autocmd filetype nnoremap is not working

Question

I'm trying to create a mapping based on the file type. (as explained in this chapter https://learnvimscriptthehardway.stevelosh.com/chapters/12.htm)

:autocmd FileType javascript nnoremap <buffer> <localleader>c I//<esc>

To test this mapping, I open a new buffer (through :vsplit) containing a javascript file but the mapping does not work at all and I don't understand why.

When I hit , and c simultaneously I expect vim to add // in the left of my line.

The relevant settings:

let mapleader=,
set ft? --> filetype=javascript

Any ideas?

UPDATE

This is what I get when I hit :nmap

n  ,c          *@I//<Esc>
n  %             <Plug>(MatchitNormalForward)
n  (             [
n  )             ]
n  ,ts         * :PHPUnitSwitchFile<CR>
n  ,tf         * :PHPUnitRunCurrentFile<CR>
n  ,ta         * :PHPUnitRunAll<CR>
n  ,da         * :call PhpDocAll()<CR>
n  ,cog        * :call PhpCreateGetters()<CR>
n  ,sg         * :call PhpCreateSettersAndGetters()<CR>
n  ,du         * :call PhpDetectUnusedUseStatements()<CR>
n  ,np         * :call PhpCreateProperty()<CR>
n  ,ep         * :call PhpExtractClassProperty()<CR>
n  ,rm         * :call PhpRenameMethod()<CR>
n  ,eu         * :call PhpExtractUse()<CR>
n  ,rcv        * :call PhpRenameClassVariable()<CR>
n  ,rlv        * :call PhpRenameLocalVariable()<CR>
n  ,r            <Plug>(FerretAcks)
n  ,l            <Plug>(FerretLack)
n  ,sv         * :source $MYVIMRC<CR>
n  ,ev         * :vsplit $MYVIMRC<CR>G
n  ,bs           :CtrlPMRU<CR>
n  ,bm           :CtrlPMixed<CR>
n  ,bb           :CtrlPBuffer<CR>
n  ,p            :CtrlP<CR>
n  ,A          * :exec "Rg ".expand("<cword>")<CR>
n  ,a          * :Rg<Space>
n  ,f            :NERDTreeFind<CR>
n  ,t            :NERDTreeToggle<CR>
n  ,s          * :%s/\<<C-R><C-W>\>/
n  <p          & :<C-U>call <SNR>52_putline(v:count1 . ']p', 'Below')<CR><']
n  <P          & :<C-U>call <SNR>52_putline(v:count1 . '[p', 'Above')<CR><']
n  <leader-o>  * :CtrlPTag<CR>
n  =p          & :<C-U>call <SNR>52_putline(v:count1 . ']p', 'Below')<CR>=']
n  =P          & :<C-U>call <SNR>52_putline(v:count1 . '[p', 'Above')<CR>=']
n  =op         * <Nop>
n  =o            <SNR>52_legacy_option_map(nr2char(getchar()))
n  >p          & :<C-U>call <SNR>52_putline(v:count1 . ']p', 'Below')<CR>>']
n  >P          & :<C-U>call <SNR>52_putline(v:count1 . '[p', 'Above')<CR>>']
n  [C            9999[c
n  [c            <Plug>(signify-prev-hunk)
n  [xx           <Plug>unimpaired_line_xml_encode
n  [x            <Plug>unimpaired_xml_encode
n  [uu           <Plug>unimpaired_line_url_encode
n  [u            <Plug>unimpaired_url_encode
n  [yy           <Plug>unimpaired_line_string_encode
n  [y            <Plug>unimpaired_string_encode
n  [P            <Plug>unimpairedPutAbove
-- Plus --

Answer 1

The problem you're having happens because you have two mappings that conflict with each other, in that you have a second longer mapping that starts with ,c, so when you type ,c Vim doesn't know right away whether you want to invoke the commenting mapping (which is ,c) or whether you are about to type the additional keys to invoke the longer mapping.

The image you posted with the output to :nmap shows the two mappings (although not next to each other):

n   ,c            *@I//<Esc>
n   ,cog          * call PhpCreateGetters()<CR>

In that case, and assuming that 'timeout' is set (which is the default), then Vim will wait up to 'timeoutlen' (defaults to one second) before it decides to execute the shorter mapping. See :help 'timeout' for more detail on these two settings.

To solve this issue, you can either disable or reconfigure the plug-in that is creating the longer mapping with the shared prefix (looks like a PHP language plug-in.) Reconfiguring that plug-in to use a different leader or prefix or a different mapping altogether for that particular feature would solve the problem.

Or you can pick a different key sequence for your comment mapping. For example, many plug-ins implementing commenting lines and blocks, such as vim-commentary or NERDCommenter will use gcc (with no leader) to comment a single line, and other mappings starting with gc to comment other kinds of blocks. Perhaps picking the same key sequence would make sense here? (Or just adopt one of the plug-ins, if you're more interested in the commenting feature than in learning Vimscript.)