Shared modules for reusable components in Angular - Performance & Optimization

Performance: Shared modules for reusable components

MEDIUM IMPACT

This affects initial page load speed and bundle size by controlling how reusable components are loaded and shared across the app.

Reusing components across multiple feature modules

Angular

import { CommonModule } from '@angular/common';
import { ComponentA } from './component-a.component';

@NgModule({
  declarations: [ComponentA],
  imports: [CommonModule],
  exports: [ComponentA]
})
export class SharedModule {}

@NgModule({
  imports: [CommonModule, SharedModule]
})
export class FeatureModule1 {}

@NgModule({
  imports: [CommonModule, SharedModule]
})
export class FeatureModule2 {}

ComponentA is declared once in SharedModule and imported where needed, avoiding duplication and reducing bundle size.

📈 Performance GainSaves bundle size by sharing code, reducing initial load time and improving LCP.

Reusing components across multiple feature modules

Angular

import { CommonModule } from '@angular/common';
import { ComponentA } from './component-a.component';

@NgModule({
  declarations: [ComponentA],
  imports: [CommonModule],
  exports: [ComponentA]
})
export class FeatureModule1 {}

@NgModule({
  declarations: [ComponentA],
  imports: [CommonModule],
  exports: [ComponentA]
})
export class FeatureModule2 {}

ComponentA is declared and bundled separately in multiple feature modules, causing code duplication and larger bundles.

📉 Performance CostAdds duplicate component code to bundles, increasing bundle size and blocking rendering longer.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Duplicate component declarations in multiple modules	No extra DOM nodes but duplicated JS code	No direct reflows caused	Increased paint cost due to slower JS execution	[X] Bad
Single shared module for reusable components	No extra DOM nodes, shared JS code	No direct reflows caused	Lower paint cost due to smaller JS bundle	[OK] Good

Rendering Pipeline

Shared modules reduce duplicated component code in bundles, which decreases JavaScript parsing and execution time during the loading phase.

→Parsing & Compilation

→Script Evaluation

→Rendering

⚠️ BottleneckParsing & Compilation of duplicated code increases load time.

Core Web Vital Affected

LCP

This affects initial page load speed and bundle size by controlling how reusable components are loaded and shared across the app.

Optimization Tips

1Declare reusable components once in a shared module to avoid duplication.

2Import shared modules in feature modules instead of redeclaring components.

3Smaller bundles from shared modules improve Largest Contentful Paint (LCP).

Performance Quiz - 3 Questions

What is the main performance benefit of using a shared module for reusable components in Angular?

A Automatically lazy loads components on demand

B Reduces duplicate code in bundles, improving load speed

C Increases DOM nodes for better rendering

D Removes the need for imports in feature modules

DevTools: Network and Performance panels

How to check: Open DevTools, go to Network tab, filter by JS files, and compare bundle sizes. Use Performance tab to record page load and check scripting time.

What to look for: Look for smaller JS bundle sizes and reduced scripting time indicating shared modules reduce load cost.

Practice

1. What is the main purpose of a shared module in Angular?

easy

A. To store environment configuration settings

B. To define the root component of the application

C. To group reusable components, directives, and pipes for easy sharing

D. To handle routing between different pages

Solution

1. Step 1: Understand the role of shared modules

   Shared modules are designed to hold components, directives, and pipes that multiple parts of the app can use.

2. Step 2: Compare with other module purposes

   Root modules handle app startup, routing modules manage navigation, and environment files store settings, not reusable UI parts.

3. Final Answer:

   To group reusable components, directives, and pipes for easy sharing -> Option C

4. Quick Check:

   Shared module = reusable parts [OK]

Hint: Shared modules bundle reusable UI parts for easy import [OK]

Common Mistakes:

• Confusing shared modules with root or routing modules
• Thinking shared modules store app settings
• Assuming shared modules handle navigation

2. Which of the following is the correct way to export a component from a shared module?

easy

A. Add the component to both declarations and exports arrays in @NgModule

B. Add the component only to the imports array

C. Add the component only to the providers array

D. Add the component only to the bootstrap array

Solution

1. Step 1: Understand Angular module metadata

   Components must be declared in declarations to be recognized, and exported in exports to be usable outside the module.

