Smart and dumb component pattern in Angular - Mini Project: Build & Apply

Smart and Dumb Component Pattern in Angular

📖 Scenario: You are building a simple Angular app to display a list of books. You want to separate the logic and data handling from the display. This helps keep your code clean and easy to manage.

🎯 Goal: Create a smart component that holds the book data and a dumb component that only shows the book titles. The smart component will pass the book list to the dumb component.

📋 What You'll Learn

Create a smart component called BookListComponent with a list of books

Create a dumb component called BookDisplayComponent that receives the list of books as an input

Use Angular's @Input() decorator in the dumb component to accept the book list

Display the book titles in the dumb component using an *ngFor loop

Use standalone components and Angular 17+ syntax with signals and new control flow directives

💡 Why This Matters

🌍 Real World

Separating smart and dumb components helps keep Angular apps organized and easier to maintain, especially as they grow.

💼 Career

Understanding this pattern is important for Angular developers to write clean, reusable, and testable code in professional projects.

1

Create the smart component with book data

Create a standalone Angular component called BookListComponent. Inside it, create a signal called books that holds this array of strings: ["The Hobbit", "1984", "Pride and Prejudice"].

Angular

import { Component, signal } from '@angular/core';

@Component({
  selector: 'app-book-list',
  standalone: true,
  template: ``,
})
export class BookListComponent {
  // Your code here
}

Hint

Use signal to create a reactive variable holding the book titles array.

2

Create the dumb component to display books

Create a standalone Angular component called BookDisplayComponent. Add an @Input() property called books of type string[]. The template should use *ngFor to show each book title inside a <li> element.

Angular

import { Component, Input } from '@angular/core';

@Component({
  selector: 'app-book-display',
  standalone: true,
  template: `
    
      
    
  `,
})
export class BookDisplayComponent {
  @Input() books!: string[];
  // Your code here
}

Hint

Use @Input() to receive data and *ngFor to loop over the books array.

3

Connect the smart component to the dumb component

In the BookListComponent template, add the <app-book-display> tag. Bind the books signal value to the dumb component's books input using Angular's property binding syntax.

Angular

import { Component, signal } from '@angular/core';
import { BookDisplayComponent } from './book-display.component';

@Component({
  selector: 'app-book-list',
  standalone: true,
  imports: [BookDisplayComponent],
  template: `
    
  `,
})
export class BookListComponent {
  books = signal(["The Hobbit", "1984", "Pride and Prejudice"]);
}

Hint

Use [books]="books()" to bind the signal value to the dumb component input.

4

Add accessibility and finalize the dumb component

In the BookDisplayComponent template, add role="list" to the <ul> and role="listitem" to each <li>. This improves accessibility for screen readers.

Angular

import { Component, Input } from '@angular/core';

@Component({
  selector: 'app-book-display',
  standalone: true,
  template: `
    
      {{ book }}
    
  `,
})
export class BookDisplayComponent {
  @Input() books!: string[];
  // Your code here
}

Hint

Add role="list" to the <ul> and role="listitem" to each <li> for better accessibility.

Practice

1. What is the main role of a smart component in Angular's smart and dumb component pattern?

easy

A. To handle user input events only

B. To only display data without logic

C. To manage data and business logic

D. To style the user interface

Solution

1. Step 1: Understand smart component responsibilities

   Smart components are designed to handle data fetching, state management, and business logic.

2. Step 2: Differentiate from dumb components

   Dumb components focus on displaying data and emitting events, not managing data or logic.

3. Final Answer:

   To manage data and business logic -> Option C

4. Quick Check:

   Smart component = data and logic [OK]

Hint: Smart components handle data and logic, dumb ones display only [OK]

Common Mistakes:

• Confusing dumb components as managing data
• Thinking smart components only display UI
• Assuming smart components handle styling only

2. Which of the following is the correct way to pass data from a smart component to a dumb component in Angular?

easy

A. Use a service to directly modify dumb component variables

B. @Output() data: any; in dumb component and bind in template

C. Use @ViewChild to access dumb component data

D. @Input() data: any; in dumb component and bind in template

Solution

1. Step 1: Identify data flow direction

   Data flows from smart to dumb components via inputs.

