Add docs on how to migrate from Gatsby. (#17491)

Addresses #17453.

docs/manifest.json

@@ -187,6 +187,15 @@
           "title": "Upgrade Guide",
           "path": "/docs/upgrading.md"
         },
+        {
+          "title": "Migrating to Next.js",
+          "routes": [
+            {
+              "title": "Migrating from Gatsby",
+              "path": "/docs/migrating/from-gatsby.md"
+            }
+          ]
+        },
         { "title": "FAQ", "path": "/docs/faq.md" }
       ]
     },

docs/migrating/from-gatsby.md

@@ -0,0 +1,253 @@
+---
+description: Learn how to transition an existing Gatsby project to Next.js.
+---
+
+# Migrating from Gatsby
+
+This guide will help you understand how to transition from an existing Gatsby project to Next.js. Migrating to Next.js will allow you to:
+
+- Choose which [data fetching](/docs/basic-features/data-fetching.md) strategy you want on a per-page basis.
+- Use [Incremental Static Regeneration](/docs/basic-features/data-fetching.md#incremental-static-regeneration) to update _existing_ pages by re-rendering them in the background as traffic comes in.
+- Use [API Routes](/docs/api-routes/introduction.md).
+
+And more! Let’s walk through a series of steps to complete the migration.
+
+## Updating `package.json` and dependencies
+
+The first step towards migrating to Next.js is to update `package.json` and dependencies. You should:
+
+- Remove all Gatsby-related packages (but keep `react` and `react-dom`).
+- Install `next`.
+- Add Next.js related commands to `scripts`. One is `next dev`, which runs a development server at `localhost:3000`. You should also add `next build` and `next start` for creating and starting a production build.
+
+Here's an example `package.json` ([view diff](https://github.com/leerob/gatsby-to-nextjs/pull/1/files#diff-b9cfc7f2cdf78a7f4b91a753d10865a2)):
+
+```json
+{
+  "scripts": {
+    "dev": "next dev",
+    "build": "next build",
+    "start": "next start"
+  },
+  "dependencies": {
+    "next": "latest",
+    "react": "latest",
+    "react-dom": "latest"
+  }
+}
+```
+
+## Static Assets and Compiled Output
+
+Gatsby uses the `public` directory for the compiled output, whereas Next.js uses it for static assets. Here are the steps for migration ([view diff](https://github.com/leerob/gatsby-to-nextjs/pull/1/files#diff-a084b794bc0759e7a6b77810e01874f2)):
+
+- Remove `.cache/` and `public` from `.gitignore` and delete both directories.
+- Rename Gatsby’s `static` directory as `public`.
+- Add `.next` to `.gitignore`.
+
+## Creating Routes
+
+Both Gatsby and Next support a `pages` directory, which uses [file-system based routing](/docs/routing/introduction.md). Gatsby's directory is `src/pages`, which is also [supported by Next.js](/docs/advanced-features/src-directory.md).
+
+Gatsby creates dynamic routes using the `createPages` API inside of `gatsby-node.js`. With Next, we can use [Dynamic Routes](/docs/routing/dynamic-routes.md) inside of `pages` to achieve the same effect. Rather than having a `template` directory, you can use the React component inside your dynamic route file. For example:
+
+- **Gatsby:** `createPages` API inside `gatsby-node.js` for each blog post, then have a template file at `src/templates/blog-post.js`.
+- **Next:** Create `pages/blog/[slug].js` which contains the blog post template. The value of `slug` is accessible through a [query parameter](/docs/routing/dynamic-routes.md). For example, the route `/blog/first-post` would forward the query object `{ 'slug': 'first-post' }` to `pages/blog/[slug].js` ([learn more here](/docs/basic-features/data-fetching.md#getstaticpaths-static-generation`)).
+
+## Styling
+
+With Gatsby, global CSS imports are included in `gatsby-browser.js`. With Next, you should create a [custom `_app.js`](/docs/advanced-features/custom-app.md) for global CSS. When migrating, you can copy over your CSS imports directly and update the relative file path, if necessary. Next.js has [built-in CSS support](/docs/basic-features/built-in-css-support.md).
+
+## Links
+
+The Gatsby `Link` and Next.js [`Link`](/docs/api-reference/next/link.md) component have a slightly different API. First, you will need to update any import statements referencing `Link` from Gatsby to:
+
+```js
+import Link from 'next/link'
+```
+
+Next, you can find and replace usages of `to="/route"` with `href="/route"`.
+
+## Data Fetching
+
+The largest difference between Gatsby and Next.js is how data fetching is implemented. Gatsby is opinionated with GraphQL being the default strategy for retrieving data across your application. With Next.js, you get to choose which strategy you want (GraphQL is one supported option).
+
+Gatsby uses the `graphql` tag to query data in the pages of your site. This may include local data, remote data, or information about your site configuration. Gatsby only allows the creation of static pages. With Next.js, you can choose on a [per-page basis](/docs/basic-features/pages.md) which [data fetching strategy](/docs/basic-features/data-fetching.md) you want. For example, `getServerSideProps` allows you to do server-side rendering. If you wanted to generate a static page, you'd export `getStaticProps` / `getStaticPaths` inside the page, rather than using `pageQuery`. For example:
+
+```js
+// src/pages/[slug].js
+
+// Install remark and remark-html
+import remark from 'remark'
