fix(router): throw an error when encounter undefined route (#12389)

modules/@angular/router/src/config.ts

@@ -350,10 +350,26 @@ export interface Route {
 }
 
 export function validateConfig(config: Routes): void {
-  config.forEach(validateNode);
+  // forEach doesn't iterate undefined values
+  for (let i = 0; i < config.length; i++) {
+    validateNode(config[i]);
+  }
 }
 
 function validateNode(route: Route): void {
+  if (!route) {
+    throw new Error(`
+      Invalid route configuration: Encountered undefined route.
+      The reason might be an extra comma.
+       
+      Example: 
+      const routes: Routes = [
+        { path: '', redirectTo: '/dashboard', pathMatch: 'full' },
+        { path: 'dashboard',  component: DashboardComponent },, << two commas
+        { path: 'detail/:id', component: HeroDetailComponent }
+      ];
+    `);
+  }
   if (Array.isArray(route)) {
     throw new Error(`Invalid route configuration: Array cannot be specified`);
   }

modules/@angular/router/src/router_module.ts

@@ -8,7 +8,6 @@
 
 import {APP_BASE_HREF, HashLocationStrategy, Location, LocationStrategy, PathLocationStrategy, PlatformLocation} from '@angular/common';
 import {ANALYZE_FOR_ENTRY_COMPONENTS, APP_BOOTSTRAP_LISTENER, ApplicationRef, Compiler, Inject, Injector, ModuleWithProviders, NgModule, NgModuleFactoryLoader, OpaqueToken, Optional, Provider, SkipSelf, SystemJsNgModuleLoader} from '@angular/core';
-
 import {Route, Routes} from './config';
 import {RouterLink, RouterLinkWithHref} from './directives/router_link';
 import {RouterLinkActive} from './directives/router_link_active';

modules/@angular/router/test/config.spec.ts

@@ -15,6 +15,12 @@ describe('config', () => {
       validateConfig([{path: 'a', redirectTo: 'b'}, {path: 'b', component: ComponentA}]);
     });
 
+    it('should throw for undefined route', () => {
+      expect(() => {
+        validateConfig([{path: 'a', component: ComponentA}, , {path: 'b', component: ComponentB}]);
+      }).toThrowError();
+    });
+
     it('should throw when Array is passed', () => {
       expect(() => {
         validateConfig([