Unleashing the Power of Angular: Mastering Lazy Loading

Introduction

Angular, an open-source framework developed by Google, has become a popular choice for creating high-performance, user-friendly web applications. One of its prominent features is lazy loading , a design pattern that defers the initialization of an object until it's needed. This blog will delve into the concept of lazy loading, why it's beneficial, and how to implement it in Angular.

What is Lazy Loading?

In the context of web applications, lazy loading is a technique where you delay loading some parts of your application until they're required by the user. This technique not only improves the initial loading speed of your application but also optimizes the usage of system resources.

Benefits of Lazy Loading

Here are a few reasons why you should consider using lazy loading in your Angular applications:

1. Improved performance : By only loading the necessary components or modules when required, you can drastically reduce the initial load time of your application. This can lead to a smoother user experience, especially for large applications.

2. Optimized resource usage : Lazy loading can significantly reduce the amount of memory your application uses, as components and modules are only loaded into memory when necessary.

3. Ease of maintenance : With lazy loading, you can work on individual components or modules without having to load the entire application, making your application easier to maintain and develop.

Implementing Lazy Loading in Angular

Now let's go through a step-by-step guide on how to implement lazy loading in an Angular application. For this tutorial, we'll assume you have basic knowledge of Angular and have Angular CLI installed.

Step 1: Create a new Angular application

To start, we need an Angular application. You can create a new one using Angular CLI with the following command:

codeng new lazy-loading-app 

Step 2: Generate modules

Now, let's create two modules for our application, HomeModule and UsersModule , which we'll lazy load. Use the Angular CLI to generate these modules:

codeng generate module home --route home --module app.module 
ng generate module users --route users --module app.module 

This command will create home and users modules, and it'll also create routing modules ( home-routing.module.ts and users-routing.module.ts ) for them. The --route flag specifies the default route path, and the --module flag denotes which module the routing module should be registered with (in this case, app.module ).

Step 3: Configure Routes

Next, we configure routes to use lazy loading. Angular uses the loadChildren property to set up routes for lazy loaded modules. Here's how you set it up in app-routing.module.ts :

codeconst routes: Routes = [ 
    { path: 'home', loadChildren: () => import('./home/home.module').then(m => m.HomeModule) }, 
    { path: 'users', loadChildren: () => import('./users/users.module').then(m => m.UsersModule) }, 
    { path: '', redirectTo: 'home', pathMatch: 'full' }, 
]; 

This configures Angular to load the HomeModule and UsersModule only when the user navigates to /home or /users , respectively.

Step 4: Add components and links

For each of your lazy-loaded modules, create a component and add some dummy content:

codeng generate component home/home-content 
ng generate component users/users-content 

Then, create links to these routes in your app.component.html :

code<a routerLink="/home">Home</a> 
<a routerLink="/users">Users</a> 
<router-outlet></router-outlet> 

The routerLink directive is used to link to routes, and the router-outlet directive is where the content of each route is displayed.

And that's it! You have now implemented lazy loading in your Angular application. When you start your application and navigate to /home or /users , Angular will lazy load the corresponding module.

Preloading Strategy

While lazy loading provides significant performance improvements, it has one drawback: when a user navigates to a lazy-loaded module for the first time, they have to wait for the module to load. Preloading is a strategy that you can use to overcome this issue.

Angular provides two preloading strategies out-of-the-box:

1. NoPreloading (default): This strategy doesn't preload any modules. This is the default strategy when you enable lazy loading.

2. PreloadAllModules : This strategy preloads all lazy-loaded modules after all eager-loaded modules are loaded.

You can specify a preloading strategy in your routing configuration:

codeimport { PreloadAllModules } from '@angular/router'; 
        
@NgModule({ 
    imports: [ 
        RouterModule.forRoot(routes, { preloadingStrategy: PreloadAllModules }) 
    ], 
    exports: [RouterModule] 
}) 
    
export class AppRoutingModule { } 

Here, after all the eager-loaded modules are loaded, Angular will start preloading all the lazy-loaded modules.

You can also create your own custom preloading strategies. For example, you can create a preloading strategy that only preloads specific modules or preloads modules during idle time.

