Lazy loading modules with routes in Angular

Introduction

Lazy loading helps your app load faster by only loading parts when needed. It saves data and improves user experience.

When your app has many pages or features that users may not visit right away.

To speed up the initial loading time of your app.

When you want to organize your app into smaller, manageable parts.

To reduce memory use by loading code only when required.

Syntax

Angular

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

The loadChildren property tells Angular to load the module only when the route is visited.

Use a function that returns a dynamic import() statement for lazy loading.

Examples

Loads the Dashboard module only when the user visits '/dashboard'.

Angular

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

Redirects empty path to 'home' and lazy loads the Home module.

Angular

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

Sample Program

This example shows an Angular app routing setup where the 'products' module is lazy loaded. The main router loads the ProductsModule only when the user visits '/products'. The ProductsModule has its own route and component.

Angular

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

const routes: Routes = [
  {
    path: 'products',
    loadChildren: () => import('./products/products.module').then(m => m.ProductsModule)
  },
  {
    path: '',
    redirectTo: 'products',
    pathMatch: 'full'
  }
];

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

// products.module.ts
import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';
import { RouterModule, Routes } from '@angular/router';
import { ProductsComponent } from './products.component';

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

@NgModule({
  declarations: [ProductsComponent],
  imports: [CommonModule, RouterModule.forChild(routes)]
})
export class ProductsModule {}

// products.component.ts
import { Component } from '@angular/core';

@Component({
  selector: 'app-products',
  template: `<h2>Products Page Loaded Lazily!</h2>`
})
export class ProductsComponent {}

Important Notes

Lazy loading works best with feature modules, not root modules.

Make sure the lazy loaded module has its own routing configured with RouterModule.forChild().

Use browser DevTools Network tab to see when modules load.

Summary

Lazy loading delays loading parts of your app until needed.

Use loadChildren with dynamic import() in routes.

This improves app speed and user experience.

Practice

1. What is the main purpose of lazy loading modules in Angular routing?

easy

A. To preload all modules when the app starts

B. To load modules only when the user navigates to their route

C. To load modules in alphabetical order

D. To load all modules at once for faster navigation

Solution

1. Step 1: Understand lazy loading concept

   Lazy loading means delaying the loading of a module until it is needed, usually when the user navigates to a route that requires it.

2. Step 2: Identify the purpose in routing

   In Angular routing, lazy loading helps improve app speed by loading modules only on demand, not all at once.

3. Final Answer:

   To load modules only when the user navigates to their route -> Option B

4. Quick Check:

   Lazy loading = load on demand [OK]

Hint: Lazy loading means load modules only when needed [OK]

Common Mistakes:

• Thinking lazy loading loads all modules at start
• Confusing lazy loading with preloading
• Assuming lazy loading loads modules alphabetically

2. Which of the following is the correct syntax to lazy load a module named AdminModule in Angular routes?

easy

A. { path: 'admin', loadChildren: import('./admin/admin.module') }

B. { path: 'admin', loadChildren: './admin/admin.module#AdminModule' }

C. { path: 'admin', component: AdminModule }

D. { path: 'admin', loadChildren: () => import('./admin/admin.module').then(m => m.AdminModule) }

Solution

1. Step 1: Recall Angular lazy loading syntax

   Angular uses dynamic import with a function returning a promise that resolves to the module class.

2. Step 2: Match syntax to options

   { path: 'admin', loadChildren: () => import('./admin/admin.module').then(m => m.AdminModule) } uses the correct arrow function with import and then to get AdminModule. { path: 'admin', loadChildren: './admin/admin.module#AdminModule' } is old deprecated string syntax. Options C and D are incorrect usage.

3. Final Answer:

   { path: 'admin', loadChildren: () => import('./admin/admin.module').then(m => m.AdminModule) } -> Option D

4. Quick Check:

   Dynamic import with arrow function = correct syntax [OK]

Hint: Use arrow function with import().then() for lazy loading [OK]

Common Mistakes:

• Using deprecated string syntax for loadChildren
• Assigning component instead of loadChildren
• Omitting the arrow function wrapper

3. Given this route configuration snippet:

{ path: 'shop', loadChildren: () => import('./shop/shop.module').then(m => m.ShopModule) }

What happens when the user navigates to /shop for the first time?

medium

A. The ShopModule is already loaded at app start, so navigation is instant

B. Navigation fails because loadChildren requires a component

C. The ShopModule is loaded asynchronously and its routes are activated

D. The entire app reloads to load ShopModule

Solution

1. Step 1: Understand lazy loading behavior on navigation

   When navigating to a lazy loaded route for the first time, Angular loads the module asynchronously.

2. Step 2: Identify what happens on first navigation

   The ShopModule is fetched, loaded, and its routes become active, enabling the user to see the shop page.

3. Final Answer:

   The ShopModule is loaded asynchronously and its routes are activated -> Option C

4. Quick Check:

   First navigation triggers async module load [OK]

Hint: First visit loads module asynchronously, not at app start [OK]

Common Mistakes:

• Assuming module loads at app start
• Confusing loadChildren with component property
• Thinking app reloads on lazy load

4. Identify the error in this Angular route configuration for lazy loading:

{ path: 'dashboard', loadChildren: './dashboard/dashboard.module#DashboardModule' }

medium

A. Using string syntax for loadChildren is deprecated and causes errors

B. Missing component property for the route

C. Path should be capitalized as 'Dashboard'

D. loadChildren should be a component, not a string

Solution

1. Step 1: Check loadChildren syntax

   The string syntax with '#ModuleName' is deprecated in Angular and may cause errors in recent versions.

2. Step 2: Identify correct syntax

   Modern Angular requires loadChildren to be a function returning a dynamic import promise, not a string.

3. Final Answer:

   Using string syntax for loadChildren is deprecated and causes errors -> Option A

4. Quick Check:

   Use dynamic import function, not string [OK]

Hint: Avoid string syntax; use dynamic import function [OK]

Common Mistakes:

• Using old string syntax for loadChildren
• Confusing component and loadChildren properties
• Ignoring case sensitivity in paths

5. You want to lazy load a feature module ProfileModule only when the user navigates to /profile. The module has child routes. Which is the best way to configure the main app routing to achieve this and ensure accessibility and performance?

hard

A. { path: 'profile', loadChildren: () => import('./profile/profile.module').then(m => m.ProfileModule), canActivate: [AuthGuard] }

B. { path: 'profile', component: ProfileModule }

C. { path: 'profile', loadChildren: () => import('./profile/profile.module').then(m => m.ProfileModule) }

D. { path: 'profile', loadChildren: './profile/profile.module#ProfileModule' }

Solution

1. Step 1: Identify modern lazy loading syntax

   The correct syntax uses dynamic import with an arrow function returning a promise: loadChildren: () => import('./profile/profile.module').then(m => m.ProfileModule).

2. Step 2: Consider best practices for accessibility and performance

   For protected features like a user profile, adding canActivate: [AuthGuard] ensures secure access while lazy loading maintains performance.

3. Step 3: Eliminate incorrect configurations

   Using component property fails for modules with child routes. String syntax like './profile/profile.module#ProfileModule' is deprecated.

4. Final Answer:

   { path: 'profile', loadChildren: () => import('./profile/profile.module').then(m => m.ProfileModule), canActivate: [AuthGuard] } -> Option A

5. Quick Check:

   Lazy load with guard for best practice [OK]

Hint: Lazy load with import and add guards for security [OK]

Common Mistakes:

• Using component instead of loadChildren for modules
• Using deprecated string syntax
• Forgetting route guards for protected routes