Support for SSG preview mode (#50)

* add support for preview mode of pre-rendered pages using getStaticProps

* remove README caveat

* Add Cypress tests: Optional catch-all route at root-level

Test that prerendered pages and static files from /public are correctly 
served under the redirect engine used with cookie-based redirects when 
having an optional catch-all route at root-level.

* Cleanup: Remove unnecessary require

We do not need `existsSync` in the `setupNetlifyFunctionForPage` helper.

* miscellaneous finishing touches for ssg preview mode

* move preview mode note in readme and update isroutewithfallback helper

Co-authored-by: Finn Woelm <[email protected]>

Author: lindsaylevine

.gitignore

@@ -39,3 +39,6 @@ jspm_packages
 
 # OS
 .DS_Store
+
+# Local Netlify folder
+.netlify

README.md

@@ -35,7 +35,6 @@
   - [Custom Netlify Redirects](#custom-netlify-redirects)
   - [Custom Netlify Functions](#custom-netlify-functions)
 - [Caveats](#caveats)
-  - [Preview Mode](#preview-mode)
   - [Fallbacks for Pages with `getStaticPaths`](#fallbacks-for-pages-with-getstaticpaths)
 - [Credits](#credits)
 - [Showcase](#showcase)
@@ -157,6 +156,12 @@ From now on, whenever you want to preview your application locally, just run:
 1. `npm run build`: This will run `next build` to build your Next.js app and `next-on-netlify` to prepare your Next.js app for compatibility with Netlify
 1. `netlify dev`: This will emulate Netlify on your computer and let you preview your app on `http://localhost:8888`.
 
+*Note:*
+
+Preview Mode is not yet available locally, running `netlify dev`, for static pages without revalidate or fallback. This will be supported soon.
+
+For now, Preview Mode *is* supported in production for all Next.js page types.
+
 #### Custom Netlify Redirects
 
 You can define custom redirects in a `_redirects` and/or in your `netlify.toml` file.
@@ -175,10 +180,6 @@ SSR pages and API endpoints. It is currently not possible to create custom Netli
 
 ## Caveats
 
-### Preview Mode
-
-[Next.js Preview Mode](https://nextjs.org/docs/advanced-features/preview-mode) does not work on pages that are pre-rendered (pages with `getStaticProps`). Netlify currently does not support cookie-based redirects, which are needed for supporting preview mode on pre-rendered pages. Preview mode works correctly on any server-side-rendered pages (pages with `getInitialProps` or `getServerSideProps`). See: [Issue #10](https://github.com/netlify/next-on-netlify/issues/10)
-
 ### Fallbacks for Pages with `getStaticPaths`
 
 [Fallback pages](https://nextjs.org/docs/basic-features/data-fetching#fallback-true) behave differently with `next-on-netlify` than they do with Next.js. On Next.js, when navigating to a path that is not defined in `getStaticPaths`, it first displays the fallback page. Next.js then generates the HTML in the background and caches it for future requests.

cypress/fixtures/pages-with-optionalCatchAll-at-root/[[...slug]].js

@@ -0,0 +1,9 @@
+const Page = () => <p>root-level optional-catch-all</p>;
+
+export async function getServerSideProps() {
+  return {
+    props: {},
+  };
+}
+
+export default Page;

cypress/fixtures/pages-with-optionalCatchAll-at-root/static.js

@@ -0,0 +1,9 @@
+const Static = () => <p>static page</p>;
+
+export async function getStaticProps() {
+  return {
+    props: {},
+  };
+}
+
+export default Static;

cypress/fixtures/pages-with-optionalCatchAll-at-root/subfolder/[id].js

@@ -0,0 +1,27 @@
+const Static = () => <p>static page in subfolder</p>;
+
+export async function getStaticPaths() {
+  return {
+    paths: [
+      {
+        params: {
+          id: "static",
+        },
+      },
+      {
+        params: {
+          id: "test",
+        },
+      },
+    ],
+    fallback: true,
+  };
+}
+
+export async function getStaticProps() {
+  return {
+    props: {},
+  };
+}
+
+export default Static;

cypress/fixtures/pages/api/enterPreviewStatic.js

@@ -0,0 +1,12 @@
+export default async function preview(req, res) {
+  const { query } = req;
+  const { id } = query;
+
+  // Enable Preview Mode by setting the cookies
+  res.setPreviewData({});
+
+  // Redirect to the path from the fetched post
+  // We don't redirect to req.query.slug as that might lead to open redirect vulnerabilities
+  res.writeHead(307, { Location: `/previewTest/static` });
+  res.end();
+}

cypress/fixtures/pages/index.js

@@ -230,6 +230,11 @@ const Index = ({ shows }) => (
           <a>previewTest/222</a>
         </Link>
       </li>
+      <li>
+        <Link href="/previewTest/static">
+          <a>previewTest/static</a>
+        </Link>
+      </li>
     </ul>
 
     <h1>6. Static Pages Stay Static</h1>

cypress/fixtures/pages/previewTest/static.js

@@ -0,0 +1,45 @@
+import Link from "next/link";
+
+const StaticTest = ({ number }) => {
+  return (
+    <div>
+      <p>
+        This page uses getStaticProps() and is SSRed when in preview mode.
+        <br />
+        <br />
+        By default, it shows the TV show by ID (as static HTML).
+        <br />
+        But when in preview mode, it shows person by ID instead (SSRed).
+      </p>
+
+      <hr />
+
+      <h1>Number: {number}</h1>
+
+      <Link href="/">
+        <a>Go back home</a>
+      </Link>
+    </div>
+  );
+};
+
+export const getStaticProps = async ({ preview }) => {
+  let number;
+
+  // In preview mode, use odd number
+  if (preview) {
+    number = 3;
+  }
+  // In normal mode, use even number
+  else {
+    number = 4;
+  }
+
+  return {
+    props: {
+      number,
+    },
+  };
+};
+
+export default StaticTest;

cypress/fixtures/public/file.txt

@@ -0,0 +1 @@
+a text file

cypress/fixtures/public/folder/file.txt

@@ -0,0 +1 @@
+a text file in a folder