diff --git a/jest.config.js b/jest.config.js
index cdafc241..7a8f4c72 100644
--- a/jest.config.js
+++ b/jest.config.js
@@ -1,9 +1,9 @@
 module.exports = {
   moduleNameMapper: {
-    "\\.css$": "<rootDir>/modules/__mocks__/styleMock.js"
+    'entry-manifest': '<rootDir>/modules/__mocks__/entryManifest.js',
+    '\\.png$': '<rootDir>/modules/__mocks__/imageMock.js',
+    '\\.css$': '<rootDir>/modules/__mocks__/styleMock.js'
   },
-  setupTestFrameworkScriptFile:
-    "<rootDir>/modules/__tests__/setupTestFramework.js",
-  testMatch: ["**/__tests__/*-test.js"],
-  testURL: "http://localhost/"
+  testMatch: ['**/__tests__/*-test.js'],
+  testURL: 'http://localhost/'
 };
diff --git a/modules/.babelrc b/modules/.babelrc
new file mode 100644
index 00000000..4df14a99
--- /dev/null
+++ b/modules/.babelrc
@@ -0,0 +1,3 @@
+{
+  "presets": [["@babel/preset-env", { "loose": true, "targets": "node 8" }]]
+}
diff --git a/modules/__mocks__/entryManifest.js b/modules/__mocks__/entryManifest.js
new file mode 100644
index 00000000..d6d1738d
--- /dev/null
+++ b/modules/__mocks__/entryManifest.js
@@ -0,0 +1 @@
+export default [];
diff --git a/modules/__mocks__/imageMock.js b/modules/__mocks__/imageMock.js
new file mode 100644
index 00000000..08d725cd
--- /dev/null
+++ b/modules/__mocks__/imageMock.js
@@ -0,0 +1 @@
+export default '';
diff --git a/modules/__tests__/invalidPackageNames-test.js b/modules/__tests__/invalidPackageNames-test.js
new file mode 100644
index 00000000..3e8a8bbf
--- /dev/null
+++ b/modules/__tests__/invalidPackageNames-test.js
@@ -0,0 +1,19 @@
+import request from 'supertest';
+
+import createServer from '../createServer';
+
+describe('Invalid package names', () => {
+  let server;
+  beforeEach(() => {
+    server = createServer();
+  });
+
+  it('are rejected', done => {
+    request(server)
+      .get('/_invalid/index.js')
+      .end((err, res) => {
+        expect(res.statusCode).toBe(403);
+        done();
+      });
+  });
+});
diff --git a/modules/__tests__/invalidQueryParams-test.js b/modules/__tests__/invalidQueryParams-test.js
new file mode 100644
index 00000000..a454792f
--- /dev/null
+++ b/modules/__tests__/invalidQueryParams-test.js
@@ -0,0 +1,20 @@
+import request from 'supertest';
+
+import createServer from '../createServer';
+
+describe('Invalid query params', () => {
+  let server;
+  beforeEach(() => {
+    server = createServer();
+  });
+
+  it('redirect to the same path w/out those params', done => {
+    request(server)
+      .get('/d3?module&invalid-param')
+      .end((err, res) => {
+        expect(res.statusCode).toBe(302);
+        expect(res.headers.location).toBe('/d3?module');
+        done();
+      });
+  });
+});
diff --git a/modules/__tests__/legacyURLs-test.js b/modules/__tests__/legacyURLs-test.js
new file mode 100644
index 00000000..90658fd5
--- /dev/null
+++ b/modules/__tests__/legacyURLs-test.js
@@ -0,0 +1,30 @@
+import request from 'supertest';
+
+import createServer from '../createServer';
+
+describe('Legacy URLs', () => {
+  let server;
+  beforeEach(() => {
+    server = createServer();
+  });
+
+  it('redirect /_meta to ?meta', done => {
+    request(server)
+      .get('/_meta/react')
+      .end((err, res) => {
+        expect(res.statusCode).toBe(301);
+        expect(res.headers.location).toBe('/react?meta');
+        done();
+      });
+  });
+
+  it('redirect ?json to ?meta', done => {
+    request(server)
+      .get('/react?json')
+      .end((err, res) => {
+        expect(res.statusCode).toBe(301);
+        expect(res.headers.location).toBe('/react?meta');
+        done();
+      });
+  });
+});
diff --git a/modules/__tests__/server-test.js b/modules/__tests__/server-test.js
deleted file mode 100644
index 0b4a8520..00000000
--- a/modules/__tests__/server-test.js
+++ /dev/null
@@ -1,49 +0,0 @@
-import request from 'supertest';
-
-import createServer from '../createServer';
-
-describe('The server', () => {
-  let server;
-  beforeEach(() => {
-    server = createServer();
-  });
-
-  it('redirects /_meta to ?meta', done => {
-    request(server)
-      .get('/_meta/react')
-      .end((err, res) => {
-        expect(res.statusCode).toBe(301);
-        expect(res.headers.location).toBe('/react?meta');
-        done();
-      });
-  });
-
-  it('redirects ?json to ?meta', done => {
-    request(server)
-      .get('/react?json')
-      .end((err, res) => {
-        expect(res.statusCode).toBe(301);
-        expect(res.headers.location).toBe('/react?meta');
-        done();
-      });
-  });
-
-  it('redirects invalid query params', done => {
-    request(server)
-      .get('/react?main=index&invalid')
-      .end((err, res) => {
-        expect(res.statusCode).toBe(302);
-        expect(res.headers.location).toBe('/react?main=index');
-        done();
-      });
-  });
-
-  it('rejects invalid package names', done => {
-    request(server)
-      .get('/_invalid/index.js')
-      .end((err, res) => {
-        expect(res.statusCode).toBe(403);
-        done();
-      });
-  });
-});
diff --git a/modules/actions/serveAutoIndexPage.js b/modules/actions/serveAutoIndexPage.js
index 83cf8736..1686d582 100644
--- a/modules/actions/serveAutoIndexPage.js
+++ b/modules/actions/serveAutoIndexPage.js
@@ -1,14 +1,14 @@
 import { renderToString, renderToStaticMarkup } from 'react-dom/server';
 import semver from 'semver';
 
