kushagharahi / prerenderer

Fast, flexible, framework-agnostic prerendering for sites and SPAs.

Prerenderer

Fast, flexible, framework-agnostic prerendering for sites and SPAs.

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Monorepo for the following NPM packages

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About prerenderer

The goal of this package is to provide a simple, framework-agnostic prerendering solution that is easily extensible and usable for any site or single-page-app.

Now, if you're not familiar with the concept of prerendering, you might predictably ask...

What is Prerendering?

Recently, SSR (Server Side Rendering) has taken the JavaScript front-end world by storm. The fact that you can now render your sites and apps on the server before sending them to your clients is an absolutely revolutionary idea (and totally not what everyone was doing before JS client-side apps got popular in the first place...)

However, the same criticisms that were valid for PHP, ASP, JSP, (and such) sites are valid for server-side rendering today. It's slow, breaks fairly easily, and is difficult to implement properly.

Thing is, despite what everyone might be telling you, you probably don't need SSR. You can get almost all the advantages of it (without the disadvantages) by using prerendering. Prerendering is basically firing up a headless browser, loading your app's routes, and saving the results to a static HTML file. You can then serve it with whatever static-file-serving solution you were using previously. It just works with HTML5 navigation and the likes. No need to change your code or add server-side rendering workarounds.

In the interest of transparency, there are some use-cases where prerendering might not be a great idea.

• Tons of routes - If your site has hundreds or thousands of routes, prerendering will be really slow. Sure you only have to do it once per update, but it could take ages. Most people don't end up with thousands of static routes, but just in-case...
• Dynamic Content - If your render routes that have content that's specific to the user viewing it or other dynamic sources, you should make sure you have placeholder components that can display until the dynamic content loads on the client-side. Otherwise, it might be a tad weird.

Example prerenderer Usage

(It's much simpler if you use prerenderer with webpack or another build system.)

Input

app/
├── index.html
└── index.js // Whatever JS controls the SPA, loaded by index.html

Output

app/
├── about
│   └── index.html // Static rendered /about route.
├── index.html // Static rendered / route.
├── index.js // Whatever JS controls the SPA, loaded by index.html
└── some
    └── deep
        └── nested
            └── route
                └── index.html // Static rendered nested route.

const fs = require('fs')
const path = require('path')
const mkdirp = require('mkdirp')
const Prerenderer = require('@prerenderer/prerenderer')
// Make sure you install a renderer as well!
const JSDOMRenderer = require('@prerenderer/renderer-jsdom')

const prerenderer = new Prerenderer({
  // Required - The path to the app to prerender. Should have an index.html and any other needed assets.
  staticDir: path.join(__dirname, 'app'),
  // The plugin that actually renders the page.
  renderer: new JSDOMRenderer(),
  postProcess (renderedRoute) {
    // Replace all http with https urls and localhost to your site url
    renderedRoute.html = renderedRoute.html.replace(
      /http:/i,
      'https:',
    ).replace(
      /(https:\/\/)?(localhost|127\.0\.0\.1):\d*/i,
      (process.env.CI_ENVIRONMENT_URL || ''),
    );
  },
})

// Initialize is separate from the constructor for flexibility of integration with build systems.
prerenderer.initialize()
  .then(() => {
    // List of routes to render.
    return prerenderer.renderRoutes(['/', '/about', '/some/deep/nested/route'])
  })
  .then(renderedRoutes => {
    // renderedRoutes is an array of objects in the format:
    // {
    //   route: String (The route rendered)
    //   html: String (The resulting HTML)
    // }
    renderedRoutes.forEach(renderedRoute => {
      try {
        // A smarter implementation would be required, but this does okay for an example.
        // Don't copy this directly!!!
        const outputDir = path.join(__dirname, 'app', renderedRoute.route)
        const outputFile = `${outputDir}/index.html`

        mkdirp.sync(outputDir)
        fs.writeFileSync(outputFile, renderedRoute.html.trim())
      } catch (e) {
        // Handle errors.
      }
    })

    // Shut down the file server and renderer.
    return prerenderer.destroy()
  })
  .catch(err => {
    // Shut down the server and renderer.
    return prerenderer.destroy()
    // Handle errors.
  })

Available Renderers

• @prerenderer/renderer-jsdom - Uses jsdom. Fast, but unreliable and cannot handle advanced usages. May not work with all front-end frameworks and apps.
• @prerenderer/renderer-puppeteer - Uses puppeteer to render pages in headless Chrome. Simpler and more reliable than the previous ChromeRenderer.

