Add code from express-unpkg repo

server/middleware/PackageUtils.js

@@ -0,0 +1,59 @@
+const { parse: parseURL } = require('url')
+
+const URLFormat = /^\/((?:@[^\/@]+\/)?[^\/@]+)(?:@([^\/]+))?(\/.*)?$/
+
+const decodeParam = (param) =>
+  param && decodeURIComponent(param)
+
+const ValidQueryKeys = {
+  main: true,
+  json: true
+}
+
+const queryIsValid = (query) =>
+  Object.keys(query).every(key => ValidQueryKeys[key])
+
+const parsePackageURL = (url) => {
+  const { pathname, search, query } = parseURL(url, true)
+
+  if (!queryIsValid(query))
+    return null
+
+  const match = URLFormat.exec(pathname)
+
+  if (match == null)
+    return null
+
+  const packageName = match[1]
+  const version = decodeParam(match[2]) || 'latest'
+  const filename = decodeParam(match[3])
+
+  return {        // If the URL is /@scope/name@version/path.js?main=browser:
+    pathname,     // /@scope/name@version/path.js
+    search,       // ?main=browser
+    query,        // { main: 'browser' }
+    packageName,  // @scope/name
+    version,      // version
+    filename      // /path.js
+  }
+}
+
+const createPackageURL = (packageName, version, filename, search) => {
+  let pathname = `/${packageName}`
+
+  if (version != null)
+    pathname += `@${version}`
+
+  if (filename != null)
+    pathname += filename
+
+  if (search)
+    pathname += search
+
+  return pathname
+}
+
+module.exports = {
+  parsePackageURL,
+  createPackageURL
+}