-import AutoIndexApp from '../client/autoIndex/App';
+import AutoIndexApp from '../client/autoIndex/App.js';
 
-import createElement from './utils/createElement';
-import createHTML from './utils/createHTML';
-import createScript from './utils/createScript';
-import getEntryPoint from './utils/getEntryPoint';
-import getGlobalScripts from './utils/getGlobalScripts';
-import MainTemplate from './utils/MainTemplate';
+import MainTemplate from './utils/MainTemplate.js';
+import createElement from './utils/createElement.js';
+import createHTML from './utils/createHTML.js';
+import createScript from './utils/createScript.js';
+import getEntryPoint from './utils/getEntryPoint.js';
+import getGlobalScripts from './utils/getGlobalScripts.js';
 
 const doctype = '<!DOCTYPE html>';
 const globalURLs =
diff --git a/modules/actions/serveFile.js b/modules/actions/serveFile.js
index 838a793e..3d0c52c4 100644
--- a/modules/actions/serveFile.js
+++ b/modules/actions/serveFile.js
@@ -1,7 +1,7 @@
-import serveAutoIndexPage from './serveAutoIndexPage';
-import serveMetadata from './serveMetadata';
-import serveModule from './serveModule';
-import serveStaticFile from './serveStaticFile';
+import serveAutoIndexPage from './serveAutoIndexPage.js';
+import serveMetadata from './serveMetadata.js';
+import serveModule from './serveModule.js';
+import serveStaticFile from './serveStaticFile.js';
 
 /**
  * Send the file, JSON metadata, or HTML directory listing.
diff --git a/modules/actions/serveHTMLModule.js b/modules/actions/serveHTMLModule.js
index 0b13d3d2..f6d1054b 100644
--- a/modules/actions/serveHTMLModule.js
+++ b/modules/actions/serveHTMLModule.js
@@ -1,8 +1,8 @@
 import etag from 'etag';
 import cheerio from 'cheerio';
 
-import getContentTypeHeader from '../utils/getContentTypeHeader';
-import rewriteBareModuleIdentifiers from '../utils/rewriteBareModuleIdentifiers';
+import getContentTypeHeader from '../utils/getContentTypeHeader.js';
+import rewriteBareModuleIdentifiers from '../utils/rewriteBareModuleIdentifiers.js';
 
 export default function serveHTMLModule(req, res) {
   try {
@@ -40,9 +40,7 @@ export default function serveHTMLModule(req, res) {
       .status(500)
       .type('text')
       .send(
-        `Cannot generate module for ${req.packageSpec}${
-          req.filename
-        }\n\n${debugInfo}`
+        `Cannot generate module for ${req.packageSpec}${req.filename}\n\n${debugInfo}`
       );
   }
 }
diff --git a/modules/actions/serveJavaScriptModule.js b/modules/actions/serveJavaScriptModule.js
index 5b7621ab..97c35517 100644
--- a/modules/actions/serveJavaScriptModule.js
+++ b/modules/actions/serveJavaScriptModule.js
@@ -1,7 +1,7 @@
 import etag from 'etag';
 
-import getContentTypeHeader from '../utils/getContentTypeHeader';
-import rewriteBareModuleIdentifiers from '../utils/rewriteBareModuleIdentifiers';
+import getContentTypeHeader from '../utils/getContentTypeHeader.js';
+import rewriteBareModuleIdentifiers from '../utils/rewriteBareModuleIdentifiers.js';
 
 export default function serveJavaScriptModule(req, res) {
   try {
@@ -34,9 +34,7 @@ export default function serveJavaScriptModule(req, res) {
       .status(500)
       .type('text')
       .send(
-        `Cannot generate module for ${req.packageSpec}${
-          req.filename
-        }\n\n${debugInfo}`
+        `Cannot generate module for ${req.packageSpec}${req.filename}\n\n${debugInfo}`
       );
   }
 }
diff --git a/modules/actions/serveMainPage.js b/modules/actions/serveMainPage.js
index 6e3bfc49..85aadaa0 100644
--- a/modules/actions/serveMainPage.js
+++ b/modules/actions/serveMainPage.js
@@ -1,13 +1,13 @@
 import { renderToString, renderToStaticMarkup } from 'react-dom/server';
 