2. Step 2: Use Angular syntax for input binding

   Dumb components declare @Input() properties to receive data from parents.

3. Final Answer:

   @Input() data: any; -> Option D

4. Quick Check:

   Data to dumb = @Input() [OK]

Hint: Use @Input() to pass data down from smart to dumb [OK]

Common Mistakes:

• Using @Output() to pass data down instead of events up
• Trying to access dumb component data directly via ViewChild
• Modifying dumb component state via services without inputs

3. Given the following Angular code, what will be the output displayed by the dumb component?

/* Smart component template */
<app-dumb [title]="pageTitle" (clicked)="onClicked()"></app-dumb>

/* Smart component class */
pageTitle = 'Hello World';
onClicked() { console.log('Clicked!'); }

/* Dumb component template */
<h1>{{ title }}</h1>
<button (click)="clicked.emit()">Click Me</button>

/* Dumb component class */
@Input() title: string;
@Output() clicked = new EventEmitter<void>();

medium

A. Displays 'Hello World' and logs 'Clicked!' on button click

B. Displays nothing and logs 'Clicked!' on button click

C. Displays 'Hello World' but does not log anything

D. Throws an error because of missing @Output() decorator

Solution

1. Step 1: Analyze data binding from smart to dumb

   The smart component passes 'Hello World' via [title] input, so dumb displays it.

2. Step 2: Analyze event emission and handling

   The dumb component emits clicked event on button click, smart component listens and logs 'Clicked!'.

3. Final Answer:

   Displays 'Hello World' and logs 'Clicked!' on button click -> Option A

4. Quick Check:

   Input shows title, output triggers log [OK]

Hint: Input shows data, output triggers event handled by smart [OK]

Common Mistakes:

• Thinking dumb component logs directly
• Assuming missing decorators cause runtime error here
• Ignoring event binding from dumb to smart

4. Identify the error in this dumb component code that prevents it from emitting events to the smart component:

@Component({
  selector: 'app-dumb',
  template: `<button (click)="clicked.emit()">Click</button>`
})
export class DumbComponent {
  clicked = new EventEmitter<void>();
}

medium

A. EventEmitter should be imported from '@angular/core/testing'

B. Missing @Output() decorator on the clicked property

C. The template syntax for click event is incorrect

D. The clicked property should be a function, not EventEmitter

Solution

1. Step 1: Check event emitter declaration

   The clicked property must have @Output() decorator to emit events to parent.

2. Step 2: Verify imports and syntax

   EventEmitter is correctly imported from '@angular/core', template syntax is correct.

3. Final Answer:

   Missing @Output() decorator on the clicked property -> Option B

4. Quick Check:

   @Output() missing = no event emission [OK]

Hint: Always add @Output() to EventEmitter properties [OK]

Common Mistakes:

• Forgetting @Output() decorator
• Importing EventEmitter from wrong package
• Miswriting template event binding syntax

5. You want to refactor a large Angular component that mixes data fetching, logic, and UI display into smart and dumb components. Which approach best follows the smart and dumb component pattern?

hard

A. Create a smart component to fetch data and handle logic, pass data via @Input() to dumb components that only display UI and emit events

B. Move all logic and data fetching into dumb components and keep smart components only for styling

C. Combine smart and dumb components into one to reduce complexity

D. Use dumb components to fetch data and smart components to display UI

Solution

1. Step 1: Separate concerns by responsibility

   Smart components should handle data fetching and logic, dumb components focus on UI and user interaction.

2. Step 2: Use Angular bindings correctly

   Pass data from smart to dumb via @Input() and receive events via @Output().

3. Final Answer:

   Create a smart component to fetch data and handle logic, pass data via @Input() to dumb components that only display UI and emit events -> Option A

4. Quick Check:

   Smart = logic/data, Dumb = UI only [OK]

Hint: Smart handles data/logic; dumb handles UI and events [OK]

Common Mistakes:

• Putting logic in dumb components
• Merging smart and dumb components unnecessarily
• Reversing data flow direction