Get rid of mitt.

lib/page-loader.js

@@ -1,6 +1,4 @@
 /* global window, document */
-import mitt from 'mitt'
-
 const webpackModule = module
 
 export default class PageLoader {
@@ -8,7 +6,6 @@ export default class PageLoader {
     this.buildId = buildId
     this.pageCache = {}
     this.pageLoadedHandlers = {}
-    this.registerEvents = mitt()
     this.loadingRoutes = {}
   }
 
@@ -32,37 +29,41 @@ export default class PageLoader {
       })
     }
 
-    return new Promise((resolve, reject) => {
-      const fire = ({ error, page }) => {
-        this.registerEvents.off(route, fire)
-
-        if (error) {
-          reject(error)
-        } else {
-          resolve(page)
-        }
+    if (this.loadingRoutes[route]) {
+      return this.loadingRoutes[route]
     }
 
-      this.registerEvents.on(route, fire)
+    const loadingPromise = new Promise((resolve, reject) => {
+      this.loadScript(route, (error) => {
+        delete this.loadingRoutes[route]
 
-      // Load the script if not asked to load yet.
-      if (!this.loadingRoutes[route]) {
-        this.loadScript(route)
-        this.loadingRoutes[route] = true
-      }
+        if (error) return reject(error)
+
+        const cachedPage = this.pageCache[route]
+        if (cachedPage.error) return reject(cachedPage.error)
+        return resolve(cachedPage.page)
       })
+    })
+
+    this.loadingRoutes[route] = loadingPromise
+    return loadingPromise
   }
 
-  loadScript (route) {
+  loadScript (route, fn) {
     route = this.normalizeRoute(route)
 
     const script = document.createElement('script')
     const url = `/_next/${encodeURIComponent(this.buildId)}/page${route}`
     script.src = url
     script.type = 'text/javascript'
-    script.onerror = () => {
-      const error = new Error(`Error when loading route: ${route}`)
-      this.registerEvents.emit(route, { error })
+
+    script.onerror = (e) => {
+      const error = new Error(`Network error occurred when loading route: ${route}`)
+      fn(error)
+    }
+
+    script.onload = () => {
+      fn()
     }
 
     document.body.appendChild(script)
@@ -76,7 +77,6 @@ export default class PageLoader {
 
         // add the page to the cache
         this.pageCache[route] = { error, page }
-        this.registerEvents.emit(route, { error, page })
       })
     }