Very useful for stuff like whitespace-mode or auto-fill-mode:

(hook 'c-mode-hook (lambda () (xxxxxxxx-mode -1)))
(add-hook 'js-mode-hook (lambda () (xxxxxxxx-mode -1)))

This page tells you how to setup emacs's whitespace-mode (in emacs 23 or later), and how to use it.

whitespace-mode renders {spaces, tabs, newlines} characters with a visible glyph. This feature is useful for working with “tab separated values” (TSV) that's commonly used format for importing/exporting address books or spreadsheets. It's also useful in whitespace-significant languages such as Python.

To use it, call:

• whitespace-mode → toggle on/off, for current file.
• global-whitespace-mode → toggle on/off globally for current emacs session.

There is also whitespace-newline-mode and global-whitespace-newline-mode. They only show newline chars.