# Prerender

Some of your pages don't have dynamic content; it'd be great if you could render them ahead of time, making for a faster experience for your end users.

We thought a lot about what the developer experience should be for route-based prerendering. The result is one of the smallest APIs imaginable!

How's Prerendering different from SSR/SSG/SWR/ISSG/...?

As Danny said in his Prerender demo at our Community Meetup, the thing all of these have in common is that they render your markup in a Node.js context to produce HTML. The difference is when (build or runtime) and how often.

# Prerendering a Page

Prerendering a page is as easy as it gets. Just add the prerender prop to the Route that you want to prerender:

// Routes.js

<Route path="/" page={HomePage} name="home" prerender/>

Then run yarn rw build and enjoy the performance boost!

# Prerendering all pages in a Set

Just add the prerender prop to the Set that wraps all Pages you want to prerender:

// Routes.js

<Set prerender>  <Route path="/" page={HomePage} name="home" />
  <Route path="/about" page={AboutPage} name="hello" />
</Set>

# Not found page

You can also prerender your not found page (a.k.a your 404 page). Just add—you guessed it—the prerender prop:

-      <Route notfound page={NotFoundPage} />
+      <Route notfound page={NotFoundPage} prerender/>

This will prerender your NotFoundPage to 404.html in your dist folder. Note that there's no need to specify a path.

# Cells, Private Routes, and Dynamic URLs

How does Prerendering handle dynamic data? For Cells, Redwood prerenders your Cells' <Loading/> component. Similarly, for Private Routes, Redwood prerenders your Private Routes' whileLoadingAuth prop:

<Private >  // Loading is shown while we're checking to see if the user's logged in  <Route path="/super-secret-admin-dashboard" page={SuperSecretAdminDashboard} name="ssad" whileLoadingAuth={() => <Loading />} prerender/>
</Private>

Right now prerendering won't work for dynamic URLs. We're working on this. If you try to prerender one of them, nothing will break, but nothing happens.

// web/src/Routes.js

<Route path="/blog-post/{id}" page={BlogPostPage} name="blogPost" prerender />

# Prerender Utils

Sometimes you need more fine-grained control over whether something gets prerendered. This may be because the component or library you're using needs access to browser APIs like window or localStorage. Redwood has three utils to help you handle these situations:

• <BrowserOnly>
• useIsBrowser
• isBrowser

Heads-up!

If you're prerendering a page that uses a third-party library, make sure it's "universal". If it's not, try calling the library after doing a browser check using one of the utils above.

Look for these key words when choosing a library: universal module, SSR compatible, server compatible—all these indicate that the library also works in Node.js.

# <BrowserOnly/> component

This higher-order component is great for JSX:

import { BrowserOnly } from '@redwoodjs/prerender/browserUtils'

const MyFancyComponent = () => {
  <h2>👋🏾 I render on both the server and the browser</h2>
  <BrowserOnly>
    <h2>🙋‍♀️ I only render on the browser</h2>
  </BrowserOnly>
}

# useIsBrowser hook

If you prefer hooks, you can use the useIsBrowser hook:

import { useIsBrowser } from '@redwoodjs/prerender/browserUtils'

const MySpecialComponent = () => {
  const browser = useIsBrowser()

  return (
    <div className="my-4 p-5 rounded-lg border-gray-200 border">
      <h1 className="text-xl font-bold">Render info:</h1>

      {browser ? <h2 className="text-green-500">Browser</h2> : <h2 className="text-red-500">Prerendered</h2>}
    </div>
  )
}

# isBrowser boolean

If you need to guard against prerendering outside React, you can use the isBrowser boolean. This is especially handy when running initializing code that only works in the browser:

import { isBrowser } from '@redwoodjs/prerender/browserUtils'

if (isBrowser) {
  netlifyIdentity.init()
}

# Optimization Tip

If you dynamically load third-party libraries that aren't part of your JS bundle, using these prerendering utils can help you avoid loading them at build time:

import { useIsBrowser } from '@redwoodjs/prerender/browserUtils'

const ComponentUsingAnExternalLibrary = () => {
  const browser = useIsBrowser()

  // if `browser` evaluates to false, this won't be included
  if (browser) {
    loadMyLargeExternalLibrary()
  }

  return (
    // ...
  )

# Debugging

If you just want to debug your app, or check for possible prerendering errors, after you've built it, you can run this command:

yarn rw prerender --dry-run

Since we just shipped this in v0.26, we're actively looking for feedback! Do let us know if: everything built ok? you encountered specific libraries that you were using that didn’t work?

# Images and Assets

Images and assets continue to work the way they used to. For more, see this doc.

Note that there's a subtlety in how SVGs are handled. Importing an SVG and using it in a component works great:

import logo from './my-logo.svg'
function Header() {
  return <logo />
}

But re-exporting the SVG as a component requires a small change:

// ❌ due to how Redwood handles SVGs, this syntax isn't supported.
import Logo from './Logo.svg'
export default Logo

// ✅ use this instead.
import Logo from './Logo.svg'

const LogoComponent = () => <Logo />

export default LogoComponent

# Configuring redirects

Depending on what pages you're prerendering, you may want to change your redirect settings. Using Netlify as an example:

[[redirects]]
- from = "/*"
- to = "/index.html"
- status = 200

[[redirects]]
  from = "/*"
  to = "/index.html"
- status = 200
+ status = 404

# Flash after page load

We're actively working preventing these flashes with upcoming changes to the Router.

You might notice a flash after page load. A quick workaround for this is to make sure whatever page you're seeing the flash on isn't code split. You can do this by explicitly importing the page in Routes.js:

import { Router, Route } from '@redwoodjs/router'
import HomePage from 'src/pages/HomePage'

const Routes = () => {
  return (
    <Router>
      <Route path="/" page={HomePage} name="hello" prerender />
      <Route path="/about" page={AboutPage} name="hello" />
      <Route notfound page={NotFoundPage} />
    </Router>
  )
}

export default Routes