+import html from 'remark-html'
+import { getPostBySlug, getAllPosts } from '../lib/blog'
+
+export async function getStaticProps({ params }) {
+  const post = getPostBySlug(params.slug)
+  const content = await remark()
+    .use(html)
+    .process(post.content || '')
+    .toString()
+
+  return {
+    props: {
+      ...post,
+      content,
+    },
+  }
+}
+
+export async function getStaticPaths() {
+  const posts = getAllPosts()
+
+  return {
+    paths: posts.map((post) => {
+      return {
+        params: {
+          slug: post.slug,
+        },
+      }
+    }),
+    fallback: false,
+  }
+}
+```
+
+You'll commonly see Gatsby plugins used for reading the file system (`gatsby-source-filesystem`), handling markdown files (`gatsby-transformer-remark`), and so on. For example, the popular starter blog example has [15 Gatsby specific packages](https://github.com/gatsbyjs/gatsby-starter-blog/blob/master/package.json). Next takes a different approach. It includes common features like [image optimization](https://github.com/vercel/next.js/discussions/17141) directly inside the framework, and gives the user full control over integrations with external packages. For example, rather than abstracting reading from the file system to a plugin, you can use the native Node.js `fs` package inside `getStaticProps` / `getStaticPaths` to read from the file system.
+
+```js
+// src/lib/blog.js
+
+// Install gray-matter and date-fns
+import matter from 'gray-matter'
+import { parseISO, format } from 'date-fns'
+import fs from 'fs'
+import { join } from 'path'
+
+// Add markdown files in `src/content/blog`
+const postsDirectory = join(process.cwd(), 'src', 'content', 'blog')
+
+export function getPostBySlug(slug) {
+  const realSlug = slug.replace(/\\.md$/, '')
+  const fullPath = join(postsDirectory, `${realSlug}.md`)
+  const fileContents = fs.readFileSync(fullPath, 'utf8')
+  const { data, content } = matter(fileContents)
+  const date = format(parseISO(data.date), 'MMMM dd, yyyy')
+
+  return { slug: realSlug, frontmatter: { ...data, date }, content }
+}
+
+export function getAllPosts() {
+  const slugs = fs.readdirSync(postsDirectory)
+  const posts = slugs.map((slug) => getPostBySlug(slug))
+
+  return posts
+}
+```
+
+## Site Configuration
+
+With Gatsby, your site's metadata (name, description, etc) is located inside `gatsby-config.js`. This is then exposed through the GraphQL API and consumed through a `pageQuery` or a static query inside a component.
+
+With Next.js, we recommend creating a config file similar to below. You can then import this file anywhere without having to use GraphQL to access your site's metadata.
+
+```js
+// src/config.js
+
+export default {
+  title: 'Starter Blog',
+  author: {
+    name: 'Lee Robinson',
+    summary: 'who loves Next.js.',
+  },
+  description: 'A starter blog converting Gatsby -> Next.',
+  social: {
+    twitter: 'leeerob',
+  },
+}
+```
+
+## Search Engine Optimization
+
+Most Gatsby examples use `react-helmet` to assist with adding `meta` tags for proper SEO. With Next.js, we recommend using [`next/head`](/docs/api-reference/next/head.md) to add `meta` tags to your `<head />` element. For example, here's part of an SEO component with Gatsby:
+
+```js
+// src/components/seo.js
+
+import { Helmet } from 'react-helmet'
+
+export default function SEO({ description, title, siteTitle }) {
+  return (
+    <Helmet
+      title={title}
+      titleTemplate={siteTitle ? `%s | ${siteTitle}` : null}
+      meta={[
+        {
+          name: `description`,
+          content: description,
+        },
+        {
+          property: `og:title`,
+          content: title,
+        },
+        {
+          property: `og:description`,
+          content: description,
+        },
+        {
+          property: `og:type`,
+          content: `website`,
+        },
+        {
+          name: `twitter:card`,
+          content: `summary`,
+        },
+        {
+          name: `twitter:creator`,
+          content: twitter,
+        },
+        {
+          name: `twitter:title`,
+          content: title,
+        },
+        {
+          name: `twitter:description`,
+          content: description,
+        },
+      ]}
+    />
+  )
+}
+```
+
+And here's the same example using Next.js, including reading from a site config file.
+
+```js
+// src/components/seo.js
+
+import Head from 'next/head'
+import config from '../config'
+
+export default function SEO({ description, title }) {
+  const siteTitle = config.title
+
+  return (
+    <Head>
+      <title>{`${title} | ${siteTitle}`}</title>
+      <meta name="description" content={description} />
+      <meta property="og:type" content="website" />
+      <meta property="og:title" content={title} />
+      <meta property="og:description" content={description} />
+      <meta property="og:site_name" content={siteTitle} />
+      <meta property="twitter:card" content="summary" />
+      <meta property="twitter:creator" content={config.social.twitter} />
+      <meta property="twitter:title" content={title} />
+      <meta property="twitter:description" content={description} />
+    </Head>
+  )
+}
+```
+
+## Learn more
+
+Please take a look at [this pull request](https://github.com/leerob/gatsby-to-nextjs/pull/1) to learn more. If you have questions, please ask on [our discussion board](https://github.com/vercel/next.js/discussions).