Quickstart

This page gives a quick introduction into Flycheck and an overview of its most important features. Before you start here please make sure that Flycheck is installed.

Enable Flycheck

Now add the following code to your init file to permanently enable syntax checking with Flycheck:

(add-hook 'after-init-hook #'global-flycheck-mode)

A modern setup

For an experience closer to a modern IDE, turn on inline diagnostics and let Flycheck surface your language server’s diagnostics too:

;; Check syntax everywhere
(add-hook 'after-init-hook #'global-flycheck-mode)

;; Show diagnostics inline, next to the code (in the spirit of Error Lens)
(add-hook 'after-init-hook #'global-flycheck-annotate-mode)

;; Report Eglot's LSP diagnostics through Flycheck
(global-flycheck-eglot-mode 1)

If you don’t use Eglot but want diagnostics straight from a linter’s own LSP server (such as RuboCop, Ruff, Biome or Harper), enable global-flycheck-lsp-mode instead. See Check buffers for the full story on both LSP integrations.

Install syntax checker programs

Now you need to install syntax checking programs for the languages you’d like to use Flycheck with. The list of supported languages tells you which languages Flycheck supports and what programs it uses.

For instance, you can install Pylint for Python and ESLint for Javascript:

$ pip install pylint
$ npm install eslint

Check syntax in a buffer

Now you are ready to use Flycheck in a Python or Javascript buffer. Visit a Python or Javascript file and check whether your Flycheck setup is complete with C-c ! v.

If everything is green, Flycheck will now start to check the buffer on the fly while you are editing. Whenever you make a mistake that eslint or Pylint can catch, Flycheck will highlight the corresponding place in the buffer with an error underline whose color reflects the severity of the issue. Additionally, Flycheck will put a symbol into the fringe for affected lines and show the total number of errors and warnings in the buffer in the mode line.

Navigate and list errors

With C-c ! n and C-c ! p you can now jump back and forth between erroneous places. If you keep on such a place for a little while Flycheck will show the corresponding error message in the echo area. Likewise, if you hover such a place with the mouse cursor Flycheck will show the error message in a tooltip.

Press C-c ! l to pop up a list of all errors in the current buffer. This list automatically updates itself when you fix errors or introduce new ones, and follows the currently selected buffer. If the error list is selected you can type n and p to move up and down between errors and jump to their corresponding location in the buffer.

More features

All Flycheck commands are available in the Emacs Menu at Tools -‣ Syntax checking:

The menu of Flycheck, showing all available Flycheck commands

The same menu also pops up when you click on the mode line lighter:

The mode line menu of Flycheck

What else Flycheck can do

The basics above are only the start. A few highlights worth knowing about:

• Inline error messages. Flycheck can show each message right next to the code it refers to, in the spirit of VS Code’s Error Lens - turn on flycheck-annotate-mode (see See errors in buffers).

• Fixes. Some checkers report a machine-applicable fix; apply the one at point with C-c ! f, or every fix in the buffer with C-c ! F (see Interact with errors).

• Related locations. When an error points at other places (an earlier definition, a borrow), jump to them with C-c ! j (see Interact with errors).

• Language servers. Flycheck can report a language server’s diagnostics - either through Eglot (flycheck-eglot-mode) or by talking to a diagnostics linter directly, with no full LSP client (flycheck-lsp-mode). See Check buffers.