Emacs: Hook

What is Hook

• A hook is a variable. Value is a list of functions (Symbol or Lambda).
• A hook variable is associated with a specific event. When the event happens, all functions in that hook are called.
• There are hundreds of hooks. Each Major Mode usually have at least 1 hook, designed to run when the mode is activated.

For example,

• when js-mode is loaded, js-mode-hook's functions are called.
• when any command is called, post-command-hook's functions are called.

Hook is similar to the concept of event in JavaScript . Adding functions to a hook is similar to adding event handlers. (note: emacs lisp manual also uses the term “event”, but that is lower level events to emacs.)

Hook is often used to change Keys for Major Mode . see Emacs Keys: Change Major Mode Keys

Add Function to Hook

Here's example of setting proportional width font for some modes:

;; use proportional width font for some modes
(add-hook 'js-mode-hook 'variable-pitch-mode)

Remove Function in Hook

(remove-hook 'js-mode-hook 'variable-pitch-mode)

;; remove all functions in the hook
(setq js-mode-hook nil)

Show Value of a Hook

• Emacs: Show Variable Value