How can I extend syntax highlighting with my own keywords?

Question

I already have syntax highlighting for javascript, but what I'd like to do is extend it with some additional rules.

What I've been trying to start, is using highlight "Prototype" with a cyan background.

I tried following some other answers, but they completely replace the syntax highlighting, but I want to simply add on to the current syntax highlighting.

Ideally this would be separated into their appropriate files, ie. highlighting rules in the syntax folder or whatever is appropriate.

---

This is what I currently have and it doesn't highlight Prototype on my attack.js file:

~/.vim/syntax/js.vim:

syn match sophiePrototype "Prototype"
highlight sophiePrototype ctermfg=cyan guifg=#00ffff

Answer 1

1. The correct filetype for JavaScript is javascript, so your file, which corresponds to a non-existing js filetpe, will never be sourced automatically. It should be:

   ~/.vim/syntax/javascript.vim
   

2. Vim sources syntax scripts from three standard location in a specific order:

   ~/.vim/syntax/javascript.vim
   $VIMRUNTIME/syntax/javascript.vim
   ~/.vim/after/syntax/javascript.vim
   

   Because you want your syntax highlighting to extend the existing one, you should use the last location:

   ~/.vim/after/syntax/javascript.vim
   

3. Given your use case, :help :syn-keyword seems more appropriate than :help :syn-match.

   syn keyword sophiePrototype Prototype
   

4. It is considered best practice to separate the definition of the visual attributes of syntax highlighting from the definition of the highlight groups themselves.

   If your goal is to have Prototype marked as some kind of "keyword", then you should probably use the default Keyword highlight group, which is likely to already be handled by your colorscheme:

   syn keyword Keyword Prototype
   

   If your goal is to give your keywords their own highlight group while still having them look like an existing highlight group, you can use a link:

   syn keyword sophiePrototype Prototype
   highlight link sophiePrototype SomeOtherHighlightGroup
   

   If your goal is really to use a new highlight group that doesn't look like anything else for Prototype, then you should move that :highlight command to a proper colorscheme or, barring that, to your vimrc, as per :help colorscheme-override:

   augroup my_colorschemes
       autocmd!
       autocmd Colorscheme * highlight sophiePrototype ctermfg=cyan guifg=#00ffff
   augroup END
   

Answer 2

Your solution is already pretty close. However, there are three minor issues with it:

1. The filetype for JavaScript is called "javascript", not "js". That's what your file is supposed to be called.
2. To change the background, you would use the *bg options, i.e. ctermbg and guibg.
3. To extend default syntax highlighting rather than replacing it, put a file in your "after" directory as explained in :help after-directory. Vim will load regular JavaScript syntax first.

The following lines in ~/.vim/after/syntax/javascript.vim will highlight "Prototype" with a cyan background:

syn match sophiePrototype "Prototype"
highlight sophiePrototype ctermbg=cyan guibg=#00ffff