-import MainApp from '../client/main/App';
+import MainApp from '../client/main/App.js';
 
-import createElement from './utils/createElement';
-import createHTML from './utils/createHTML';
-import createScript from './utils/createScript';
-import getEntryPoint from './utils/getEntryPoint';
-import getGlobalScripts from './utils/getGlobalScripts';
-import MainTemplate from './utils/MainTemplate';
+import MainTemplate from './utils/MainTemplate.js';
+import createElement from './utils/createElement.js';
+import createHTML from './utils/createHTML.js';
+import createScript from './utils/createScript.js';
+import getEntryPoint from './utils/getEntryPoint.js';
+import getGlobalScripts from './utils/getGlobalScripts.js';
 
 const doctype = '<!DOCTYPE html>';
 const globalURLs =
diff --git a/modules/actions/serveMetadata.js b/modules/actions/serveMetadata.js
index 9ed3f8d7..10436712 100644
--- a/modules/actions/serveMetadata.js
+++ b/modules/actions/serveMetadata.js
@@ -1,6 +1,6 @@
 import path from 'path';
 
-import addLeadingSlash from '../utils/addLeadingSlash';
+import addLeadingSlash from '../utils/addLeadingSlash.js';
 
 function getMatchingEntries(entry, entries) {
   const dirname = entry.name || '.';
diff --git a/modules/actions/serveModule.js b/modules/actions/serveModule.js
index fb408a70..10c9039a 100644
--- a/modules/actions/serveModule.js
+++ b/modules/actions/serveModule.js
@@ -1,5 +1,5 @@
-import serveHTMLModule from './serveHTMLModule';
-import serveJavaScriptModule from './serveJavaScriptModule';
+import serveHTMLModule from './serveHTMLModule.js';
+import serveJavaScriptModule from './serveJavaScriptModule.js';
 
 export default function serveModule(req, res) {
   if (req.entry.contentType === 'application/javascript') {
diff --git a/modules/actions/serveStaticFile.js b/modules/actions/serveStaticFile.js
index 5d6b1d6b..a7ddcdc0 100644
--- a/modules/actions/serveStaticFile.js
+++ b/modules/actions/serveStaticFile.js
@@ -1,7 +1,7 @@
 import path from 'path';
 import etag from 'etag';
 
-import getContentTypeHeader from '../utils/getContentTypeHeader';
+import getContentTypeHeader from '../utils/getContentTypeHeader.js';
 
 export default function serveStaticFile(req, res) {
   const tags = ['file'];
diff --git a/modules/actions/utils/MainTemplate.js b/modules/actions/utils/MainTemplate.js
index 663c767e..7df4d628 100644
--- a/modules/actions/utils/MainTemplate.js
+++ b/modules/actions/utils/MainTemplate.js
@@ -1,8 +1,8 @@
 import PropTypes from 'prop-types';
 
-import e from './createElement';
-import h from './createHTML';
-import x from './createScript';
+import e from './createElement.js';
+import h from './createHTML.js';
+import x from './createScript.js';
 
 const promiseShim =
   'window.Promise || document.write(\'\\x3Cscript src="/es6-promise@4.2.5/dist/es6-promise.min.js">\\x3C/script>\\x3Cscript>ES6Promise.polyfill()\\x3C/script>\')';
@@ -11,12 +11,12 @@ const fetchShim =
   'window.fetch || document.write(\'\\x3Cscript src="/whatwg-fetch@3.0.0/dist/fetch.umd.js">\\x3C/script>\')';
 
 export default function MainTemplate({
-  title,
-  description,
-  favicon,
+  title = 'UNPKG',
+  description = 'The CDN for everything on npm',
+  favicon = '/favicon.ico',
   data,
-  content,
-  elements
+  content = h(''),
+  elements = []
 }) {
   return e(
     'html',
@@ -47,14 +47,6 @@ export default function MainTemplate({
   );
 }
 
-MainTemplate.defaultProps = {
-  title: 'UNPKG',
-  description: 'The CDN for everything on npm',
-  favicon: '/favicon.ico',
-  content: h(''),
-  elements: []
-};
-
 if (process.env.NODE_ENV !== 'production') {
   const htmlType = PropTypes.shape({
     __html: PropTypes.string
diff --git a/modules/actions/utils/cloudflare.js b/modules/actions/utils/cloudflare.js
index 1854de32..c3ff2c28 100644
--- a/modules/actions/utils/cloudflare.js
+++ b/modules/actions/utils/cloudflare.js
@@ -1,16 +1,18 @@
 import fetch from 'isomorphic-fetch';
-import invariant from 'invariant';
 
 const cloudflareURL = 'https://api.cloudflare.com/client/v4';
 const cloudflareEmail = process.env.CLOUDFLARE_EMAIL;
 const cloudflareKey = process.env.CLOUDFLARE_KEY;
 
