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Lazy Loading Feature Modules

High level view

By default, NgModules are eagerly loaded, which means that as soon as the app loads, so do all the NgModules, whether or not they are immediately necessary. For large apps with lots of routes, consider lazy loading—a design pattern that loads NgModules as needed. Lazy loading helps keep initial bundle sizes smaller, which in turn helps decrease load times.

For the final sample app with two lazy-loaded modules that this page describes, see the live example.

There are three main steps to setting up a lazy-loaded feature module:

  1. Create the feature module with the CLI, using the --route flag.
  2. Create the feature module’s component.
  3. Configure the routes.

Set up an app

If you don’t already have an app, you can follow the steps below to create one with the CLI. If you do already have an app, skip to Configure the routes. Enter the following command where customer-app is the name of your app:

ng new customer-app --routing

This creates an app called customer-app and the --routing flag generates a file called app-routing.module.ts, which is one of the files you need for setting up lazy loading for your feature module. Navigate into the project by issuing the command cd customer-app.

Create a feature module with routing

Next, you’ll need a feature module with a component to route to. To make one, enter the following command in the terminal, where customers is the name of the feature module, and customer-list is the route path for loading the customers component:

ng generate module customers --route customer-list --module app.module

This creates a customers folder with the new lazy-loadable module CustomersModule defined in the file customers.module.ts. The command automatically adds the CustomerComponent to the new feature module.

Because the new module is meant to be lazy-loaded, the command does NOT add a reference for the new feature module to the root application's module file, app.module.ts. Instead, it adds the declared route, customer-list to the Routes array declared in the module provided as the --module option.

const routes: Routes = [
    { path: 'customer-list',
      loadChildren: () => import('./customers/customers.module').then(m => m.CustomersModule) }
    ];

Notice that the lazy-loading syntax uses loadChildren followed by a function that uses the browser's built-in import('...') syntax for dynamic imports. The import path is the relative path to the module.

Add another feature module

Use the same command to create a second lazy-loaded feature module with routing, along with its stub component.

ng generate module orders --route order-list --module app.module

This creates a new folder called orders containing an OrdersModule and OrdersRoutingModule, along with the new OrderComponent source files. The order-list route is added to the Routes array in app-routing.module.ts, using the lazy-loading syntax.

const routes: Routes = [
    { path: 'customer-list',
      loadChildren: () => import('./customers/customers.module').then(m => m.CustomersModule) },
    { path: 'order-list',
      loadChildren: () => import('./orders/orders.module').then(m => m.OrdersModule) }
    ];

Set up the UI

Though you can type the URL into the address bar, a navigation UI is easier for the user and more common. Replace the default placeholder markup in app.component.html with a custom nav so you can easily navigate to your modules in the browser:

<h1>
  {{title}}
</h1>

<button routerLink="/customers">Customers</button>
<button routerLink="/orders">Orders</button>
<button routerLink="">Home</button>

<router-outlet></router-outlet>

To see your app in the browser so far, enter the following command in the terminal window:

ng serve

Then go to localhost:4200 where you should see “app works!” and three buttons.

These buttons work, because the CLI automatically added the routes to the feature modules to the routes array in app.module.ts.

Imports and route configuration

The CLI automatically added each feature module to the routes map at the application level. Finish this off by adding the default route. In AppRoutingModule, update the routes array with the following:

const routes: Routes = [
  {
    path: 'customers',
    loadChildren: () => import('./customers/customers.module').then(mod => mod.CustomersModule)
  },
  {
    path: 'orders',
    loadChildren: () => import('./orders/orders.module').then(mod => mod.OrdersModule)
  },
  {
    path: '',
    redirectTo: '',
    pathMatch: 'full'
  }
];

The first two paths are the routes to the CustomersModule and the OrdersModule. The final entry defines a default route. The empty path matches everything that doesn't match an earlier path.

Inside the feature module

Next, take a look at customers.module.ts. If you’re using the CLI and following the steps outlined in this page, you don’t have to do anything here.

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';
import { CustomersRoutingModule } from './customers-routing.module';
import { CustomerListComponent } from './customer-list/customer-list.component';

@NgModule({
  imports: [
    CommonModule,
    CustomersRoutingModule
  ],
  declarations: [CustomerListComponent]
})
export class CustomersModule { }

The customers.module.ts file imports the CustomersRoutingModule and CustomerListComponent so the CustomersModule class can have access to them. CustomersRoutingModule is then listed in the @NgModule imports array giving CustomersModule access to its own routing module, and CustomerListComponent is in the declarations array, which means CustomerListComponent belongs to the CustomersModule.

The feature module has its own routing module, customers-routing.module.ts. The AppRoutingModule imports the feature module, CustomersModule, and CustomersModule in turn imports the CustomersRoutingModule.

The feature-specific routing module imports its own feature component, CustomerListComponent, along with the other JavaScript import statements. It also adds the route to its own component.

import { NgModule } from '@angular/core';
import { Routes, RouterModule } from '@angular/router';

import { CustomerListComponent } from './customer-list/customer-list.component';


const routes: Routes = [
  {
    path: '',
    component: CustomerListComponent
  }
];

@NgModule({
  imports: [RouterModule.forChild(routes)],
  exports: [RouterModule]
})
export class CustomersRoutingModule { }

Notice that the path is set to an empty string. This is because the path in AppRoutingModule is already set to customers, so this route in the CustomersRoutingModule, is already within the customers context. Every route in this routing module is a child route.

The other feature module's routing module is configured similarly.

import { OrderListComponent } from './order-list/order-list.component';

const routes: Routes = [
  {
    path: '',
    component: OrderListComponent
  }
];

Confirm it’s working

You can check to see that a module is indeed being lazy loaded with the Chrome developer tools. In Chrome, open the dev tools by pressing Cmd+Option+i on a Mac or Ctrl+Shift+j on a PC and go to the Network Tab.

Click on the Orders or Customers button. If you see a chunk appear, everything is wired up properly and the feature module is being lazy loaded. A chunk should appear for Orders and for Customers but will only appear once for each.

To see it again, or to test after working in the project, clear everything out by clicking the circle with a line through it in the upper left of the Network Tab:

Then reload with Cmd+r or Ctrl+r, depending on your platform.

forRoot() and forChild()

You might have noticed that the CLI adds RouterModule.forRoot(routes) to the app-routing.module.ts imports array. This lets Angular know that this module, AppRoutingModule, is a routing module and forRoot() specifies that this is the root routing module. It configures all the routes you pass to it, gives you access to the router directives, and registers the RouterService. Use forRoot() in the AppRoutingModule—that is, one time in the app at the root level.

The CLI also adds RouterModule.forChild(routes) to feature routing modules. This way, Angular knows that the route list is only responsible for providing additional routes and is intended for feature modules. You can use forChild() in multiple modules.

The forRoot() method takes care of the global injector configuration for the Router. The forChild() method has no injector configuration. It uses directives such as RouterOutlet and RouterLink. For more information, see the forRoot() pattern section of the Singleton Services guide.

More on NgModules and routing

You may also be interested in the following:

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Licensed under the Creative Commons Attribution License 4.0.
https://angular.io/guide/lazy-loading-ngmodules