Add Fast Refresh Demo (vercel#16576)

Closes vercel#16538 

Basically reverts vercel#16497 and some minor changes. Also adds a link in the docs.

This reverts commit ec281df.

docs/basic-features/fast-refresh.md

@@ -6,6 +6,13 @@ description:
 
 # Fast Refresh
 
+<details open>
+  <summary><b>Examples</b></summary>
+  <ul>
+    <li><a href="https://github.com/vercel/next.js/tree/canary/examples/fast-refresh-demo">Fast Refresh Demo</a></li>
+  </ul>
+</details>
+
 Fast Refresh is a Next.js feature that gives you instantaneous feedback on
 edits made to your React components. Fast Refresh is enabled by default in all
 Next.js applications on **9.4 or newer**. With Next.js Fast Refresh enabled,

examples/fast-refresh-demo/.gitignore

@@ -0,0 +1,34 @@
+# See https://help.github.com/articles/ignoring-files/ for more about ignoring files.
+
+# dependencies
+/node_modules
+/.pnp
+.pnp.js
+
+# testing
+/coverage
+
+# next.js
+/.next/
+/out/
+
+# production
+/build
+
+# misc
+.DS_Store
+*.pem
+
+# debug
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# local env files
+.env.local
+.env.development.local
+.env.test.local
+.env.production.local
+
+# vercel
+.vercel

examples/fast-refresh-demo/README.md

@@ -0,0 +1,23 @@
+# Fast Refresh Demo
+
+Next.js ships with [Fast Refresh](https://nextjs.org/docs/basic-features/fast-refresh) which gives you instantaneous feedback on edits made to your React components.
+
+This demos shows how the state of an auto incrementing value and a classic counter is preserved after edits or if there are errors.
+
+## Deploy your own
+
+Deploy the example using [Vercel](https://vercel.com):
+
+[![Deploy with Vercel](https://vercel.com/button)](https://vercel.com/import/project?template=https://github.com/vercel/next.js/tree/canary/examples/fast-refresh-demo)
+
+## How to use
+
+Execute [`create-next-app`](https://github.com/vercel/next.js/tree/canary/packages/create-next-app) with [npm](https://docs.npmjs.com/cli/init) or [Yarn](https://yarnpkg.com/lang/en/docs/cli/create/) to bootstrap the example:
+
+```bash
+npx create-next-app --example fast-refresh-demo fast-refresh-demo
+# or
+yarn create next-app --example fast-refresh-demo fast-refresh-demo
+```
+
+Deploy it to the cloud with [Vercel](https://vercel.com/import?filter=next.js&utm_source=github&utm_medium=readme&utm_campaign=next-example) ([Documentation](https://nextjs.org/docs/deployment)).

examples/fast-refresh-demo/components/Button.js

@@ -0,0 +1,5 @@
+import styles from './Button.module.css'
+
+export default function Button(props) {
+  return <button type="button" className={styles.btn} {...props} />
+}

examples/fast-refresh-demo/components/Button.module.css

@@ -0,0 +1,32 @@
+button.btn {
+  margin: 0;
+  border: 1px solid #d1d1d1;
+  border-radius: 5px;
+  padding: 0.5em;
+  vertical-align: middle;
+  white-space: normal;
+  background: none;
+  line-height: 1;
+  font-size: 1rem;
+  font-family: inherit;
+  transition: all 0.2s ease;
+}
+
+button.btn {
+  padding: 0.65em 1em;
+  background: #0076ff;
+  color: #fff;
+  border: none;
+  cursor: pointer;
+  transition: all 0.2s ease;
+}
+button.btn:focus {
+  outline: 0;
+  border-color: #0076ff;
+}
+button.btn:hover {
+  background: rgba(0, 118, 255, 0.8);
+}
+button.btn:focus {
+  box-shadow: 0 0 0 2px rgba(0, 118, 255, 0.5);
+}

