How do I define Vimscript functions that load automatically on demand without slowing down startup?

Explanation

Vim's autoload mechanism lets you place functions in autoload/*.vim files using a filename#funcname naming convention. The file is loaded only when a function from it is first called, keeping startup fast no matter how much helper code you have.

How it works

Create a file at ~/.vim/autoload/myutils.vim (or ~/.config/nvim/autoload/myutils.vim for Neovim):

function! myutils#TrimWhitespace() abort
  let l:saved = @/
  %s/\s\+$//e
  let @/ = l:saved
endfunction

The prefix before # must exactly match the filename (without .vim). Call it from anywhere:

:call myutils#TrimWhitespace()

Vim automatically finds and sources autoload/myutils.vim on the first call.

Example

Instead of defining a large helper function at the top of your vimrc (loaded every time), move it to autoload/:

~/.vim/
  autoload/
    myutils.vim      ← loaded only when myutils#* is first called
  vimrc              ← just has mappings and settings

Tips

• Function names must follow the pattern {filename}#FuncName exactly — mismatches cause E117
• Nested paths work: autoload/my/lib.vim uses prefix my#lib#FuncName
• This is how almost all Vim plugins expose their public API
• Combine with noremap <leader>t :call myutils#TrimWhitespace()<CR> to call autoloaded functions from mappings without loading them at startup