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Server-Side Routing in LWR on Node.js

To set server-side routes for your app with LWR, use the lwr.config.json project configuration file. Server-side routes can be specified in lwr.config.json by providing either:

  • a static path for each page
  • a JavaScript file that defines a hook to dynamically generate server-side routes for pages at server startup (this is an advanced option)

Both static and dynamic server-side routes can also have route handler functions, which let you customize the page response at runtime.

Read on for details. We start with a basic static routing example.

Static Server-Side Routing

Let’s add a page to our site and do some basic (and fast!) routing.

  1. Now that you’ve got your StaticSite project, make a copy of the about.md file. Rename the new file explore.md and leave it in the content directory.
  2. Edit the explore.md file. Change the first line from # About LWR to # Explore LWR. Save the file.
  3. Open the main_layout.njk file. On a new line, add <li><a href="/explore">Explore</a> under the about line. Save the file. Code snippet showing the Explore href addition
  4. Finally, open the lwr.config.json file. Copy the entire block of code for about, including the curly brackets, and paste it below the about block. Change about to explore throughout, and don’t forget to add the comma after the closing bracket in the about block. Save the file. Code snippet showing the Explore block added
  5. If you're still in the StaticSite directory from when you first made your site, just type npm run start in the terminal to see your changes. Your updated StaticSite project runs at http://localhost:3000/, with a new Explore LWR page and a working Explore button. Screenshot of updated site with Explore LWR page

tip: If you get a "Port 3000 is already in use" error after running npm run start, close any terminal tabs that run your site preview. Then, open a new terminal tab and run npm run start.

What just happened is that you:

  • created a Markdown page (explore.md) and gave it new heading text ("Explore LWR")
  • updated the site navigation in the Nunjucks layout (main_layout.njk) file to include a new Explore button
  • updated the JSON routing configuration file (lwr.config.json) to let LWR know how to route to the new page using a route
    • you gave an id for the page (Explore)
    • you provided the path for the page (/explore)
    • you provided a contentTemplate for the page (explore.md)
    • you told LWR the layoutTemplate to use for the page; in this case, you used main_layout.njk, which is the same as for other pages in this example, though that’s not required

Error Routes

You can set up routes to serve if LWR encounters a 404 or 500 error during the bootstrap of a route. Error routes take a status code value instead of a path value.

{
  "errorRoutes": [
    {
      "id": "not_found",
      "status": 404,
      "rootComponent": "example/notFound"
    },
    {
      "id": "server_error",
      "status": 500,
      "contentTemplate": "$contentDir/not-found.html"
    }
  ]
}

Dynamic Server-Side Routing with Configuration Hooks

note: This is an advanced topic.

Configuration hooks let you dynamically generate server-side routes for your application. On server startup, they update the configuration and global data for your app.

To set up a configuration hook, start by creating a hooks section in your lwr.config.json file. In there, add the filepath that points to the hook.

{
  "hooks": ["$rootDir/src/hooks/my-config-hook.ts"]
}

The following is an example of a configuration hook:

import path from 'path';
import { DEFAULT_LWR_BOOTSTRAP_CONFIG } from '@lwrjs/config';
import { slugify } from '@lwrjs/shared-utils';
import type { HooksPlugin, NormalizedLwrGlobalConfig, NormalizedLwrRoute, GlobalData } from '@lwrjs/types';

const DEFAULT_MAIN_LAYOUT = 'main_layout.njk';
const CACHE_TTL = '30m';
interface GuideItem {
  label: string;
  content: string;
}
interface GuideSidebarItem {
  label: string;
  id: string;
  url: string;
}
interface SiteGlobalData extends GlobalData {
  site?: {
    guide: GuideItem[];
      sidebar: GuideSidebarItem[];
  };
}

function generateGuideSidebar(guide: GuideItem[]): GuideSidebarItem[] {
  return guide.map(({ label, content }) => {
    return {
      label,
      id: `guide_${slugify(label)}`,
      url: `/${content.replace('.md', '')}`,
    };
  });
}

function generateGuideRoutes(
  guide: GuideItem[],
  { contentDir, layoutsDir }: NormalizedLwrGlobalConfig,
): NormalizedLwrRoute[] {
  return guide.map(({ label, content }) => {
    return {
      id: `guide_${slugify(label)}`,
      path: `/${content.replace('.md', '')}`,
      contentTemplate: path.join(contentDir, content),
      layoutTemplate: path.join(layoutsDir, DEFAULT_MAIN_LAYOUT),
      cache: { ttl: CACHE_TTL },
      bootstrap: DEFAULT_LWR_BOOTSTRAP_CONFIG,
    };
  });
}