examples/fast-refresh-demo/components/ClickCount.js

@@ -0,0 +1,11 @@
+import { useCallback, useState } from 'react'
+import Button from './Button'
+
+export default function ClickCount() {
+  const [count, setCount] = useState(0)
+  const increment = useCallback(() => {
+    setCount((v) => v + 1)
+  }, [setCount])
+
+  return <Button onClick={increment}>Clicks: {count}</Button>
+}

examples/fast-refresh-demo/global.css

@@ -0,0 +1,14 @@
+body {
+  font-family: 'SF Pro Text', 'SF Pro Icons', 'Helvetica Neue', 'Helvetica',
+    'Arial', sans-serif;
+  margin: 0 auto;
+  font-size: 16px;
+  line-height: 1.65;
+  word-break: break-word;
+  font-kerning: auto;
+  font-variant: normal;
+  -webkit-font-smoothing: antialiased;
+  -moz-osx-font-smoothing: grayscale;
+  text-rendering: optimizeLegibility;
+  hyphens: auto;
+}

examples/fast-refresh-demo/package.json

@@ -0,0 +1,15 @@
+{
+  "name": "fast-refresh-demo",
+  "version": "1.0.0",
+  "scripts": {
+    "dev": "next",
+    "build": "next build",
+    "start": "next start"
+  },
+  "dependencies": {
+    "next": "latest",
+    "react": "^16.13.1",
+    "react-dom": "^16.13.1"
+  },
+  "license": "MIT"
+}

examples/fast-refresh-demo/pages/_app.js

@@ -0,0 +1,5 @@
+import '../global.css'
+
+export default function MyApp({ Component, pageProps }) {
+  return <Component {...pageProps} />
+}

examples/fast-refresh-demo/pages/index.js

@@ -0,0 +1,71 @@
+import { useCallback, useEffect, useState } from 'react'
+import Button from '../components/Button'
+import ClickCount from '../components/ClickCount'
+import styles from '../styles/home.module.css'
+
+function throwError() {
+  console.log(
+    // The function body() is not defined
+    document.body()
+  )
+}
+
+function Home() {
+  const [count, setCount] = useState(0)
+  const increment = useCallback(() => {
+    setCount((v) => v + 1)
+  }, [setCount])
+
+  useEffect(() => {
+    const r = setInterval(() => {
+      increment()
+    }, 1000)
+
+    return () => {
+      clearInterval(r)
+    }
+  }, [increment])
+
+  return (
+    <main className={styles.main}>
+      <h1>Fast Refresh Demo</h1>
+      <p>
+        Fast Refresh is a Next.js feature that gives you instantaneous feedback
+        on edits made to your React components, without ever losing component
+        state.
+      </p>
+      <hr className={styles.hr} />
+      <div>
+        <p>
+          Auto incrementing value. The counter won't reset after edits or if
+          there are errors.
+        </p>
+        <p>Current value: {count}</p>
+      </div>
+      <hr className={styles.hr} />
+      <div>
+        <p>Component with state.</p>
+        <ClickCount />
+      </div>
+      <hr className={styles.hr} />
+      <div>
+        <p>
+          The button below will throw 2 errors. You'll see the error overlay to
+          let you know about the errors but it won't break the page or reset
+          your state.
+        </p>
+        <Button
+          onClick={(e) => {
+            setTimeout(() => document.parentNode(), 0)
+            throwError()
+          }}
+        >
+          Throw an Error
+        </Button>
+      </div>
+      <hr className={styles.hr} />
+    </main>
+  )
+}
+
+export default Home

examples/fast-refresh-demo/styles/home.module.css

@@ -0,0 +1,11 @@
+.main {
+  padding: 20px 20px 60px;
+  max-width: 680px;
+  margin: 0 auto;
+}
+
+.hr {
+  border: none;
+  border-bottom: 1px solid #efefef;
+  margin: 3em auto;
+}