Shared Modules

When you're using lazy loading, you might have components, directives, and pipes that are used in multiple modules. If you declare these in each lazy-loaded module, they'll be duplicated in each chunk, increasing the size of your application.

To avoid this, you can create a shared module where you declare these components, directives, and pipes, and then import the shared module into each lazy-loaded module.

Here's how you can create a shared module:

codeng generate module shared 

In shared.module.ts , declare and export the components, directives, and pipes that are used in multiple modules:

codeimport { CommonModule } from '@angular/common'; 
import { MyComponent } from './my.component'; 

@NgModule({ 
    declarations: [MyComponent], 
    imports: [CommonModule], 
    exports: [MyComponent] 
}) 

export class SharedModule { } 

Then, import the SharedModule into each of your lazy-loaded modules:

codeimport { SharedModule } from '../shared/shared.module'; 
        
@NgModule({ 
    imports: [SharedModule] 
}) 

export class HomeModule { } 

This way, you avoid duplicating code in your application, leading to smaller bundle sizes and better performance.

Route Guards with Lazy-Loaded Modules

In a real-world Angular application, there will often be a need for route guards, which allow or disallow users to access certain routes based on specific conditions. For example, you might have a route that should only be accessible to authenticated users.

In the context of lazy loading, the use of route guards is especially relevant. You might wonder: what if you don’t want to load a module until a condition has been satisfied? This is where canLoad guard comes in.

canLoad guard checks whether a module can be loaded or not, which is perfect for lazy loaded routes. If canLoad guard returns false , the bundle won't be loaded at all.

Here's a simple canLoad guard implementation:

codeimport { Injectable } from '@angular/core'; 
import { CanLoad, Route, UrlSegment } from '@angular/router'; 

@Injectable({ 
    providedIn: 'root' 
}) 

export class AuthGuard implements CanLoad { 
    userIsAuthenticated = false; // This would typically check an authentication service 
    
    canLoad(route: Route, segments: UrlSegment[]): boolean { 
        return this.userIsAuthenticated; 
    } 
} 

You can then use the AuthGuard in your routing module like so:

code{ 
    path: 'users', 
    loadChildren: () => import('./users/users.module').then(m => m.UsersModule), 
    canLoad: [AuthGuard] 
} 

This will prevent the UsersModule from being loaded unless userIsAuthenticated is true.

Lazy Loading and Ahead-of-Time (AOT) Compilation

Angular provides two types of compilation processes - Just-in-Time (JIT) and Ahead-of-Time (AOT). JIT compilation compiles your application in the browser at runtime, while AOT compilation compiles your application at build time.

When you use lazy loading, it's important to consider AOT compilation, as it can provide significant performance improvements. With AOT, Angular compiles your code and generates JavaScript before the browser downloads and runs it. This leads to faster rendering, fewer asynchronous requests, and smaller Angular framework download size because unnecessary parts are removed.

To enable AOT compilation, you can use the --aot option with the ng build or ng serve commands:

codeng build --aot 
ng serve --aot 

When AOT is enabled, Angular will compile the code for your eager-loaded modules at build time and generate separate bundles for your lazy-loaded modules that are compiled when they're loaded.

This is particularly important for large Angular applications, where AOT compilation can significantly improve performance.

Conclusion

As we reach the conclusion of this in-depth exploration of lazy loading in Angular, it becomes evident how powerful this design pattern can be. By implementing lazy loading, we can vastly enhance the performance of our Angular applications, improving user experience and system resource usage.

But it doesn't stop there. We delved into advanced concepts such as preloading strategies to combat initial load time drawbacks, shared modules to prevent duplication and further optimize load time, using route guards for conditional module loading, and the impact of AOT compilation on lazy loading.

Although these topics may seem complex, they are fundamental in mastering Angular and developing efficient, high-performance applications. With constant learning and practice, you can utilize these techniques effectively and become proficient in Angular application development.

Remember, the journey of mastering any framework is a continuous learning experience. Don't be afraid to explore and experiment with these concepts. Understanding and implementing such techniques will only bring you one step closer to becoming an Angular expert.

Keep coding, stay curious, and continue pushing your boundaries. Happy coding!