Which renderer should I use?

Use @prerenderer/renderer-puppeteer if: You're prerendering up to a couple hundred pages.

Use @prerenderer/renderer-jsdom if: You need to prerender thousands upon thousands of pages, but quality isn't all that important, and you're willing to work around issues for more advanced cases. (Programmatic SVG support, etc.)

An alternative faster dom renderer using linkedom is being considered

Documentation

All of the packages are strongly typed using typescript, if some documentation is missing or when in doubt, we recommend referring to the types which are self documenting

Prerenderer Options

Option	Type	Required?	Default	Description
staticDir	String	Yes	None	The root path to serve your app from. (If you are using a plugin, you don't need to set this, it will be taken from the configuration of webpack or rollup)
indexPath	String	No	staticDir/index.html	The index file to fall back on for SPAs.
server	Object	No	None	App server configuration options (See below)
renderer	IRenderer Instance, constructor or String to require	No	new PuppeteerRenderer()	The renderer you'd like to use to prerender the app. It's recommended that you specify this, but if not it will default to @prerenderer/renderer-puppeteer.
rendererOptions	Object	No	None	The options to pass to the renderer if it was not given as an instance, see below for a list of options
postProcess	(renderedRoute: Route, routes: Route[]) => void	No	None	Allows you to customize the HTML and output path before writing the rendered contents to a file, you can also add your own routes by pushing to the routes parameter

Server Options

Option	Type	Required?	Default	Description
port	Integer	No	First free port after 8000	The port for the app server to run on.
proxy	Object	No	No proxying	Proxy configuration. Has the same signature as webpack-dev-server
host	String	No	127.0.0.1	The host to send requests to. Use with caution, as changing this could result in external urls being rendered instead of your local server, use postProcess if you just want to change urls in the resulting .html
listenHost	String	No	127.0.0.1	The ip address the server will listen to. (0.0.0.0 would allow external access, use with caution)
before	Function	No	No operation	Deprecated: Use hookServer() instead. Function for adding custom server middleware.

Prerenderer Methods

• constructor(options: Object) - Creates a Prerenderer instance and sets up the renderer and server objects.
• hookServer(cb: (server: Express) => void, stage: Stage = 'pre-fallback') - Use this method to hook into the express server to add middlewares, routes etc
• initialize(): Promise<void> - Starts the static file server and renderer instance (where appropriate).
• getOptions(): PrerenderFinalOptions - Returns the options used to configure prerenderer
• getServer(): Server - Returns the Server class holding the express server
• destroy(): Promise<void> - Destroys the static file server and renderer, freeing the resources.
• renderRoutes(routes: Array<String>): Promise<Array<RenderedRoute>> - Renders set of routes. Returns a promise resolving to an array of rendered routes in the form of:

[
  {
    originalRoute: '/route/path', // The requested route path.
    route: '/route/redirected-path', // The final route path after redirection or history change.
    html: '<!DOCTYPE html><html>...</html>' // The prerendered HTML for the route
  },
  // ...
]

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@prerenderer/renderer-jsdom Options

None of the options are required, by default the page will render on DOMContentLoaded

Option	Type	Default	Description
maxConcurrentRoutes	Number	0 (No limit)	The number of routes allowed to be rendered at the same time. Useful for breaking down massive batches of routes into smaller chunks.
inject	Object	None	An object to inject into the global scope of the rendered page before it finishes loading. Must be JSON.stringifiy-able. The property injected to is window['__PRERENDER_INJECTED'] by default.
injectProperty	String	__PRERENDER_INJECTED	The property to mount inject to during rendering. Does nothing if inject isn't set.
renderAfterDocumentEvent	String	DOMContentLoaded	Wait to render until the specified event is fired on the document. (You can fire an event like so: document.dispatchEvent(new Event('custom-render-trigger'))
renderAfterElementExists	String (Selector)	None	Wait to render until the specified element is detected using document.querySelector
renderAfterTime	Integer (Milliseconds)	None	Wait to render until a certain amount of time has passed.
timeout	Integer (Milliseconds)	30000	If this timeout triggers while waiting for an event or an element, the rendering will abort with an error.
JSDOMOptions	BaseOptions	{ runScripts: 'dangerously', resources: 'usable', pretendToBeVisual: true }	Additional options for JSDOM.fromUrl

