Add formatter for js files

autoload/airline/extensions/tabline/formatters/jsformatter.vim

@@ -0,0 +1,10 @@
+function! airline#extensions#tabline#formatters#jsformatter#format(bufnr, buffers)
+  let buf = bufname(a:bufnr)
+  let filename = fnamemodify(buf, ':t')
+
+  if filename == 'index.js' || filename == 'index.jsx'
+    return fnamemodify(buf, ':p:h:t') . '/i'
+  else
+    return airline#extensions#tabline#formatters#unique_tail_improved#format(a:bufnr, a:buffers)
+  endif
+endfunction