Use <link rel=“prefetch”> for prefetching

packages/next-server/lib/router/router.js

@@ -3,7 +3,6 @@
 import { parse, format } from 'url'
 import EventEmitter from '../EventEmitter'
 import shallowEquals from '../shallow-equals'
-import PQueue from '../p-queue'
 import { loadGetInitialProps, getURL } from '../utils'
 import { _rewriteUrlForNextExport } from './'
 
@@ -30,7 +29,6 @@ export default class Router {
     this.events = Router.events
 
     this.pageLoader = pageLoader
-    this.prefetchQueue = new PQueue({ concurrency: 2 })
     this.ErrorComponent = ErrorComponent
     this.pathname = pathname
     this.query = query
@@ -359,7 +357,7 @@ export default class Router {
 
     const { pathname } = parse(url)
     const route = toRoute(pathname)
-    return this.prefetchQueue.add(() => this.fetchRoute(route))
+    return this.pageLoader.prefetch(route)
   }
 
   async fetchComponent (route, as) {

packages/next/lib/page-loader.js

@@ -1,6 +1,19 @@
-/* global window, document */
+/* global document */
 import EventEmitter from 'next-server/dist/lib/EventEmitter'
 
+// smaller version of https://gist.github.com/igrigorik/a02f2359f3bc50ca7a9c
+function listSupports (list, token) {
+  if (!list || !list.supports) {
+    return false
+  }
+  try {
+    return list.supports(token)
+  } catch (e) {
+    return false
+  }
+}
+
+const supportsPrefetch = listSupports(document.createElement('link').relList, 'prefetch')
 const webpackModule = module
 
 export default class PageLoader {
@@ -9,7 +22,7 @@ export default class PageLoader {
     this.assetPrefix = assetPrefix
 
     this.pageCache = {}
-    this.pageLoadedHandlers = {}
+    this.prefetchCache = new Set()
     this.pageRegisterEvents = new EventEmitter()
     this.loadingRoutes = {}
   }
@@ -110,10 +123,30 @@ export default class PageLoader {
     }
   }
 
+  async prefetch (route) {
+    route = this.normalizeRoute(route)
+    const scriptRoute = route === '/' ? '/index.js' : `${route}.js`
+    if (this.prefetchCache.has(scriptRoute)) {
+      return
+    }
+    this.prefetchCache.add(scriptRoute)
+
+    const link = document.createElement('link')
+    // Feature detection is used to see if prefetch is supported, else fall back to preload
+    // Mainly this is for Safari
+    // https://caniuse.com/#feat=link-rel-prefetch
+    // https://caniuse.com/#feat=link-rel-preload
+    link.rel = supportsPrefetch ? 'prefetch' : 'preload'
+    link.href = `${this.assetPrefix}/_next/static/${encodeURIComponent(this.buildId)}/pages${scriptRoute}`
+    link.as = 'script'
+    document.head.appendChild(link)
+  }
+
   clearCache (route) {
     route = this.normalizeRoute(route)
     delete this.pageCache[route]
     delete this.loadingRoutes[route]
+    delete this.loadingRoutes[route]
 
     const script = document.getElementById(`__NEXT_PAGE__${route}`)
     if (script) {

packages/next/package.json

@@ -50,7 +50,7 @@
     "autodll-webpack-plugin": "0.4.2",
     "babel-core": "7.0.0-bridge.0",
     "babel-loader": "8.0.2",
-    "babel-plugin-react-require": "3.0.1",
+    "babel-plugin-react-require": "3.0.0",
     "babel-plugin-transform-react-remove-prop-types": "0.4.15",
     "case-sensitive-paths-webpack-plugin": "2.1.2",
     "cross-spawn": "5.1.0",

test/integration/production/pages/prefetch.js

@@ -0,0 +1,28 @@
+import Link from 'next/link'
+
+export default () => {
+  return <div>
+    <ul>
+      <li>
+        <Link href='/' prefetch>
+          <a>index</a>
+        </Link>
+      </li>
+      <li>
+        <Link href='/process-env' prefetch>
+          <a>process env</a>
+        </Link>
+      </li>
+      <li>
+        <Link href='/counter' prefetch>
+          <a>counter</a>
+        </Link>
+      </li>
+      <li>
+        <Link href='/about' prefetch>
+          <a>about</a>
+        </Link>
+      </li>
+    </ul>
+  </div>
+}

test/integration/production/test/index.test.js

@@ -210,6 +210,21 @@ describe('Production Usage', () => {
       browser.close()
     })
 