-invariant(
-  cloudflareEmail,
-  'Missing the $CLOUDFLARE_EMAIL environment variable'
-);
+if (process.env.NODE_ENV !== 'production' && process.env.NODE_ENV !== 'test') {
+  if (!cloudflareEmail) {
+    throw new Error('Missing the $CLOUDFLARE_EMAIL environment variable');
+  }
 
-invariant(cloudflareKey, 'Missing the $CLOUDFLARE_KEY environment variable');
+  if (!cloudflareKey) {
+    throw new Error('Missing the $CLOUDFLARE_KEY environment variable');
+  }
+}
 
 function get(path, headers) {
   return fetch(`${cloudflareURL}${path}`, {
diff --git a/modules/actions/utils/createScript.js b/modules/actions/utils/createScript.js
index 6bd3ab9d..67bd1c77 100644
--- a/modules/actions/utils/createScript.js
+++ b/modules/actions/utils/createScript.js
@@ -1,5 +1,5 @@
-import createElement from './createElement';
-import createHTML from './createHTML';
+import createElement from './createElement.js';
+import createHTML from './createHTML.js';
 
 export default function createScript(script) {
   return createElement('script', {
diff --git a/modules/actions/utils/getEntryPoint.js b/modules/actions/utils/getEntryPoint.js
index c4502844..ed5fe3f1 100644
--- a/modules/actions/utils/getEntryPoint.js
+++ b/modules/actions/utils/getEntryPoint.js
@@ -1,4 +1,5 @@
 // Virtual module id; see rollup.config.js
+// eslint-disable-next-line import/no-unresolved
 import entryManifest from 'entry-manifest';
 
 export default function getEntryPoint(name, format) {
diff --git a/modules/actions/utils/getGlobalScripts.js b/modules/actions/utils/getGlobalScripts.js
index cdb3fa0b..9bfe5402 100644
--- a/modules/actions/utils/getGlobalScripts.js
+++ b/modules/actions/utils/getGlobalScripts.js
@@ -1,10 +1,13 @@
-import invariant from 'invariant';
-
-import createElement from './createElement';
+import createElement from './createElement.js';
 
 export default function getGlobalScripts(entryPoint, globalURLs) {
   return entryPoint.globalImports.map(id => {
-    invariant(globalURLs[id], 'Missing global URL for id "%s"', id);
+    if (process.env.NODE_ENV !== 'production') {
+      if (!globalURLs[id]) {
+        throw new Error('Missing global URL for id "%s"', id);
+      }
+    }
+
     return createElement('script', { src: globalURLs[id] });
   });
 }
diff --git a/modules/actions/utils/getStats.js b/modules/actions/utils/getStats.js
index 8fc82c56..6505f0e2 100644
--- a/modules/actions/utils/getStats.js
+++ b/modules/actions/utils/getStats.js
@@ -1,4 +1,4 @@
-import * as cloudflare from './cloudflare.js';
+import { getZones, getZoneAnalyticsDashboard } from './cloudflare.js';
 
 function extractPublicInfo(data) {
   return {
@@ -28,12 +28,8 @@ function extractPublicInfo(data) {
 const DomainNames = ['unpkg.com', 'npmcdn.com'];
 
 export default async function getStats(since, until) {
-  const zones = await cloudflare.getZones(DomainNames);
-  const dashboard = await cloudflare.getZoneAnalyticsDashboard(
-    zones,
-    since,
-    until
-  );
+  const zones = await getZones(DomainNames);
+  const dashboard = await getZoneAnalyticsDashboard(zones, since, until);
 
   return {
     timeseries: dashboard.timeseries.map(extractPublicInfo),
diff --git a/modules/client/.babelrc b/modules/client/.babelrc
index 4c18fb3e..6b6125ce 100644
--- a/modules/client/.babelrc
+++ b/modules/client/.babelrc
@@ -1,9 +1,7 @@
 {
   "presets": [
-    ["@babel/preset-env", { "loose": true }],
+    ["@babel/preset-env", { "loose": true, "targets": "> 0.25%, not dead" }],
     "@babel/preset-react"
   ],
-  "plugins": [
-    ["@babel/plugin-proposal-class-properties", { "loose": true }]
-  ]
+  "plugins": [["@babel/plugin-proposal-class-properties", { "loose": true }]]
 }
diff --git a/modules/client/.eslintrc b/modules/client/.eslintrc
index d17408da..b1e8fbe0 100644
--- a/modules/client/.eslintrc
+++ b/modules/client/.eslintrc
@@ -1,4 +1,7 @@
 {
+  "env": {
+    "browser": true
+  },
   "plugins": [
     "react"
   ],
@@ -10,8 +13,5 @@
     "react": {
       "version": "16"
     }
-  },
-  "env": {
-    "browser": true
   }
 }
diff --git a/modules/createServer.js b/modules/createServer.js
new file mode 100644
index 00000000..4080e89f
--- /dev/null
+++ b/modules/createServer.js
@@ -0,0 +1,49 @@
+import express from 'express';
+
+import serveFile from './actions/serveFile.js';
+import serveMainPage from './actions/serveMainPage.js';
+import serveStats from './actions/serveStats.js';
+
+import cors from './middleware/cors.js';
+import fetchPackage from './middleware/fetchPackage.js';
+import findFile from './middleware/findFile.js';
+import logger from './middleware/logger.js';
+import redirectLegacyURLs from './middleware/redirectLegacyURLs.js';
+import staticFiles from './middleware/staticFiles.js';
+import validatePackageURL from './middleware/validatePackageURL.js';
+import validatePackageName from './middleware/validatePackageName.js';
+import validateQuery from './middleware/validateQuery.js';
+
+export default function createServer() {
+  const app = express();
+
+  app.disable('x-powered-by');
+  app.enable('trust proxy');
+
+  app.use(logger);
+  app.use(cors);
+  app.use(staticFiles);
+
+  // Special startup request from App Engine
+  // https://cloud.google.com/appengine/docs/standard/nodejs/how-instances-are-managed
+  app.get('/_ah/start', (req, res) => {
+    res.status(200).end();
+  });
+
+  app.get('/', serveMainPage);
+  app.get('/api/stats', serveStats);
+
+  app.use(redirectLegacyURLs);
+
+  app.get(
+    '*',
+    validatePackageURL,
+    validatePackageName,
+    validateQuery,
+    fetchPackage,
+    findFile,
+    serveFile
+  );
+
+  return app;
+}
diff --git a/modules/middleware/fetchPackage.js b/modules/middleware/fetchPackage.js
index a021f259..827607d0 100644
--- a/modules/middleware/fetchPackage.js
+++ b/modules/middleware/fetchPackage.js
@@ -1,9 +1,9 @@
 import semver from 'semver';
 
