How can a function check which major mode is enabled?

Question

The accepted answer to this question says:

When the function is invoked, it can test the current mode and do something appropriate. For example, for some modes, or all modes except some modes, it can be a no-op or it can raise an error.

How can this be done?

Answer 1

Inspect the value of the variable major-mode. From the docs:

Symbol for current buffer’s major mode. The default value (normally ‘fundamental-mode’) affects new buffers. A value of nil means to use the current buffer’s major mode, provided it is not marked as "special".

Example:

(defun example ()
  (cond ((eq major-mode 'foo-mode) (foo))
        ((eq major-mode 'bar-mode) (bar))))

Answer 2

Fran's answer is perfectly acceptable but it may be worth bringing up a point made by Bozhidar Batsov, on Emacs Redux:

[...] You’re doing an exact match for a particular mode, but major modes can be inherited by other modes. Consider clojure-mode - it’s the parent of modes like clojurescript-mode and clojurec-mode, and it inherits from prog-mode.

Enter derived-mode-p:

;; Assuming the current major mode is clojurescript-mode

(derived-mode-p 'clojurescript-mode)
;; => t

(derived-mode-p 'clojure-mode)
;; => t

(derived-mode-p 'prog-mode)
;; => t

From https://www.gnu.org/software/emacs/manual/html_node/elisp/Derived-Modes.html:

Function: derived-mode-p &rest modes

This function returns non-nil if the current major mode is derived from any of the major modes given by the symbols modes.