+    it('should add prefetch tags when Link prefetch prop is used', async () => {
+      const browser = await webdriver(appPort, '/prefetch')
+      const elements = await browser.elementsByCss('link[rel=prefetch]')
+      expect(elements.length).toBe(4)
+      await Promise.all(
+        elements.map(async (element) => {
+          const rel = await element.getAttribute('rel')
+          const as = await element.getAttribute('as')
+          expect(rel).toBe('prefetch')
+          expect(as).toBe('script')
+        })
+      )
+      browser.close()
+    })
+
     it('should reload the page on page script error with prefetch', async () => {
       const browser = await webdriver(appPort, '/counter')
       const counter = await browser
@@ -220,7 +235,14 @@ describe('Production Usage', () => {
       // Let the browser to prefetch the page and error it on the console.
       await waitFor(3000)
       const browserLogs = await browser.log('browser')
-      expect(browserLogs[0].message).toMatch(/\/no-such-page.js - Failed to load resource/)
+      let foundLog = false
+      browserLogs.forEach((log) => {
+        if (log.message.match(/\/no-such-page\.js - Failed to load resource/)) {
+          foundLog = true
+        }
+      })
+
+      expect(foundLog).toBe(true)
 
       // When we go to the 404 page, it'll do a hard reload.
       // So, it's possible for the front proxy to load a page from another zone.

test/unit/router.test.js

@@ -5,6 +5,7 @@ class PageLoader {
   constructor (options = {}) {
     this.options = options
     this.loaded = {}
+    this.prefetched = {}
   }
 
   loadPage (route) {
@@ -14,6 +15,10 @@ class PageLoader {
       return new Promise((resolve) => setTimeout(resolve, this.options.delay))
     }
   }
+
+  prefetch (route) {
+    this.prefetched[route] = true
+  }
 }
 
 describe('Router', () => {
@@ -26,10 +31,10 @@ describe('Router', () => {
       const route = '/routex'
       await router.prefetch(route)
 
-      expect(pageLoader.loaded['/routex']).toBeTruthy()
+      expect(pageLoader.prefetched['/routex']).toBeTruthy()
     })
 
-    it('should only run two jobs at a time', async () => {
+    it('should call prefetch correctly', async () => {
       global.__NEXT_DATA__ = {}
       // delay loading pages for an hour
       const pageLoader = new PageLoader({ delay: 1000 * 3600 })
@@ -40,11 +45,8 @@ describe('Router', () => {
       router.prefetch('route3')
       router.prefetch('route4')
 
-      // Wait for a bit
-      await new Promise((resolve) => setTimeout(resolve, 50))
-
-      expect(Object.keys(pageLoader.loaded).length).toBe(2)
-      expect(Object.keys(pageLoader.loaded)).toEqual(['route1', 'route2'])
+      expect(Object.keys(pageLoader.prefetched).length).toBe(4)
+      expect(Object.keys(pageLoader.prefetched)).toEqual(['route1', 'route2', 'route3', 'route4'])
     })
 
     it('should run all the jobs', async () => {
@@ -60,7 +62,7 @@ describe('Router', () => {
       await router.prefetch(routes[2])
       await router.prefetch(routes[3])
 
-      expect(Object.keys(pageLoader.loaded)).toEqual(routes)
+      expect(Object.keys(pageLoader.prefetched)).toEqual(routes)
     })
   })
 })