-import addLeadingSlash from '../utils/addLeadingSlash';
-import createPackageURL from '../utils/createPackageURL';
-import createSearch from '../utils/createSearch';
-import { getPackageInfo as getNpmPackageInfo } from '../utils/npm';
+import addLeadingSlash from '../utils/addLeadingSlash.js';
+import createPackageURL from '../utils/createPackageURL.js';
+import createSearch from '../utils/createSearch.js';
+import { getPackageInfo as getNpmPackageInfo } from '../utils/npm.js';
 
 function tagRedirect(req, res) {
   const version = req.packageInfo['dist-tags'][req.packageVersion];
@@ -114,41 +114,41 @@ function filenameRedirect(req, res) {
  * version if the request targets a tag or uses a semver version, or to the
  * exact filename if the request omits the filename.
  */
-export default function fetchPackage(req, res, next) {
-  getNpmPackageInfo(req.packageName).then(
-    packageInfo => {
-      if (packageInfo == null || packageInfo.versions == null) {
-        return res
-          .status(404)
-          .type('text')
-          .send(`Cannot find package "${req.packageName}"`);
-      }
-
-      req.packageInfo = packageInfo;
-      req.packageConfig = req.packageInfo.versions[req.packageVersion];
+export default async function fetchPackage(req, res, next) {
+  let packageInfo;
+  try {
+    packageInfo = await getNpmPackageInfo(req.packageName);
+  } catch (error) {
+    console.error(error);
 
-      if (!req.packageConfig) {
-        // Redirect to a fully-resolved version.
-        if (req.packageVersion in req.packageInfo['dist-tags']) {
-          return tagRedirect(req, res);
-        } else {
-          return semverRedirect(req, res);
-        }
-      }
+    return res
+      .status(500)
+      .type('text')
+      .send(`Cannot get info for package "${req.packageName}"`);
+  }
 
-      if (!req.filename) {
-        return filenameRedirect(req, res);
-      }
+  if (packageInfo == null || packageInfo.versions == null) {
+    return res
+      .status(404)
+      .type('text')
+      .send(`Cannot find package "${req.packageName}"`);
+  }
 
-      next();
-    },
-    error => {
-      console.error(error);
+  req.packageInfo = packageInfo;
+  req.packageConfig = req.packageInfo.versions[req.packageVersion];
 
-      return res
-        .status(500)
-        .type('text')
-        .send(`Cannot get info for package "${req.packageName}"`);
+  if (!req.packageConfig) {
+    // Redirect to a fully-resolved version.
+    if (req.packageVersion in req.packageInfo['dist-tags']) {
+      return tagRedirect(req, res);
+    } else {
+      return semverRedirect(req, res);
     }
-  );
+  }
+
+  if (!req.filename) {
+    return filenameRedirect(req, res);
+  }
+
+  next();
 }
diff --git a/modules/middleware/findFile.js b/modules/middleware/findFile.js
index 95871dda..b1e363f7 100644
--- a/modules/middleware/findFile.js
+++ b/modules/middleware/findFile.js
@@ -1,11 +1,11 @@
 import path from 'path';
 
