Update code to ES6

* var  ->  let / const
* [].forEach  ->  for (... of ...)
* '...' + var  ->  template strings  `...${var}`
* function  ->  arrow functions `=>`
* use === and !==  instead of == and !=

src/serve_font.js

@@ -1,66 +1,61 @@
 'use strict';
 
-var clone = require('clone'),
-    express = require('express'),
-    fs = require('fs'),
-    path = require('path');
+const clone = require('clone');
+const express = require('express');
+const fs = require('fs');
+const path = require('path');
 
-var utils = require('./utils');
+const utils = require('./utils');
 
-module.exports = function(options, allowedFonts) {
-  var app = express().disable('x-powered-by');
+module.exports = (options, allowedFonts) => {
+  const app = express().disable('x-powered-by');
 
-  var lastModified = new Date().toUTCString();
+  const lastModified = new Date().toUTCString();
 
-  var fontPath = options.paths.fonts;
+  const fontPath = options.paths.fonts;
 
-  var existingFonts = {};
-  var fontListingPromise = new Promise(function(resolve, reject) {
-    fs.readdir(options.paths.fonts, function(err, files) {
+  const existingFonts = {};
+  const fontListingPromise = new Promise((resolve, reject) => {
+    fs.readdir(options.paths.fonts, (err, files) => {
       if (err) {
         reject(err);
         return;
       }
-      files.forEach(function(file) {
-        fs.stat(path.join(fontPath, file), function(err, stats) {
+      for (const file of files) {
+        fs.stat(path.join(fontPath, file), (err, stats) => {
           if (err) {
             reject(err);
             return;
           }
           if (stats.isDirectory() &&
-              fs.existsSync(path.join(fontPath, file, '0-255.pbf'))) {
+            fs.existsSync(path.join(fontPath, file, '0-255.pbf'))) {
             existingFonts[path.basename(file)] = true;
           }
         });
-      });
+      }
       resolve();
     });
   });
 
-  app.get('/fonts/:fontstack/:range([\\d]+-[\\d]+).pbf',
-      function(req, res, next) {
-    var fontstack = decodeURI(req.params.fontstack);
-    var range = req.params.range;
+  app.get('/fonts/:fontstack/:range([\\d]+-[\\d]+).pbf', (req, res, next) => {
+    const fontstack = decodeURI(req.params.fontstack);
+    const range = req.params.range;
 
     utils.getFontsPbf(options.serveAllFonts ? null : allowedFonts,
-      fontPath, fontstack, range, existingFonts).then(function(concated) {
+      fontPath, fontstack, range, existingFonts).then(concated => {
         res.header('Content-type', 'application/x-protobuf');
         res.header('Last-Modified', lastModified);
         return res.send(concated);
-      }, function(err) {
-        return res.status(400).send(err);
-      }
+      }, err => res.status(400).send(err)
     );
   });
 
-  app.get('/fonts.json', function(req, res, next) {
+  app.get('/fonts.json', (req, res, next) => {
     res.header('Content-type', 'application/json');
     return res.send(
       Object.keys(options.serveAllFonts ? existingFonts : allowedFonts).sort()
     );
   });
 
-  return fontListingPromise.then(function() {
-    return app;
-  });
+  return fontListingPromise.then(() => app);
 };