Preloading strategies in Angular - Mini Project: Build & Apply

Implementing Preloading Strategies in Angular

📖 Scenario: You are building an Angular application with multiple feature modules. To improve user experience, you want to preload some modules after the app loads so navigation feels faster.

🎯 Goal: Learn how to set up Angular routing with lazy-loaded feature modules and apply different preloading strategies to optimize loading behavior.

📋 What You'll Learn

Create an Angular routing module with two lazy-loaded feature modules named AdminModule and UserModule

Add a configuration variable called preloadStrategy to select the preloading strategy

Use the RouterModule.forRoot() method with the preloadingStrategy option to apply the selected preloading strategy

Complete the routing module setup to export the configured RouterModule

💡 Why This Matters

🌍 Real World

Preloading strategies help Angular apps load feature modules in the background, improving navigation speed and user experience.

💼 Career

Understanding Angular routing and preloading is essential for frontend developers building scalable, performant single-page applications.

1

Set up lazy-loaded routes

Create a constant called routes that is an array with two route objects. The first route has path set to 'admin' and lazy loads AdminModule using loadChildren: () => import('./admin/admin.module').then(m => m.AdminModule). The second route has path set to 'user' and lazy loads UserModule similarly.

Angular

# Your code here to create the routes array with lazy-loaded AdminModule and UserModule

Hint

Use the loadChildren property with dynamic imports to lazy load modules.

2

Add preloading strategy configuration

Create a constant called preloadStrategy and set it to PreloadAllModules imported from @angular/router.

Angular

import { PreloadAllModules } from '@angular/router';

const routes: Routes = [
  { path: 'admin', loadChildren: () => import('./admin/admin.module').then(m => m.AdminModule) },
  { path: 'user', loadChildren: () => import('./user/user.module').then(m => m.UserModule) }
];

# Define preloadStrategy constant here

Hint

Import PreloadAllModules and assign it to preloadStrategy.

3

Configure RouterModule with preloading strategy

Import RouterModule from @angular/router and create a constant called AppRoutingModule by calling RouterModule.forRoot(routes, { preloadingStrategy }).

Angular

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

const routes: Routes = [
  { path: 'admin', loadChildren: () => import('./admin/admin.module').then(m => m.AdminModule) },
  { path: 'user', loadChildren: () => import('./user/user.module').then(m => m.UserModule) }
];

const preloadStrategy = PreloadAllModules;

# Create AppRoutingModule using RouterModule.forRoot with routes and preloadStrategy

Hint

Use RouterModule.forRoot with the preloadingStrategy option.

4

Export the routing module

Export the constant AppRoutingModule using export { AppRoutingModule }; to complete the routing setup.

Angular

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

const routes: Routes = [
  { path: 'admin', loadChildren: () => import('./admin/admin.module').then(m => m.AdminModule) },
  { path: 'user', loadChildren: () => import('./user/user.module').then(m => m.UserModule) }
];

const preloadStrategy = PreloadAllModules;

const AppRoutingModule = RouterModule.forRoot(routes, { preloadingStrategy });

# Export AppRoutingModule here

Hint

Use the export keyword to make AppRoutingModule available for import.

Practice

1. What is the main purpose of preloading strategies in Angular?

easy

A. To load lazy modules in the background to improve navigation speed

B. To prevent any modules from loading until explicitly requested

C. To compile all modules at build time for faster startup

D. To automatically update Angular to the latest version

Solution

1. Step 1: Understand lazy loading in Angular

   Lazy loading delays loading modules until needed, which can slow navigation initially.

2. Step 2: Role of preloading strategies

   Preloading strategies load lazy modules in the background after app startup to speed up future navigation.

3. Final Answer:

   To load lazy modules in the background to improve navigation speed -> Option A

4. Quick Check:

   Preloading = background loading for faster navigation [OK]

Hint: Preloading means loading modules quietly before needed [OK]

Common Mistakes:

• Confusing preloading with eager loading
• Thinking preloading disables lazy loading
• Assuming preloading updates Angular versions

2. Which of the following is the correct way to enable preloading of all lazy modules in Angular routing?

easy

A. RouterModule.forRoot(routes, { preloadingStrategy: PreloadAllModules })

B. RouterModule.forRoot(routes, { preload: true })

C. RouterModule.forRoot(routes, { lazyLoad: true })

D. RouterModule.forRoot(routes, { preloadingStrategy: NoPreloading })

Solution

1. Step 1: Identify Angular's preloading strategy option

   Angular uses the preloadingStrategy property in router config to set preloading behavior.