2. Step 2: Check other arrays usage

   Imports bring in other modules, providers register services, bootstrap is for root components only.

3. Final Answer:

   Add the component to both declarations and exports arrays in @NgModule -> Option A

4. Quick Check:

   Declare + export = share component [OK]

Hint: Declare and export components to share them outside module [OK]

Common Mistakes:

• Putting components only in imports or providers
• Forgetting to export after declaring
• Using bootstrap array in shared modules

3. Given this shared module code, what will be the output when FeatureComponent uses SharedButtonComponent?

@NgModule({
  declarations: [SharedButtonComponent],
  exports: [SharedButtonComponent]
})
export class SharedModule {}

@NgModule({
  imports: [SharedModule],
  declarations: [FeatureComponent]
})
export class FeatureModule {}

If FeatureComponent template contains <app-shared-button>Click</app-shared-button>, what happens?

medium

A. The button renders correctly with text 'Click'

B. Error: 'app-shared-button' is not a known element

C. The button renders but without text

D. The app crashes at runtime

Solution

1. Step 1: Check module imports and exports

   SharedButtonComponent is declared and exported in SharedModule, which is imported by FeatureModule.

2. Step 2: Confirm component usage in FeatureComponent

   Since FeatureModule imports SharedModule, FeatureComponent can use SharedButtonComponent selector without error.

3. Final Answer:

   The button renders correctly with text 'Click' -> Option A

4. Quick Check:

   Imported shared module exports = component usable [OK]

Hint: Import shared module to use its exported components [OK]

Common Mistakes:

• Not importing SharedModule in FeatureModule
• Forgetting to export component in SharedModule
• Using component selector incorrectly

4. Identify the error in this shared module setup:

@NgModule({
  declarations: [SharedCardComponent],
  imports: [CommonModule]
})
export class SharedModule {}

@NgModule({
  imports: [SharedModule],
  declarations: [DashboardComponent]
})
export class DashboardModule {}

DashboardComponent template uses <app-shared-card> but Angular throws an error.

medium

A. CommonModule should not be imported in SharedModule

B. DashboardComponent should import SharedCardComponent directly

C. DashboardModule should declare SharedCardComponent again

D. SharedCardComponent is not exported from SharedModule

Solution

1. Step 1: Check SharedModule exports

   SharedCardComponent is declared but not exported, so other modules can't use it.

2. Step 2: Understand Angular module sharing rules

   To use a component from another module, it must be exported by that module.

3. Final Answer:

   SharedCardComponent is not exported from SharedModule -> Option D

4. Quick Check:

   Declare + export needed to share component [OK]

Hint: Always export components to share them outside module [OK]

Common Mistakes:

• Forgetting to export declared components
• Trying to redeclare components in feature modules
• Misunderstanding imports vs exports

5. You want to create a shared module that provides a reusable button component and a custom pipe. Which setup correctly allows any feature module to use both without redeclaring them?

// SharedModule
@NgModule({
  declarations: [SharedButtonComponent, CustomPipe],
  exports: [SharedButtonComponent, CustomPipe],
  imports: [CommonModule]
})
export class SharedModule {}

// FeatureModule
@NgModule({
  imports: [SharedModule],
  declarations: [FeatureComponent]
})
export class FeatureModule {}

What must you do in FeatureComponent template to use both?

hard

A. Import SharedButtonComponent and CustomPipe directly in FeatureComponent

B. Use <app-shared-button> and apply | customPipe in template expressions

C. Declare SharedButtonComponent and CustomPipe again in FeatureModule

D. Use <shared-button> and | CustomPipe with capital letters

Solution

1. Step 1: Confirm shared module exports

   SharedButtonComponent and CustomPipe are declared and exported in SharedModule, which FeatureModule imports.

2. Step 2: Use correct selectors and pipe names

   FeatureComponent can use selector and apply | customPipe (pipe names are case-sensitive and lowercase by convention).

3. Final Answer:

   Use <app-shared-button> and apply | customPipe in template expressions -> Option B

4. Quick Check:

   Import shared module + use selectors and pipe names correctly [OK]

Hint: Use exported selectors and pipe names exactly as declared [OK]

Common Mistakes:

• Redeclaring shared components or pipes in feature modules
• Using wrong selector or pipe name casing
• Trying to import components or pipes directly