diff --git a/server/middleware/fetchFile.js b/server/middleware/fetchFile.js
index e3ad572..e72a744 100644
--- a/server/middleware/fetchFile.js
+++ b/server/middleware/fetchFile.js
@@ -1,6 +1,7 @@
 const fs = require("fs");
 const path = require("path");
 const semver = require("semver");
+
 const createPackageURL = require("../utils/createPackageURL");
 const createSearch = require("./utils/createSearch");
 const getPackageInfo = require("./utils/getPackageInfo");
@@ -69,11 +70,12 @@ function fetchFile(req, res, next) {
         .send(`Cannot get info for package "${req.packageName}"`);
     }
 
-    if (packageInfo == null || packageInfo.versions == null)
+    if (packageInfo == null || packageInfo.versions == null) {
       return res
         .status(404)
         .type("text")
         .send(`Cannot find package "${req.packageName}"`);
+    }
 
     req.packageInfo = packageInfo;
 
@@ -84,6 +86,7 @@ function fetchFile(req, res, next) {
       getPackage(req.packageConfig, (error, outputDir) => {
         if (error) {
           console.error(error);
+
           res
             .status(500)
             .type("text")
@@ -95,9 +98,9 @@ function fetchFile(req, res, next) {
           let useIndex = true;
 
           if (req.query.module != null) {
-            // They want an ES module. Try "module", "jsnext:main", and "/"
-            // https://github.com/rollup/rollup/wiki/pkg.module
+            // They want an ES module.
             if (!filename) {
+              // See https://github.com/rollup/rollup/wiki/pkg.module
               filename =
                 req.packageConfig.module ||
                 req.packageConfig["jsnext:main"] ||
@@ -105,15 +108,18 @@ function fetchFile(req, res, next) {
             }
           } else if (filename) {
             // They are requesting an explicit filename. Only try to find an
-            // index file if they are NOT requesting an HTML directory listing.
-            useIndex = filename[filename.length - 1] !== "/";
+            // index.js if they are NOT requesting an HTML directory listing.
+            useIndex = filename.charAt(filename.length - 1) !== "/";
           } else if (
             req.query.main &&
             typeof req.packageConfig[req.query.main] === "string"
           ) {
             // They specified a custom ?main field.
+            // Deprecated, see https://github.com/unpkg/unpkg/issues/63
             filename = req.packageConfig[req.query.main];
 
+            // Count which packages are using this so we can warn them when we
+            // remove this functionality.
             incrementCounter(
               "package-json-custom-main",
               req.packageSpec + "?main=" + req.query.main,
@@ -121,15 +127,15 @@ function fetchFile(req, res, next) {
             );
           } else if (typeof req.packageConfig.unpkg === "string") {
             // The "unpkg" field allows packages to explicitly declare the
-            // file to serve at the bare URL (see #59).
+            // file to serve at the bare URL.
             filename = req.packageConfig.unpkg;
           } else if (typeof req.packageConfig.browser === "string") {
             // Fall back to the "browser" field if declared (only support strings).
+            // Deprecated, see https://github.com/unpkg/unpkg/issues/63
             filename = req.packageConfig.browser;
 
             // Count which packages + versions are actually using this fallback
             // so we can warn them when we deprecate this functionality.
-            // See https://github.com/unpkg/unpkg/issues/63
             incrementCounter(
               "package-json-browser-fallback",
               req.packageSpec,
@@ -166,6 +172,7 @@ function fetchFile(req, res, next) {
                 // Need to redirect to the module file so relative imports resolve
                 // correctly. Cache module redirects for 1 minute.
                 delete req.query.main;
+
                 res
                   .set({
                     "Cache-Control": "public, max-age=60",
diff --git a/server/middleware/utils/createMutex.js b/server/middleware/utils/createMutex.js
index 106276a..9be3687 100644
--- a/server/middleware/utils/createMutex.js
+++ b/server/middleware/utils/createMutex.js
@@ -1,21 +1,18 @@
 function createMutex(doWork) {
   const mutex = {};
 