2. Step 2: Recognize the correct strategy for all modules

   PreloadAllModules is the built-in strategy to preload all lazy modules after app loads.

3. Final Answer:

   RouterModule.forRoot(routes, { preloadingStrategy: PreloadAllModules }) -> Option A

4. Quick Check:

   PreloadAllModules = preload all lazy modules [OK]

Hint: Use preloadingStrategy: PreloadAllModules to preload all [OK]

Common Mistakes:

• Using 'preload: true' which is invalid
• Confusing NoPreloading with preloading all
• Using lazyLoad option which doesn't exist

3. Given this Angular routing setup:

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

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

What happens after the app loads?

medium

A. Only 'home' module loads eagerly, 'admin' loads on demand

B. 'admin' module preloads, 'home' module does not

C. Neither module preloads; both load on demand

D. Both 'home' and 'admin' modules preload in the background

Solution

1. Step 1: Analyze the preloading strategy used

   The routing uses PreloadAllModules, which preloads all lazy modules after app startup.

2. Step 2: Identify lazy loaded modules

   Both 'home' and 'admin' modules are lazy loaded via loadChildren.

3. Final Answer:

   Both 'home' and 'admin' modules preload in the background -> Option D

4. Quick Check:

   PreloadAllModules preloads all lazy modules [OK]

Hint: PreloadAllModules preloads every lazy module automatically [OK]

Common Mistakes:

• Assuming only first lazy module preloads
• Thinking modules load only on demand with PreloadAllModules
• Confusing eager loading with preloading

4. Identify the error in this Angular routing configuration:

const routes = [
  { path: 'dashboard', loadChildren: './dashboard/dashboard.module#DashboardModule' }
];

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

medium

A. Missing 'pathMatch' property in route

B. Using string syntax for loadChildren is deprecated; should use dynamic import

C. Preloading strategy must be NoPreloading when using lazy loading

D. RouterModule.forRoot should not have a second argument

Solution

1. Step 1: Check loadChildren syntax

   The string syntax ('./dashboard/dashboard.module#DashboardModule') is deprecated in Angular; dynamic import is required.

2. Step 2: Confirm preloading strategy and other options

   Preloading strategy can be used with lazy loading; no error there. 'pathMatch' is optional here.

3. Final Answer:

   Using string syntax for loadChildren is deprecated; should use dynamic import -> Option B

4. Quick Check:

   Use dynamic import() for loadChildren [OK]

Hint: Use dynamic import() syntax for loadChildren [OK]

Common Mistakes:

• Using old string syntax for loadChildren
• Thinking preloading strategy conflicts with lazy loading
• Assuming pathMatch is mandatory for all routes

5. You want to preload only the 'admin' module but not the 'user' module in your Angular app. Which approach correctly implements this custom preloading strategy?

hard

A. Set preloadingStrategy to NoPreloading and preload 'admin' module eagerly

B. Use PreloadAllModules and then lazy load 'user' module manually

C. Create a class implementing PreloadingStrategy that preloads only routes with data.preload = true

D. Mark 'admin' module as eager loaded and 'user' as lazy loaded without preloading

Solution

1. Step 1: Understand custom preloading strategy

   Angular allows creating a class implementing PreloadingStrategy to control which modules preload.

2. Step 2: Use route data property to select modules

   By checking data.preload flag in routes, the strategy can preload only selected modules like 'admin'.

3. Final Answer:

   Create a class implementing PreloadingStrategy that preloads only routes with data.preload = true -> Option C

4. Quick Check:

   Custom strategy uses data.preload flag [OK]

Hint: Use data.preload flag with custom PreloadingStrategy [OK]

Common Mistakes:

• Using PreloadAllModules when selective preload is needed
• Trying to preload modules eagerly without strategy
• Confusing eager loading with preloading