-import addLeadingSlash from '../utils/addLeadingSlash';
-import createPackageURL from '../utils/createPackageURL';
-import createSearch from '../utils/createSearch';
-import { fetchPackage as fetchNpmPackage } from '../utils/npm';
-import getIntegrity from '../utils/getIntegrity';
-import getContentType from '../utils/getContentType';
+import addLeadingSlash from '../utils/addLeadingSlash.js';
+import createPackageURL from '../utils/createPackageURL.js';
+import createSearch from '../utils/createSearch.js';
+import { fetchPackage as fetchNpmPackage } from '../utils/npm.js';
+import getIntegrity from '../utils/getIntegrity.js';
+import getContentType from '../utils/getContentType.js';
 
 function indexRedirect(req, res, entry) {
   // Redirect to the index file so relative imports
@@ -143,62 +143,59 @@ const trailingSlash = /\/$/;
  * Fetch and search the archive to try and find the requested file.
  * Redirect to the "index" file if a directory was requested.
  */
-export default function findFile(req, res, next) {
-  fetchNpmPackage(req.packageConfig).then(tarballStream => {
-    const wantsIndex = trailingSlash.test(req.filename);
-
-    // The name of the file/directory we're looking for.
-    const entryName = req.filename
-      .replace(multipleSlash, '/')
-      .replace(trailingSlash, '')
-      .replace(leadingSlash, '');
-
-    searchEntries(tarballStream, entryName, wantsIndex).then(
-      ({ entries, foundEntry }) => {
-        if (!foundEntry) {
-          return res
-            .status(404)
-            .set({
-              'Cache-Control': 'public, max-age=31536000', // 1 year
-              'Cache-Tag': 'missing, missing-entry'
-            })
-            .type('text')
-            .send(`Cannot find "${req.filename}" in ${req.packageSpec}`);
-        }
+export default async function findFile(req, res, next) {
+  const wantsIndex = trailingSlash.test(req.filename);
+
+  // The name of the file/directory we're looking for.
+  const entryName = req.filename
+    .replace(multipleSlash, '/')
+    .replace(trailingSlash, '')
+    .replace(leadingSlash, '');
+
+  const tarballStream = await fetchNpmPackage(req.packageConfig);
+  const { entries, foundEntry } = await searchEntries(
+    tarballStream,
+    entryName,
+    wantsIndex
+  );
+
+  if (!foundEntry) {
+    return res
+      .status(404)
+      .set({
+        'Cache-Control': 'public, max-age=31536000', // 1 year
+        'Cache-Tag': 'missing, missing-entry'
+      })
+      .type('text')
+      .send(`Cannot find "${req.filename}" in ${req.packageSpec}`);
+  }
+
+  // If the foundEntry is a directory and there is no trailing slash
+  // on the request path, we need to redirect to some "index" file
+  // inside that directory. This is so our URLs work in a similar way
+  // to require("lib") in node where it searches for `lib/index.js`
+  // and `lib/index.json` when `lib` is a directory.
+  if (foundEntry.type === 'directory' && !wantsIndex) {
+    const indexEntry =
+      entries[path.join(entryName, 'index.js')] ||
+      entries[path.join(entryName, 'index.json')];
+
+    if (indexEntry && indexEntry.type === 'file') {
+      return indexRedirect(req, res, indexEntry);
+    }
 
-        // If the foundEntry is a directory and there is no trailing slash
-        // on the request path, we need to redirect to some "index" file
-        // inside that directory. This is so our URLs work in a similar way
-        // to require("lib") in node where it searches for `lib/index.js`
-        // and `lib/index.json` when `lib` is a directory.
-        if (foundEntry.type === 'directory' && !wantsIndex) {
-          const indexEntry =
-            entries[path.join(entryName, 'index.js')] ||
-            entries[path.join(entryName, 'index.json')];
-
-          if (indexEntry && indexEntry.type === 'file') {
-            return indexRedirect(req, res, indexEntry);
-          } else {
-            return res
-              .status(404)
-              .set({
-                'Cache-Control': 'public, max-age=31536000', // 1 year
-                'Cache-Tag': 'missing, missing-index'
-              })
-              .type('text')
-              .send(
-                `Cannot find an index in "${req.filename}" in ${
-                  req.packageSpec
-                }`
-              );
-          }
-        }
+    return res
+      .status(404)
+      .set({
+        'Cache-Control': 'public, max-age=31536000', // 1 year
+        'Cache-Tag': 'missing, missing-index'
+      })
+      .type('text')
+      .send(`Cannot find an index in "${req.filename}" in ${req.packageSpec}`);
+  }
 
-        req.entries = entries;
-        req.entry = foundEntry;
+  req.entries = entries;
+  req.entry = foundEntry;
 
-        next();
-      }
-    );
-  });
+  next();
 }
diff --git a/modules/middleware/redirectLegacyURLs.js b/modules/middleware/redirectLegacyURLs.js
index 03f46f9c..afe2d0e0 100644
--- a/modules/middleware/redirectLegacyURLs.js
+++ b/modules/middleware/redirectLegacyURLs.js
@@ -1,15 +1,20 @@
+import createSearch from '../utils/createSearch.js';
+
 /**
  * Redirect old URLs that we no longer support.
  */
 export default function redirectLegacyURLs(req, res, next) {
-  // Permanently redirect /_meta/path to /_metadata/path
+  // Permanently redirect /_meta/path to /path?meta
   if (req.path.match(/^\/_meta\//)) {
-    return res.redirect(301, '/_metadata' + req.path.substr(6));
+    req.query.meta = '';
+    return res.redirect(301, req.path.substr(6) + createSearch(req.query));
   }
 
