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# Routing and Code-Splitting

# Routing with vue-router

You may have noticed that our server code uses a * handler which accepts arbitrary URLs. This allows us to pass the visited URL into our Vue app, and reuse the same routing config for both client and server!

It is recommended to use the official vue-router(opens new window) library for this purpose. Let's first create a file where we create the router. Note that similar to application instance, we also need a fresh router instance for each request, so the file exports a createRouter function:

// router.js
import { createRouter, createMemoryHistory, createWebHistory } from 'vue-router'
import MyUser from './components/MyUser.vue'

const isServer = typeof window === 'undefined'

const history = isServer ? createMemoryHistory() : createWebHistory()

const routes = [{ path: '/user', component: MyUser }]

export default function() {
  return createRouter({ routes, history })
}
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And update our app.js, client and server entries:

// app.js
import { createSSRApp } from 'vue'
import App from './App.vue'
import createRouter from './router'

export default function(args) {
  const app = createSSRApp(App)
  const router = createRouter()

  app.use(router)

  return {
    app,
    router
  }
}
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// entry-client.js
const { app, router } = createApp({
  /*...*/
})
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// entry-server.js
const { app, router } = createApp({
  /*...*/
})
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# Code-Splitting

Code-splitting, or lazy-loading part of your app, helps reduce the size of assets that need to be downloaded by the browser for the initial render, and can greatly improve TTI (time-to-interactive) for apps with large bundles. The key is "loading just what is needed" for the initial screen.

Vue Router provides lazy-loading support(opens new window) , allowing webpack to code-split at that point(opens new window) . All you need to do is:

// change this...
import MyUser from './components/MyUser.vue'
const routes = [{ path: '/user', component: MyUser }]

// to this:
const routes = [
  { path: '/user', component: () => import('./components/MyUser.vue') }
]
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On both client and server we need to wait for router to resolve async route components ahead of time in order to properly invoke in-component hooks. For this we will be using router.isReady(opens new window) method Let's update our client entry:

// entry-client.js
import createApp from './app'

const { app, router } = createApp({
  /* ... */
})

router.isReady().then(() => {
  app.mount('#app')
})
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We also need to update our server.js script:

// server.js
const path = require('path')

const appPath = path.join(__dirname, './dist', 'server', manifest['app.js'])
const createApp = require(appPath).default

server.get('*', async (req, res) => {
  const { app, router } = createApp()

  router.push(req.url)
  await router.isReady()

  const appContent = await renderToString(app)

  fs.readFile(path.join(__dirname, '/dist/client/index.html'), (err, html) => {
    if (err) {
      throw err
    }

    html = html
      .toString()
      .replace('<div id="app">', `<div id="app">${appContent}`)
    res.setHeader('Content-Type', 'text/html')
    res.send(html)
  })
})
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