Vim: center current window in screen

center.vim

let g:centerinscreen_active = 0

function! ToggleCenterInScreen(desired_width)
    if g:centerinscreen_active == 0
        let l:window_width = winwidth(winnr())
        let l:sidepanel_width = (l:window_width - a:desired_width) / 2

        exec("silent leftabove " . l:sidepanel_width . "vsplit new")
        wincmd l
        exec("silent rightbelow " . l:sidepanel_width . "vsplit new")
        wincmd h
        let g:centerinscreen_active = 1
    else
        wincmd h
        close
        wincmd l
        close
        
        let g:centerinscreen_active = 0
    endif
endfunction

nnoremap <Leader>r :exec ToggleCenterInScreen(100)<CR>

I suggest using the l: prefix for function-local variables because the a: prefix is intended for arguments passed to the function.

Done, thanks for the tip!