fix undo mapping commands

- only append if there's already a command
- use exe to unmap mappings

Author: Konfekt

ftplugin/svelte.vim

@@ -1,7 +1,12 @@
 if exists("b:did_ftplugin") | finish | endif
 let b:did_ftplugin = 1
 
-let b:undo_ftplugin = get(b:, 'undo_ftplugin', '')
+" Indent correctly with template string for vim-javascript/builtin
+" indentexpr
+let b:syng_str = '^\%(.*template\)\@!.*string\|special'
+let b:syng_strcom = '^\%(.*template\)\@!.*string\|comment\|regex\|special\|doc'
+
+let b:undo_ftplugin = (exists('b:undo_ftplugin') ? b:undo_ftplugin . " | " : "") . "unlet b:syng_str b:syng_strcom"
 
 if !has('nvim')
   setlocal matchpairs+=<:>
@@ -16,24 +21,19 @@ if exists("loaded_matchit")
         \ '<\@<=\([^/][^ \t>]*\)[^>]*\%(>\|$\):<\@<=/\1>,' .
         \ '{#\(if\|each\)[^}]*}:{\:else[^}]*}:{\/\(if\|each\)},' .
         \ '{#await[^}]*}:{\:then[^}]*}:{\/await},'
-  let b:undo_ftplugin .= "| unlet b:match_ignorecase b:match_words"
+  let b:undo_ftplugin .= " | unlet b:match_ignorecase b:match_words"
 endif
 
-" Indent correctly with template string for vim-javascript/builtin
-" indentexpr
-let b:syng_str = '^\%(.*template\)\@!.*string\|special'
-let b:syng_strcom = '^\%(.*template\)\@!.*string\|comment\|regex\|special\|doc'
-let b:undo_ftplugin .= "| unlet b:syng_str b:syng_strcom"
-
 if executable('npx') && !empty(globpath(&runtimepath, 'compiler/svelte-check.vim'))
   compiler svelte-check
-  let b:undo_ftplugin .= "| compiler make"
+  let b:undo_ftplugin .= " | compiler make"
 endif
 if exists(':Open') == 2
   " let &keywordprg = ':Open https://devdocs.io/\#q='..&filetype
   nnoremap <buffer> <expr> <F1> '<cmd>Open https://devdocs.io/\#q='..&filetype..' '..expand('<cword>')..'<CR>'
+  let b:undo_ftplugin .= " | exe 'nunmap <buffer> <F1>'"
   if exists('*getregion')
     vnoremap <buffer> <expr> <F1> '<cmd>Open https://devdocs.io/\#q='..&filetype..' '..join(getregion(getpos('v'), getpos('.')))..'<CR>'
+    let b:undo_ftplugin .= " | exe 'vunmap <buffer> <F1>'"
   endif
-  let b:undo_ftplugin .= "| nunmap <buffer> <F1> | vnunmap <buffer> <F1>"
 endif