// Export an Application Configuration hook
// Configured in lwr.config.json[hooks]
export default class MyAppHooks implements HooksPlugin {
  initConfigs(lwrConfig: NormalizedLwrGlobalConfig, globalData: SiteGlobalData): void {
    if (!globalData.site) {
      throw 'Expected site global data to be defined';
    }

    // The guide is an ordered list of files we want to display
    // Hardcoded here: src/data/site/guide.json
    const guide = globalData.site.guide;

    // Generate sidebar add it to the globalData object
    // The data is accessed in src/layouts/partials/guide-sidebar.njk
    globalData.site.sidebar = generateGuideSidebar(guide);

    // Dynamically add a new route for each guide
    // Note: other routes are statically declared in lwr.config.json[routes]
    lwrConfig.routes.push(...generateGuideRoutes(guide, lwrConfig));
  }
}

Dynamic server-side routes can use route handler functions to customize the page response at runtime.

Route Handler Functions

note: This is an advanced topic.

Route handler functions are part of the LWR context object. They’re a server-side way to alter the current route and customize the page response at runtime. You can use route handler functions with both static and dynamic server-side routing.

tip: Route handler functions, which are used in LWR's server-side routing, aren’t the same as route handler modules, which are used in client-side routing.

There are a couple of differences between configuration hooks and route handler functions that are worth noting:

  • A configuration hook is called once on server startup, while a route handler is triggered with each incoming page request.
  • A configuration hook applies to your entire app, whereas each route handler applies only to its specified path.

You provide the path to a route handler function in lwr.config.json, like this:

{
  "id": "custom_route_handler",
  "path": "/custom/:param",
  "routeHandler": "$rootDir/src/routeHandlers/custom.ts"
}

A route handler function follows this syntax. For more information about syntax and properties of RouteHandlerFunction and the RouteHandlerViewReponse that it returns, see the Server-Side Routing Reference.

type RouteHandlerFunction = (
    viewRequest: LocalizedViewRequest,
    handlerContext: HandlerContext,
) => Promise<RouteHandlerViewResponse>;

Things to note:

  • Cache size. All the ViewDefinitionResponse.viewParams are added to the cache key for a page view response. To control cache size, monitor the number of items added.
  • Static route properties. The ViewDefinitionResponse.viewParams replace the static route properties, so if the static route properties are needed, you must merge them into the viewParams in the route handler function.
  • Markdown. The dynamic ViewDefinitionResponse.viewParams are available in Markdown content templates. This is notable because in general context isn't passed into Markdown templates, unless you use a custom route handler.
  • View. LWR merges the ViewDefinitionResponse.view with the id and bootstrap values from the current route.
  • Time-to-live. The CacheResponse.ttl is a number, in seconds, or a time string to use as the max-age on the Cache-Control header.
  • Supported languages. You can use both TypeScript and JavaScript to create your route handler.

Customized Page Response Example

The following is an example of using a route handler function to customize a page response. The LWR server constructs the page response from this function.

import { HandlerContext, RouteHandlerViewResponse, ViewRequest } from '@lwrjs/types';

// Return customized input, from which the LWR server constructs a response
// viewRequest = { url, requestPath, params?, query? }
// handlerContext = { route, viewApi: { hasViewResponse, getViewResponse } }
export default function echoRouteHandler(
  viewRequest: ViewRequest,
  handlerContext: HandlerContext,
): RouteHandlerViewResponse {
  const routeProperties = handlerContext.route.properties || {};
  const message = viewRequest.params.message;

  // return a "ViewDefinitionResponse"
  return {
    // Required: customize the current route by setting:
    // { rootComponent?, contentTemplate?, layoutTemplate? }
    view: {
      contentTemplate: '$contentDir/echo.html',
    },
    // Required: pass context to the templates
    viewParams: {
      message, // pass the "message" path param
      ...routeProperties, // pass the static route properties
    },
    // Optional: rendering options { skipMetadataCollection?, freezeAssets?, skipCaching? }
    renderOptions: {
      freezeAssets: true,
    },
    // Optional: caching options { ttl? }
    cache: {
      ttl: 200,
    },
  };
}

Overridden Page Response Example

The following is an example of using a route handler function to completely override a page response. The LWR server constructs the page response from this function.

import { ViewRequest, ViewResponse } from 'lwr';

// Return a completely custom response
// containing some JSON data, based on the current path and query parameters
// viewRequest = { url, requestPath, params?, query? }
export default function jsonRouteHandler(viewRequest: ViewRequest): ViewResponse {
  const myPathParam = viewRequest.params.my;
  const someQueryParams = viewRequest.query?.some;

  // return a "ViewResponse": { status?, body, cache?, headers? }
  return {
    // Required: return the response body
    body: {
      my: myPathParam,
      some: someQueryParams,
    },
    // Optional: HTTP header map
    headers: {
      'Content-Type': 'application/json',
    },
  };
}