How To Use Lazy Loading Routes in Angular

Introduction

Lazy loading is an approach to limit the modules that are loaded to the ones that the user currently needs. This can improve your application’s performance and reduce the initial bundle size.
By default, Angular uses eager loading to load modules. This means that all the modules must be loaded before the application can be run. While this may be adequate for many use cases, there may be situations where this load time begins to affect performance.

Note: The following covers lazy loading modules in Angular 8+ apps.

In this article, you will use lazy loading routes in an Angular application.

Prerequisites

To complete this tutorial, you will need:

Node.js installed locally, which you can do by following How to Install Node.js and Create a Local Development Environment.
Some familiarity with setting up an Angular project.

This tutorial was verified with Node v16.4.0, npm v7.19.0, @angular/core v12.1.0, and @angular/router v12.1.0.

Step 1 – Setting Up the Project

Lazy loaded routes need to be outside of the root app module. You will want to have your lazy loaded features in feature modules.
First, let’s use Angular CLI to create a new project with Angular Router:

ng new angular-lazy-loading-example --routing --style=css --skip-tests

Then navigate to the new project directory:

cd angular-lazy-loading-example

Let’s create a new feature module:

ng generate module shop --route shop --module app.module

Now let’s also create 3 components inside our shop feature module:
The first will be a cart component:

ng generate component shop/cart

The second will be a checkout component:

ng generate component shop/checkout

The third will be a confirm component:

ng generate component shop/confirm

All three components will be located in the shop directory.

Note: Do not import feature modules that should be lazy-loaded in your app module, otherwise they will be eager loaded.

At this point, you should have a new Angular project with a shop module and 3 components.

Step 2 – Using loadChildren

In your main routing configuration, you will want to do something like the following:
src/app/app-routing.module.ts

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

const routes: Routes = [
  { path: 'shop', loadChildren: () => import('./shop/shop.module').then(m => m.ShopModule) },
];

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

New with Angular 8, loadChildren expects a function that uses the dynamic import syntax to import your lazy-loaded module only when it’s needed. The dynamic import is promise-based and gives you access to the module, where the module’s class can be called.

Step 3 – Setting Route Configuration in the Feature Module

Now all that’s left to do is to configure routes specific to the feature module.
Here’s an example:
src/app/shop/shop-routing.module.ts

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

import { CartComponent } from './cart/cart.component';
import { CheckoutComponent } from './checkout/checkout.component';
import { ConfirmComponent } from './confirm/confirm.component';

const routes: Routes = [
  { path: '', component: CartComponent },
  { path: 'checkout', component: CheckoutComponent },
  { path: 'confirm', component: ConfirmComponent },
];

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

And finally, in the feature module itself, you’ll include your routes with RouterModule’s forChild method instead of forRoot:
shop/shop.module.ts

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';

import { ShopRoutingModule } from './shop-routing.module';
import { ShopComponent } from './shop.component';
import { CartComponent } from './cart/cart.component';
import { CheckoutComponent } from './checkout/checkout.component';
import { ConfirmComponent } from './confirm/confirm.component';

@NgModule({
  declarations: [
    ShopComponent,
    CartComponent,
    CheckoutComponent,
    ConfirmComponent,
  ],
  imports: [
    CommonModule,
    ShopRoutingModule
  ]
})
export class ShopModule { }

Now you can use the routerLink directive to navigate to /shop, /shop/checkout, or /shop/confirm and the module will be loaded the first time one of these paths are navigated to.
In your terminal, start the server:

ng serve

This will generate a main.js file and a src_app_shop_shop_module_ts.js file:

Initial Chunk Files            | Names         |      Size
vendor.js                      | vendor        |   2.38 MB
polyfills.js                   | polyfills     | 128.58 kB
main.js                        | main          |  57.18 kB
runtime.js                     | runtime       |  12.55 kB
styles.css                     | styles        | 119 bytes

                               | Initial Total |   2.58 MB

Lazy Chunk Files               | Names         |      Size
src_app_shop_shop_module_ts.js | -             |  10.62 kB

Next, use your browser to visit localhost:4200.
Verify that lazy loading works by opening the browser’s DevTools and looking at the Network tab. When the application initially loads at the application root, you should not observe lazy chunk files. When you navigate to a route like /shop, you should observe src_app_shop_shop_module_ts.js.

Note: If it is not working right away, try restarting your server.

Your application now supports lazy loading.

Conclusion

In this article, you used lazy loading routes in an Angular application.
Continue your learning with testing components with dependencies, testing services as well as using mocks, stubs, and spies.
You can also refer to the official documentation for more information on lazy loading.