Smart and dumb component pattern in Angular

Introduction

This pattern helps separate how data is handled from how it looks. It makes your app easier to understand and change.

When you want to keep your UI simple and focused only on showing data.

When you want to manage data and logic in one place and share it with many UI parts.

When you want to test your UI and logic separately without mixing them.

When your app grows and you want to keep code organized and easy to fix.

When you want to reuse UI parts without copying data logic.

Syntax

Angular

/* Smart component (container) */
@Component({
  selector: 'app-smart',
  standalone: true,
  imports: [DumbComponent],
  template: `<app-dumb [items]="items" (select)="onSelect($event)"></app-dumb>`
})
export class SmartComponent {
  items = ['Apple', 'Banana', 'Cherry'];

  onSelect(item: string) {
    console.log('Selected:', item);
  }
}

/* Dumb component (presentational) */
@Component({
  selector: 'app-dumb',
  standalone: true,
  imports: [CommonModule],
  template: `
    <ul>
      <li *ngFor="let item of items" (click)="select.emit(item)" tabindex="0" role="button" aria-label="Select {{item}}">
        {{item}}
      </li>
    </ul>
  `
})
export class DumbComponent {
  @Input() items: string[] = [];
  @Output() select = new EventEmitter<string>();
}

The smart component holds data and logic.

The dumb component only shows data and sends user actions back.

Examples

This dumb component just shows a message passed from outside.

Angular

/* Dumb component only shows data */
@Component({
  selector: 'app-dumb',
  standalone: true,
  template: `<p>{{message}}</p>`,
  inputs: ['message']
})
export class DumbComponent {
  @Input() message = '';
}

The smart component sends a greeting to the dumb component to display.

Angular

/* Smart component passes data and handles events */
@Component({
  selector: 'app-smart',
  standalone: true,
  imports: [DumbComponent],
  template: `<app-dumb [message]="greeting"></app-dumb>`
})
export class SmartComponent {
  greeting = 'Hello from smart component!';
}

Sample Program

The smart component holds a list of fruits and passes it to the dumb component. The dumb component shows the list and lets the user click an item. When clicked, the dumb component tells the smart component which item was selected. The smart component then logs the selection.

Angular

import { Component, EventEmitter, Input, Output } from '@angular/core';
import { CommonModule } from '@angular/common';

@Component({
  selector: 'app-dumb',
  standalone: true,
  imports: [CommonModule],
  template: `
    <ul>
      <li *ngFor="let item of items" (click)="select.emit(item)" tabindex="0" role="button" aria-label="Select {{item}}">
        {{item}}
      </li>
    </ul>
  `
})
export class DumbComponent {
  @Input() items: string[] = [];
  @Output() select = new EventEmitter<string>();
}

@Component({
  selector: 'app-smart',
  standalone: true,
  imports: [DumbComponent],
  template: `<app-dumb [items]="items" (select)="onSelect($event)"></app-dumb>`
})
export class SmartComponent {
  items = ['Apple', 'Banana', 'Cherry'];

  onSelect(item: string) {
    console.log('Selected:', item);
  }
}

Important Notes

Use @Input() to send data to dumb components.

Use @Output() with EventEmitter to send events back to smart components.

Keep dumb components free of data fetching or complex logic.

Summary

Smart components manage data and logic.

Dumb components focus on displaying data and user interaction.

This pattern makes your app easier to maintain and test.

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