:filetype on 启用文件类型识别后，VIM会尝试设置文件的filetype选项，这将会触发FileType 事件，然后能够正确启用语法高亮等。识别文件文件是一般是通过文件扩展名（当前也根据文件内容）autocmd BufNewFile,BufRead *.tsx setf typescriptreact ，:filetype on 应该是启用了是否发送FileType 事件。

:filetype on 启用后，VIM默认使用$VIMRUNTIME/filetype.vim 来检测文件类型

根据filetype加载的目录

• ftpplugin filetype plugin on
• indent filetype indent on
• syntax syntax on

多类型H2

When a dot appears in the value then this separates two filetype names. Example: / vim: set filetype=c.doxygen : / ~ This will use the “c” filetype first, then the “doxygen” filetype.

例如ALE插件

jsx
function! ale#linter#Get(original_filetypes) abort
    let l:possibly_duplicated_linters = []

    " Handle dot-separated filetypes.
    for l:original_filetype in split(a:original_filetypes, '\.')
        let l:filetype = ale#linter#ResolveFiletype(l:original_filetype)
        let l:linter_names = s:GetLinterNames(l:original_filetype)
        let l:all_linters = ale#linter#GetAll(l:filetype)
        let l:filetype_linters = []

        if type(l:linter_names) is v:t_string && l:linter_names is# 'all'
            let l:filetype_linters = l:all_linters
        elseif type(l:linter_names) is v:t_list
            " Select only the linters we or the user has specified.
            for l:linter in l:all_linters
                let l:name_list = [l:linter.name] + l:linter.aliases

                for l:name in l:name_list
                    if index(l:linter_names, l:name) >= 0
                        call add(l:filetype_linters, l:linter)
                        break
                    endif
                endfor
            endfor
        endif