   // Permanently redirect /path?json => /path?meta
   if (req.query.json != null) {
-    return res.redirect(301, '/_metadata' + req.path);
+    delete req.query.json;
+    req.query.meta = '';
+    return res.redirect(301, req.path + createSearch(req.query));
   }
 
   next();
diff --git a/modules/middleware/validatePackageURL.js b/modules/middleware/validatePackageURL.js
index 878e9d6e..153adf75 100644
--- a/modules/middleware/validatePackageURL.js
+++ b/modules/middleware/validatePackageURL.js
@@ -1,4 +1,4 @@
-import parsePackageURL from '../utils/parsePackageURL';
+import parsePackageURL from '../utils/parsePackageURL.js';
 
 /**
  * Parse the URL and add various properties to the request object to
diff --git a/modules/middleware/validateQuery.js b/modules/middleware/validateQuery.js
index 9b3fade5..a5789441 100644
--- a/modules/middleware/validateQuery.js
+++ b/modules/middleware/validateQuery.js
@@ -1,4 +1,4 @@
-import createSearch from '../utils/createSearch';
+import createSearch from '../utils/createSearch.js';
 
 const knownQueryParams = {
   main: true, // Deprecated, see #63
diff --git a/modules/plugins/__tests__/unpkgRewrite-test.js b/modules/plugins/__tests__/unpkgRewrite-test.js
index f5146e48..d827be0f 100644
--- a/modules/plugins/__tests__/unpkgRewrite-test.js
+++ b/modules/plugins/__tests__/unpkgRewrite-test.js
@@ -1,6 +1,6 @@
-import babel from 'babel-core';
+import * as babel from '@babel/core';
 
-import unpkgRewrite from '../unpkgRewrite';
+import unpkgRewrite from '../unpkgRewrite.js';
 
 const testCases = [
   {
@@ -66,6 +66,7 @@ const testCases = [
   }
 ];
 
+const origin = 'https://unpkg.com';
 const dependencies = {
   react: '15.6.1',
   '@angular/router': '4.3.5',
@@ -75,9 +76,9 @@ const dependencies = {
 
 describe('Rewriting imports/exports', () => {
   testCases.forEach(testCase => {
-    it(`successfully rewrites '${testCase.before}'`, () => {
+    it(`rewrites '${testCase.before}' => '${testCase.after}'`, () => {
       const result = babel.transform(testCase.before, {
-        plugins: [unpkgRewrite(dependencies)]
+        plugins: [unpkgRewrite(origin, dependencies)]
       });
 
       expect(result.code).toEqual(testCase.after);
diff --git a/modules/server.js b/modules/server.js
index a0bf2e1f..48253ad4 100644
--- a/modules/server.js
+++ b/modules/server.js
@@ -1,59 +1,8 @@
-import express from 'express';
-
-import serveFile from './actions/serveFile';
-import serveMainPage from './actions/serveMainPage';
-import serveStats from './actions/serveStats';
-
-import cors from './middleware/cors';
-import fetchPackage from './middleware/fetchPackage';
-import findFile from './middleware/findFile';
-import logger from './middleware/logger';
-import redirectLegacyURLs from './middleware/redirectLegacyURLs';
-import staticFiles from './middleware/staticFiles';
-import validatePackageURL from './middleware/validatePackageURL';
-import validatePackageName from './middleware/validatePackageName';
-import validateQuery from './middleware/validateQuery';
-
-import createRouter from './utils/createRouter';
+import createServer from './createServer.js';
 
+const server = createServer();
 const port = process.env.PORT || '8080';
 
-const app = express();
-
-app.disable('x-powered-by');
-app.enable('trust proxy');
-
-app.use(logger);
-app.use(cors);
-app.use(staticFiles);
-
-// Special startup request from App Engine
-// https://cloud.google.com/appengine/docs/standard/nodejs/how-instances-are-managed
-app.get('/_ah/start', (req, res) => {
-  res.status(200).end();
-});
-
-app.get('/', serveMainPage);
-
-app.use(redirectLegacyURLs);
-
-app.use(
-  '/api',
-  createRouter(app => {
-    app.get('/stats', serveStats);
-  })
-);
-
-app.get(
-  '*',
-  validatePackageURL,
-  validatePackageName,
-  validateQuery,
-  fetchPackage,
-  findFile,
-  serveFile
-);
-
-app.listen(port, () => {
+server.listen(port, () => {
   console.log('Server listening on port %s, Ctrl+C to quit', port);
 });
diff --git a/modules/utils/__tests__/createSearch-test.js b/modules/utils/__tests__/createSearch-test.js
index c55050cb..0a887a0f 100644
--- a/modules/utils/__tests__/createSearch-test.js
+++ b/modules/utils/__tests__/createSearch-test.js
@@ -1,4 +1,4 @@
-import createSearch from '../createSearch';
+import createSearch from '../createSearch.js';
 
