Lazy Loading Modules

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

1. Create the child module.
2. Create the child module’s routing module.
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 child module with routing
Next, you’ll need a child module to route to. To make one, enter the following command at the terminal window prompt where customers is the name of the module:

ng generate module customers --routing

This creates a customers folder with two files inside; CustomersModule and CustomersRoutingModule.
CustomersModule will act as the gatekeeper for anything that concerns customers. CustomersRoutingModule will handle any customer-related routing. This keeps the app’s structure organized as the app grows and allows you to reuse this module while easily keeping its routing intact.

The CLI imports the CustomersRoutingModule into the CustomersModule by adding a JavaScript import statement at the top of the file and adding CustomersRoutingModule to the @NgModule imports array.

Add a component to the child module
In order to see the module being lazy loaded in the browser, create a component to render some HTML when the app loads CustomersModule. At the command line, enter the following:

ng generate component customers/customer-list

This creates a folder inside of customers called customer-list with the four files that make up the component.

Just like with the routing module, the CLI imports the CustomerListComponent into the CustomersModule.

Add another child module
For another place to route to, create a second child module with routing:

ng generate module orders --routing

This makes a new folder called orders containing an OrdersModule and an OrdersRoutingModule.
Now, just like with the CustomersModule, give it some content:

ng generate component orders/order-list

Set up the UI
Though you can type the URL into the address bar, a nav 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:

src/app/app.component.html

<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.

To make the buttons work, you need to configure the routing modules.

Configure the routes
The two child modules, OrdersModule and CustomersModule, have to be wired up to the AppRoutingModule so the router knows about them. The structure is as follows:

Each child module acts as a doorway via the router. In the AppRoutingModule, you configure the routes to the child modules, in this case OrdersModule and CustomersModule. This way, the router knows to go to the child module. The child module then connects the AppRoutingModule to the CustomersRoutingModule or the OrdersRoutingModule. Those routing modules tell the router where to go to load relevant components.

Routes at the app level

In AppRoutingModule, update the routes array with the following:

src/app/app-routing.module.ts

const routes: Routes = [

  {

    path: 'customers',

    loadChildren: () => import('app/customers/customers-routing.module')                                                     .then(m => m.CustomersModule),

  },

  {

    path: 'orders',

    loadChildren: () => import('app/orders/orders.module').then(m => m.OrdersModule),

  },

  {

    path: '',

    redirectTo: '',

    pathMatch: 'full'

  }];

 The import statements stay the same. The first two paths are the routes to the CustomersModule and the OrdersModule respectively. Notice that the lazy loading syntax uses loadChildren followed by a string that is the path to the module, a hash mark or #, and the module’s class name.

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. The child module is like a connector between the AppRoutingModule and the child routing module. The AppRoutingModule imports the feature module, CustomersModule, and CustomersModule in turn imports the CustomersRoutingModule.

src/app/customers/customers.module.ts

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.

Configure the feature module’s routes
The next step is in customers-routing.module.ts. First, import the component at the top of the file with the other JavaScript import statements. Then, add the route to CustomerListComponent

src/app/customers/customers-routing.module.ts

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.

Repeat this last step of importing the OrdersListComponent and configuring the Routes array for the orders-routing.module.ts:

src/app/orders/orders-routing.module.ts (excerpt)

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

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

Now, if you view the app in the browser, the three buttons take you to each module.

it’s working
Now you can test your app.

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 child routing modules. This way, Angular knows that the route list is only responsible for providing additional routes and is intended for child modules. You can use forChild() in multiple modules.

forRoot() contains injector configuration which is global; such as configuring the Router. forChild() has no injector configuration, only directives such as RouterOutlet and RouterLink.

Reference: https://angular.io/