title: angular6 feature module lazy loading throwing error TypeError: undefined is not a function
body: <p>I have this code in <code>app-routing.module.ts</code>,
  as per the new documentation in angular I went through the method still it's not working, throwing some errors I can't understand.</p>
  
  <pre><code>import { NgModule } from '@angular/core';
  import { CommonModule } from '@angular/common';
  import { RouterModule, Routes } from "@angular/router";
  import { HomeComponent } from "./home/home.component";
  import { AdminModule } from "./admin/admin.module";
  
  const routes: Routes = [
    {
      path: 'admin',
      loadChildren: './admin/admin.module#AdminModule'
    },
    { path: '', redirectTo: '/home', pathMatch: 'full' },
    { path: 'home', component: HomeComponent },
  ];
  
  @NgModule({
    imports: [
      CommonModule,
      RouterModule.forRoot(routes)
    ],
    exports: [
      RouterModule
    ],
    declarations: []
   })
   export class AppRoutingModule { }
  </code></pre>
  
  <p>It's throwing an error like this.</p>
  
  <p>I have also tried the old way of loadchildren like this <code>'app/admin/admin.module#AdminModule'</code>. But it's still not working.</p>
  
  <pre><code>core.js:1601 ERROR Error: Uncaught (in promise): TypeError: undefined is not a function
  TypeError: undefined is not a function
      at Array.map (&lt;anonymous&gt;)
      at webpackAsyncContext ($_lazy_route_resource lazy namespace object:18)
      at SystemJsNgModuleLoader.push../node_modules/@angular/core/fesm5/core.js.SystemJsNgModuleLoader.loadAndCompile (core.js:5569)
      at SystemJsNgModuleLoader.push../node_modules/@angular/core/fesm5/core.js.SystemJsNgModuleLoader.load (core.js:5561)
      at RouterConfigLoader.push../node_modules/@angular/router/fesm5/router.js.RouterConfigLoader.loadModuleFactory (router.js:3294)
      at RouterConfigLoader.push../node_modules/@angular/router/fesm5/router.js.RouterConfigLoader.load (router.js:3282)
      at MergeMapSubscriber.project (router.js:1479)
      at MergeMapSubscriber.push../node_modules/rxjs/_esm5/internal/operators/mergeMap.js.MergeMapSubscriber._tryNext (mergeMap.js:117)
      at MergeMapSubscriber.push../node_modules/rxjs/_esm5/internal/operators/mergeMap.js.MergeMapSubscriber._next (mergeMap.js:107)
      at MergeMapSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.next (Subscriber.js:93)
      at Array.map (&lt;anonymous&gt;)
      at webpackAsyncContext ($_lazy_route_resource lazy namespace object:18)
      at SystemJsNgModuleLoader.push../node_modules/@angular/core/fesm5/core.js.SystemJsNgModuleLoader.loadAndCompile (core.js:5569)
      at SystemJsNgModuleLoader.push../node_modules/@angular/core/fesm5/core.js.SystemJsNgModuleLoader.load (core.js:5561)
      at RouterConfigLoader.push../node_modules/@angular/router/fesm5/router.js.RouterConfigLoader.loadModuleFactory (router.js:3294)
      at RouterConfigLoader.push../node_modules/@angular/router/fesm5/router.js.RouterConfigLoader.load (router.js:3282)
      at MergeMapSubscriber.project (router.js:1479)
      at MergeMapSubscriber.push../node_modules/rxjs/_esm5/internal/operators/mergeMap.js.MergeMapSubscriber._tryNext (mergeMap.js:117)
      at MergeMapSubscriber.push../node_modules/rxjs/_esm5/internal/operators/mergeMap.js.MergeMapSubscriber._next (mergeMap.js:107)
      at MergeMapSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.next (Subscriber.js:93)
      at resolvePromise (zone.js:814)
      at resolvePromise (zone.js:771)
      at zone.js:873
      at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (zone.js:421)
      at Object.onInvokeTask (core.js:4062)
      at ZoneDelegate.push../node_modules/zone.js/dist/zone.js.ZoneDelegate.invokeTask (zone.js:420)
      at Zone.push../node_modules/zone.js/dist/zone.js.Zone.runTask (zone.js:188)
      at drainMicroTaskQueue (zone.js:595)
      at ZoneTask.push../node_modules/zone.js/dist/zone.js.ZoneTask.invokeTask [as invoke] (zone.js:500)
      at invokeTask (zone.js:1540)
  </code></pre>