Use the automated build tool gulp to write ES6 directly after configuration

function! s:add_title()
  if &filetype == 'javascript'
    call setline(1, 'use strict')
    call append(line('.'), '')
  endif
endfunction

autocmd BufNewFile *.js execute "call s:add_title()"

This is convenient for adding similar functions to other types of files in the future. See Vim to automatically add file headers when creating new files

Or just for this usage scenario:

function! s:add_title_js()
    call setline(1,'use strict')
    call append(line('.'), '')
endfunction

autocmd FileType javascript execute "call s:add_title_js()"

What editor are you using?