The Imenu facility offers a way to find the major definitions in a file by name. It is also useful in text formatter major modes, where it treats each chapter, section, etc., as a definition. (See Xref, for a more powerful feature that handles multiple files together.)
If you type M-x imenu, it reads the name of a definition using the minibuffer, then moves point to that definition. You can use completion to specify the name; the command always displays the whole list of valid names.
Alternatively, you can bind the command
imenu to a mouse
click. Then it displays mouse menus for you to select a definition
name. You can also add the buffer's index to the menu bar by calling
imenu-add-menubar-index. If you want to have this menu bar
item available for all buffers in a certain major mode, you can do
this by adding
imenu-add-menubar-index to its mode hook. But
if you have done that, you will have to wait a little while each time
you visit a file in that mode, while Emacs finds all the definitions
in that buffer.
When you change the contents of a buffer, if you add or delete
definitions, you can update the buffer's index based on the
new contents by invoking the ‘*Rescan*’ item in the menu.
Rescanning happens automatically if you set
nil value. There is no need to rescan because of small
changes in the text.
imenu-auto-rescan will be disabled in buffers that are larger
imenu-auto-rescan-maxout in bytes.
You can customize the way the menus are sorted by setting the
imenu-sort-function. By default, names are ordered as
they occur in the buffer; if you want alphabetic sorting, use the
imenu--sort-by-name as the value. You can also
define your own comparison function by writing Lisp code.
Imenu provides the information to guide Which Function mode (see Which Function). The Speedbar can also use it (see Speedbar).