-  return function(key, payload, callback) {
+  return (key, payload, callback) => {
     if (mutex[key]) {
       mutex[key].push(callback);
     } else {
-      mutex[key] = [
-        function() {
-          delete mutex[key];
-        },
-        callback
-      ];
+      mutex[key] = [callback];
 
-      doWork(payload, function(error, value) {
+      doWork(payload, (error, value) => {
         mutex[key].forEach(callback => {
           callback(error, value);
         });
+
+        delete mutex[key];
       });
     }
   };
diff --git a/server/middleware/utils/getPackageInfo.js b/server/middleware/utils/getPackageInfo.js
index 7d2e177..a595568 100644
--- a/server/middleware/utils/getPackageInfo.js
+++ b/server/middleware/utils/getPackageInfo.js
@@ -1,11 +1,12 @@
 require("isomorphic-fetch");
+
 const createCache = require("./createCache");
 const createMutex = require("./createMutex");
 
-const RegistryURL =
+const registryURL =
   process.env.NPM_REGISTRY_URL || "https://registry.npmjs.org";
 
-const PackageInfoCache = createCache("packageInfo");
+const packageInfoCache = createCache("packageInfo");
 
 function fetchPackageInfo(packageName) {
   console.log(`info: Fetching package info for ${packageName}`);
@@ -17,7 +18,7 @@ function fetchPackageInfo(packageName) {
     encodedPackageName = encodeURIComponent(packageName);
   }
 
-  const url = `${RegistryURL}/${encodedPackageName}`;
+  const url = `${registryURL}/${encodedPackageName}`;
 
   return fetch(url, {
     headers: {
@@ -34,25 +35,25 @@ const PackageNotFound = "PackageNotFound";
 // the registry for the same package info.
 const fetchMutex = createMutex((packageName, callback) => {
   fetchPackageInfo(packageName).then(
-    function(value) {
+    value => {
       if (value == null) {
         // Cache 404s for 5 minutes. This prevents us from making
         // unnecessary requests to the registry for bad package names.
         // In the worst case, a brand new package's info will be
         // available within 5 minutes.
-        PackageInfoCache.set(packageName, PackageNotFound, 300, function() {
+        packageInfoCache.set(packageName, PackageNotFound, 300, () => {
           callback(null, value);
         });
       } else {
         // Cache valid package info for 1 minute.
-        PackageInfoCache.set(packageName, value, 60, function() {
+        packageInfoCache.set(packageName, value, 60, () => {
           callback(null, value);
         });
       }
     },
-    function(error) {
+    error => {
       // Do not cache errors.
-      PackageInfoCache.del(packageName, function() {
+      packageInfoCache.del(packageName, () => {
         callback(error);
       });
     }
@@ -60,7 +61,7 @@ const fetchMutex = createMutex((packageName, callback) => {
 });
 
 function getPackageInfo(packageName, callback) {
-  PackageInfoCache.get(packageName, function(error, value) {
+  packageInfoCache.get(packageName, (error, value) => {
     if (error || value != null) {
       callback(error, value === PackageNotFound ? null : value);
     } else {
diff --git a/server/utils/parsePackageURL.js b/server/utils/parsePackageURL.js
index 6019786..4a47694 100644
--- a/server/utils/parsePackageURL.js
+++ b/server/utils/parsePackageURL.js
@@ -1,7 +1,8 @@
 const url = require("url");
+
 const isValidPackageName = require("./isValidPackageName");
 
-const URLFormat = /^\/((?:@[^/@]+\/)?[^/@]+)(?:@([^/]+))?(\/.*)?$/;
+const packageURLFormat = /^\/((?:@[^/@]+\/)?[^/@]+)(?:@([^/]+))?(\/.*)?$/;
 
 function decodeParam(param) {
   if (param) {
@@ -18,7 +19,7 @@ function decodeParam(param) {
 function parsePackageURL(originalURL) {
   const { pathname, search, query } = url.parse(originalURL, true);
 
-  const match = URLFormat.exec(pathname);
+  const match = packageURLFormat.exec(pathname);
 
   // Disallow invalid URL formats.
   if (match == null) {