How to Lazy Load Components in Angular 4 in Three Steps

Lazy loading is a technique in Angular that allows you to load JavaScript components asynchronously when a specific route is activated. This can add some initial performance during the initial load, especially if you have many components with complex routing. There are some good posts about lazy loading in angular, but I wanted to simplify it further. This lesson will show you how to enable lazy loading in 3 easy steps with a brand new app.

Step 1 - Create a new App with Routing

Our app will load the AppComponent by default at the root URL, then when the user navigates to lazy/load-me, the lazy module will be loaded asynchronously.

ng new lazyDemo --routing

Now add a link to the lazy url in the app component.

<button routerLink="/lazy/load-me"></button>
<router-outlet></router-outlet>

Step 2 - Generate the “Lazy” Module

Let’s create a module that will be lazy loaded, along with a couple components. The --flat flag prevents a directory from being created, then we can easily add components to the module via the Angular CLI.

ng g module lazy --flat
ng g component lazy-parent --module lazy
ng g component lazy-child --module lazy

Inside the lazy module we need to import the RouterModule. Two things to notice here:

1. The route path to the component is path: 'load-me', even though it will actually be activated on lazy/load-me. This is a child route, which you will see come together in step 3
2. The RouterModule calls forChild(routes).

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';
import { LazyParentComponent } from './lazy-parent/lazy-parent.component';
import { LazyChildComponent } from './lazy-child/lazy-child.component';

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

const routes: Routes = [
    { path: 'load-me', component: LazyParentComponent }
];

@NgModule({
  imports: [
    CommonModule,
    RouterModule.forChild(routes)
  ],
  declarations: [
    LazyParentComponent,
    LazyChildComponent
  ]
})
export class LazyModule { }

And here’s what the LazyParentComponent looks like. Just looping over the child component a few times.

<div *ngFor="let name of ['Foo', 'Bar', 'Baz']">
  <p>Hi, my name is {{name}}. I'm a lazy child component.</p>

  <lazy-child></lazy-child>

</div>

Your Angular CLI config might append an app prefix to the component selector name. If so, make sure remove the prefix and for the last section to work properly. For example…

@Component({ selector: 'lazy-child' })

Step 3 - Point the App Router to the Lazy Module

The final step is to point the lazy route to the lazy module from the app router. We can do this with the loadChildren property with the path to the module file, then reference the module itself with a hash #. This tells angular to only load LazyModule when the lazy url is activated.

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

const routes: Routes = [
    { path: 'lazy', loadChildren: './lazy.module#LazyModule'}
];

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

Verify Lazy Loading is Working

Let’s make sure Lazy Loading is working. In chrome, open developer tools and click the network tab. When you navigate to the lazy url, you should see a 0.chunk.js file rendered. In this demo, you can see it took 2ms to load.

The lazy loaded module is the last chunk.js file in the chrome network logs

When you take a look inside this file, you should see a bunch of webpack JavaScript code with related to the lazy module and its components.

Looking inside, we see that is a bunch of JavaScript webpack code

That’s it for lazy loading components in Angular. Good luck!