Sometimes Vim needs to reach outside of itself to use an existing program. For example, wiring up auto-formatting of JavaScript code requires Vim to call out to the prettier binary.

We want our .vimrc files and plugins to, generally, be as portable as possible. What happens if you haven't yet installed a particular program? Vim will likely experience a runtime exception. One way to get around this is to check for the presence of that program on the path. If it isn't there, don't do the thing. We can use the executable() function for this.

if executable('prettier')
  ...
endif

It will return 1 (true) if prettier is an executable on the path, otherwise it will return 0 (false).

See :help executable() for more details.