add Polka server example (#3588)

* add Polka server example

* Update package.json

* Fix linting

examples/custom-server-polka/package.json

@@ -0,0 +1,15 @@
+{
+  "name": "custom-server-polka",
+  "version": "1.0.0",
+  "scripts": {
+    "dev": "node server.js",
+    "build": "next build",
+    "start": "NODE_ENV=production node server.js"
+  },
+  "dependencies": {
+    "next": "latest",
+    "polka": "0.2.3",
+    "react": "16.2.0",
+    "react-dom": "16.2.0"
+  }
+}

examples/custom-server-polka/pages/a.js

@@ -0,0 +1,3 @@
+import React from 'react'
+
+export default () => <div>a</div>

examples/custom-server-polka/pages/b.js

@@ -0,0 +1,3 @@
+import React from 'react'
+
+export default () => <div>b</div>

examples/custom-server-polka/pages/index.js

@@ -0,0 +1,17 @@
+import React from 'react'
+import Link from 'next/link'
+
+export default () => (
+  <ul>
+    <li>
+      <Link href='/b' as='/a'>
+        <a>a</a>
+      </Link>
+    </li>
+    <li>
+      <Link href='/a' as='/b'>
+        <a>b</a>
+      </Link>
+    </li>
+  </ul>
+)

examples/custom-server-polka/server.js

@@ -0,0 +1,21 @@
+const polka = require('polka')
+const next = require('next')
+
+const port = parseInt(process.env.PORT, 10) || 3000
+const dev = process.env.NODE_ENV !== 'production'
+const app = next({ dev })
+const handle = app.getRequestHandler()
+
+app.prepare().then(() => {
+  const server = polka()
+
+  server.get('/a', (req, res) => app.render(req, res, '/b', req.query))
+
+  server.get('/b', (req, res) => app.render(req, res, '/a', req.query))
+
+  server.get('*', (req, res) => handle(req, res))
+
+  server
+    .listen(port)
+    .then(() => console.log(`> Ready on http://localhost:${port}`))
+})