"))
  (let ((newbuf (generate-new-buffer bufname)))
    (set-buffer-major-mode newbuf)
    (switch-to-buffer newbuf)))

(global-set-key (kbd "C-n") #'new-buffer-by-name)