Use the locally installed node modules for eslint and flow checking

syntastic_javascript_local_exec.vim

" return full path with the trailing slash or empty string
fun! s:GetNodeModulesAbsPath ()
  let lcd_saved = fnameescape(getcwd())
  silent! exec "lcd" expand('%:p:h')
  let path = finddir('node_modules', '.;')
  exec "lcd" lcd_saved

  " fnamemodify will return full path with trailing slash;
  " if no node_modules found, we're safe
  return path is '' ? '' : fnamemodify(path, ':p')
endfun

" return full path of local bin or empty string
fun! s:GetNodeModuleBin (node_modules, bin)
  let guess = a:node_modules is '' ? '' : a:node_modules . '.bin/' . a:bin
  return exepath(guess)
endfun

fun! s:main ()
  let node_modules = s:GetNodeModulesAbsPath()

  let eslint_exec = s:GetNodeModuleBin(node_modules, 'eslint')
  if a:eslint_exec isnot ''
    let b:syntastic_javascript_eslint_exec = a:eslint_exec
  endif

  let flowtype_exec = s:GetNodeModuleBin(node_modules, 'flow')
  if a:flowtype_exec isnot ''
    let b:syntastic_javascript_flowtype_exe = a:flowtype_exec
  endif
endfun

call s:main()