Lazy Loading in Angular

What is Lazy Loading?

Lazy loading in Angular is a technique to load only the required pages. Isn't it amazing? You might have faced difficulty viewing pages having a lot of media elements. It's because as soon as the application opens all the modules, components, images, and videos start loading immediately. This is eager loading. Lazy loading is the opposite of it. It will load the elements when you are about to scroll them thus saving bandwidth and load time.

In this Angular tutorial, we'll learn this important technique thoroughly. We'll see the implementation, benefits, use cases, etc. of lazy loading along with the comparison of lazy loading and eager loading. So, go along with us and understand the concept.

Read More: Top 50+ Angular Interview Questions & Answers

How Lazy Loading Works?

Angular performs lazy loading by using the Angular Router. When a user navigates to a specific route associated with a lazy-loaded module, Angular fetches and loads that module immediately

To lazy load Angular modules, use loadChildren in your AppRoutingModule routes configuration as follows.

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

Add a route for the component in the lazy-loaded routing module.

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

Don't forget to remove the ItemsModule from the AppModule.

Implementing Lazy Loading in Angular

Prerequisites

1. Node.js must have been installed.
2. Basic knowledge of Angular

1. Create an Angular Project

Use the Angular CLI to create your project. Install the CLI using npm by running the command:

npm install -g @angular/cli

Now create a project named Lazy Loading Example

ng new lazy-loading-example --routing

The Angular project named Lazy Loading Example was created. The src/app folder contains the code for your app. The main routing file, app-routing.module.ts is located in this folder.

2. Create a Feature Module with Routes

This feature module will load lazily. Run the command

ng generate module blog --route blog --module app.module

This command creates a module named BlogModule, along with routing. You can see this in src/app/app-routing.module.ts:

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

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

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

In the above code, the following line defines the routes for lazy loading:

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

The module defines its child routes, such as blog/**, in its routing.module.ts file. The blog module you generated looks like this:

import { NgModule } from '@angular/core';
import { RouterModule, Routes } from '@angular/router';
import { BlogComponent } from './blog.component';

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

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

The above routing file contains a single route, ''. This resolves for /blog and points to the BlogComponent. You can add more components and define those routes in this file.

For example, to add a component that would pull details about a particular blog post, you could create the component with this command:

ng generate component blog/detail

This generates the component for the blog detail and adds it to the blog module. To add a route for it, you can add it to your routes array:

const routes: Routes = [{ path: '', component: BlogComponent },
                        {path:"/:title",component: DetailComponent}];

Verify Lazy Loading

You can easily check whether lazy loading is working or not. For this, run the below command:

ng serve

You will get the following output

You can observe in the above output that Initial Chunk Files are loaded when the page first loads. Lazy Chunk Files are lazy-loaded. The blog module is listed in this part.

Benefits of Lazy Loading in Angular

• Improved initial load time: As everything doesn't load at the beginning the application loads fast.
• Less memory usage: Only required modules and components are loaded hence less memory consumption.
• Organized code: There is modularity in the code hence easy to maintain.

Lazy Loading Vs. Eager Loading

Lazy Loading in Angular: Examples and Use Cases

• Feature Modules

  Break down your application into feature modules (e.g., user management, shopping cart).

• Large and Infrequently Used Features

  If a feature is complex or rarely accessed (e.g., admin dashboard), lazy load it to reduce initial load time.

• Dynamic Content

  Load content based on user interaction (e.g., clicking a tab to display a specific section).

• Code Splitting for Improved Build Times

  Lazy loading modules help break down the overall application bundle for faster build and development cycles.

Summary

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