improve: folder listing perf and sorting

Added explicit sorting since order is undefined on Windows (nodejs/node#3232)

server/lib/getFolders.js

@@ -1,35 +1,10 @@
-/* eslint-disable promise/no-nesting */
 const path = require('path')
 const fs = require('fs')
 const { promisify } = require('util')
 const readdir = promisify(fs.readdir)
-const stat = promisify(fs.stat)
 
-// based on http://stackoverflow.com/a/38314404/2533525
-const getFolders = function (dir, filterFn) {
-  // default filter function accepts all folders
-  filterFn = filterFn || function () { return true }
-
-  return readdir(dir).then(list => {
-    return Promise.all(list.map(file => {
-      file = path.resolve(dir, file)
-      return stat(file).then(stat => {
-        if (stat.isDirectory()) {
-          return filterFn(file) ? file : ''
-        }
-      }).catch(err => {
-        // stat failed for this folder
-        console.error(err.message)
-      })
-    })).then(results => {
-      return results.filter(f => {
-        return !!f
-      })
-    })
-  }).then(results => {
-    // flatten the array of arrays
-    return Array.prototype.concat.apply([], results)
-  })
-}
+const getFolders = dir => readdir(dir, { withFileTypes: true })
+  .then(list => Promise.all(list.map(ent => ent.isDirectory() ? path.resolve(dir, ent.name) : null)))
+  .then(list => list.filter(f => !!f).sort())
 
 module.exports = getFolders