use-package and js2-mode: setting fill-column

Question

I'm using use-package and I want my fill-column to be set to 100 for files edited with js2-mode.

I tried to do so in the :config part, like:

(use-package js2-mode
  :config
  (setq fill-column 100))

(among other configuration) but it didn't work, fill-column was still 70 after loading a JS file (which activated js2-mode).

The way I made it work was by setting fill-column inside a hook like:

(use-package js2-mode
  :preface
  (defun me/js2-hook ()
    (setq fill-column 100))
  :config
  (add-hook 'js2-mode-hook #'me/js2-hook))

So, my question is, what was wrong with setting fill-column straight into :config? Why is the hook required?

Thanks!

Answer 1

fill-column immediately becomes buffer-local when set. So, when your config is evaluated, that buffer's fill-column is set... but then nothing else happens. If you set it with a hook, then fill-column will be set any time that hook is fired -- that is, every time a new buffer is created in js2-mode, and then fill-column will be set for that buffer.

You can, if you want, change the global default value of fill-column from 70 (which is silly) to another number, outside of use-package, like so:

(setq-default fill-column 79)