 describe('createSearch', () => {
   it('omits the trailing = for empty string values', () => {
diff --git a/modules/utils/__tests__/getContentType-test.js b/modules/utils/__tests__/getContentType-test.js
index ee5f4f25..e878aa66 100644
--- a/modules/utils/__tests__/getContentType-test.js
+++ b/modules/utils/__tests__/getContentType-test.js
@@ -1,4 +1,4 @@
-import getContentType from '../getContentType';
+import getContentType from '../getContentType.js';
 
 it('gets a content type of text/plain for LICENSE|README|CHANGES|AUTHORS|Makefile', () => {
   expect(getContentType('AUTHORS')).toBe('text/plain');
diff --git a/modules/utils/__tests__/parsePackageURL-test.js b/modules/utils/__tests__/parsePackageURL-test.js
index b89eef27..3ead80ad 100644
--- a/modules/utils/__tests__/parsePackageURL-test.js
+++ b/modules/utils/__tests__/parsePackageURL-test.js
@@ -1,4 +1,4 @@
-import parsePackageURL from '../parsePackageURL';
+import parsePackageURL from '../parsePackageURL.js';
 
 describe('parsePackageURL', () => {
   it('parses plain packages', () => {
diff --git a/modules/utils/bufferStream.js b/modules/utils/bufferStream.js
index 421d715e..34531f68 100644
--- a/modules/utils/bufferStream.js
+++ b/modules/utils/bufferStream.js
@@ -1,10 +1,10 @@
 export default function bufferStream(stream) {
-  return new Promise((resolve, reject) => {
+  return new Promise((accept, reject) => {
     const chunks = [];
 
     stream
       .on('error', reject)
       .on('data', chunk => chunks.push(chunk))
-      .on('end', () => resolve(Buffer.concat(chunks)));
+      .on('end', () => accept(Buffer.concat(chunks)));
   });
 }
diff --git a/modules/utils/createRouter.js b/modules/utils/createRouter.js
deleted file mode 100644
index 898c9e77..00000000
--- a/modules/utils/createRouter.js
+++ /dev/null
@@ -1,7 +0,0 @@
-import express from 'express';
-
-export default function createRouter(configureRouter) {
-  const router = express.Router();
-  configureRouter(router);
-  return router;
-}
diff --git a/modules/utils/createSearch.js b/modules/utils/createSearch.js
index 44aa19bb..bfa89918 100644
--- a/modules/utils/createSearch.js
+++ b/modules/utils/createSearch.js
@@ -3,9 +3,7 @@ export default function createSearch(query) {
   const params = keys.reduce(
     (memo, key) =>
       memo.concat(
-        query[key] === ''
-          ? key // Omit the trailing "=" from key=
-          : `${key}=${encodeURIComponent(query[key])}`
+        query[key] ? `${key}=${encodeURIComponent(query[key])}` : key
       ),
     []
   );
diff --git a/modules/utils/npm.js b/modules/utils/npm.js
index 9d59bc04..9d58f2f4 100644
--- a/modules/utils/npm.js
+++ b/modules/utils/npm.js
@@ -4,8 +4,8 @@ import gunzip from 'gunzip-maybe';
 import tar from 'tar-stream';
 import LRUCache from 'lru-cache';
 
-import debug from './debug';
-import bufferStream from './bufferStream';
+import debug from './debug.js';
+import bufferStream from './bufferStream.js';
 
 const npmRegistryURL =
   process.env.NPM_REGISTRY_URL || 'https://registry.npmjs.org';
diff --git a/modules/utils/rewriteBareModuleIdentifiers.js b/modules/utils/rewriteBareModuleIdentifiers.js
index d540747b..be9fbcc2 100644
--- a/modules/utils/rewriteBareModuleIdentifiers.js
+++ b/modules/utils/rewriteBareModuleIdentifiers.js
@@ -1,6 +1,6 @@
 import babel from '@babel/core';
 
-import unpkgRewrite from '../plugins/unpkgRewrite';
+import unpkgRewrite from '../plugins/unpkgRewrite.js';
 
 const origin = process.env.ORIGIN || 'https://unpkg.com';
 
diff --git a/package.json b/package.json
index 637f320a..74f42a90 100644
--- a/package.json
+++ b/package.json
@@ -23,7 +23,6 @@
     "etag": "^1.8.1",
     "express": "^4.16.4",
     "gunzip-maybe": "^1.4.1",
-    "invariant": "^2.2.4",
     "isomorphic-fetch": "^2.2.1",
     "lru-cache": "^5.1.1",
     "mime": "^2.4.0",
diff --git a/yarn.lock b/yarn.lock
index 63c98779..81c3326a 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -3142,7 +3142,7 @@ inquirer@^3.0.6:
     strip-ansi "^4.0.0"
     through "^2.3.6"
 
-invariant@^2.2.0, invariant@^2.2.2, invariant@^2.2.4:
+invariant@^2.2.0, invariant@^2.2.2:
   version "2.2.4"
   resolved "https://registry.yarnpkg.com/invariant/-/invariant-2.2.4.tgz#610f3c92c9359ce1db616e538008d23ff35158e6"
   integrity sha512-phJfQVBuaJM5raOpJjSfkiD6BpbCE4Ns//LaXl6wGYtUBY83nWS6Rf9tXm2e8VaK60JEjYldbPif/A2B1C2gNA==