@prerenderer/renderer-puppeteer Options

None of the options are required, by default the page will render when puppeteer is ready which is when DOMContentLoaded fires

Option	Type	Default	Description
maxConcurrentRoutes	Number	0 (No limit)	The number of routes allowed to be rendered at the same time. Useful for breaking down massive batches of routes into smaller chunks.
inject	Object	None	An object to inject into the global scope of the rendered page before it finishes loading. Must be JSON.stringifiy-able. The property injected to is window['__PRERENDER_INJECTED'] by default.
injectProperty	String	__PRERENDER_INJECTED	The property to mount inject to during rendering. Does nothing if inject isn't set.
renderAfterDocumentEvent	String	DOMContentLoaded	Wait to render until the specified event is fired on the document. (You can fire an event like so: document.dispatchEvent(new Event('custom-render-trigger'))
renderAfterTime	Integer (Milliseconds)	None	Wait to render until a certain amount of time has passed.
renderAfterElementExists	String (Selector)	None	Wait to render until the specified element is detected using document.querySelector
elementVisible	Boolean	None	If we should wait until the renderAfterElementExists is visible
elementHidden	Boolean	None	If we should wait until the renderAfterElementExists is hidden
timeout	Integer (Milliseconds)	30000	If this timeout triggers while waiting for an event or an element, the rendering will abort with an error.
skipThirdPartyRequests	Boolean	false	Automatically block any third-party requests. (This can make your pages load faster by not loading non-essential scripts, styles, or fonts.)
headless	Boolean	true	Whether to run the browser in headless mode
consoleHandler	function(route: String, message: ConsoleMessage)	None	Allows you to provide a custom console.* handler for pages. Argument one to your function is the route being rendered, argument two is the Puppeteer ConsoleMessage object.
viewport	Viewport	None	Those options will be passed to puppeteer.launch().
launchOptions	LaunchOptions	None	Those options will be passed to puppeteer.launch().
navigationOptions	WaitForOptions	None	Those options will be passed to page.goto(), such as timeout: 30000ms.

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@prerenderer/webpack-plugin Options

You MUST use HtmlWebpackPlugin as this plugin is triggered after.

None of the options are required, by default the renderer-puppeteer will be used and render only the entry file

Option	Type	Default	Description
routes	Array	['/']	The list of routes to prerender
fallback	Boolean	String	false
renderer	string or instance of a renderer	'@prerenderer/renderer-puppeteer'	The instance of the renderer or the name of the renderer to require
rendererOptions	Object	None	The options to pass to the renderer if it was not given as an instance, see above for a list of options
postProcess	(renderedRoute: Route) => void	None	Allows you to customize the HTML and output path before writing the rendered contents to a file
urlModifier	(url: string) => string	None	Hook to be able to modify the url to retrieve the compiled asset
entryPath	String	indexPath option	The entry html file to use
...			Additional Prerenderer Options

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@prerenderer/rollup-plugin Options

The @prerenderer/rollup-plugin and @prerenderer/webpack-plugin aims to have the same feature set and api for an easy migration

As such the options are the same as the webpack-plugin

Caveats

• For obvious reasons, prerenderer only works for SPAs that route using the HTML5 history API. index.html#/hash/route URLs will unfortunately not work.
• Whatever client-side rendering library you're using should be able to at least replace any server-rendered content or diff with it.
  • For Vue.js 1 use replace: false on root components.
  • For Vue.js 2 and 3 Ensure your root component has the same id as the prerendered element it's replacing. Otherwise you'll end up with duplicated content.

Contributing

This is a monorepo, using lerna, so you'll need to clone the repository, then run npm install inside the directory

Run npm run test to make sure that everything is working correctly

Maintainers

Adrien Foulon

Joshua Bemenderfer