Input signals and model signals in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when an input signal changes in Angular?

Consider an Angular standalone component that receives an input signal. What is the behavior when the input signal value changes?

Angular

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

@Component({
  selector: 'app-counter',
  standalone: true,
  template: `
    <p>Count: {{ count() }}</p>
  `
})
export class CounterComponent {
  @Input() count = signal(0);
}

A The component automatically updates the displayed count when the input signal changes.

B The component requires manual subscription to the signal to update the display.

C The component does not update because signals cannot be passed as inputs.

D The component updates only if a lifecycle hook manually triggers change detection.

💡 Hint

Think about how Angular signals propagate changes automatically.

Input signals in Angular automatically notify the component when their value changes, so the template updates reactively without manual subscriptions or lifecycle hooks.

❓ state_output

intermediate

What is the output of this Angular component using model signals?

Given the following Angular component code, what will be displayed after the button is clicked twice?

Angular

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

@Component({
  selector: 'app-clicker',
  standalone: true,
  template: `
    <p>Clicks: {{ clicks() }}</p>
    <button (click)="increment()">Click me</button>
  `
})
export class ClickerComponent {
  clicks = signal(0);

  increment() {
    this.clicks.update(c => c + 1);
  }
}

A Clicks: 0

B Clicks: undefined

C Clicks: 1

D Clicks: 2

💡 Hint

Each button click calls the increment method which updates the signal.

The signal 'clicks' starts at 0 and increments by 1 each time the button is clicked. After two clicks, it will be 2.

📝 Syntax

advanced

Which option correctly defines an input signal in Angular?

Choose the correct syntax to define an input signal property in an Angular standalone component.

A @Input() count = signal(0);

B @Input signal count = 0;

C @Input() count: Signal<number> = signal(0);

D count = @Input(signal(0));

💡 Hint

Remember to use the correct TypeScript type annotation and decorator syntax.

Option C correctly uses the @Input decorator with a typed Signal property initialized with signal(0). Option C misses the type annotation, B and C have invalid syntax.

🔧 Debug

advanced

Why does this Angular component fail to update when the input signal changes?

Examine the code below. The component receives an input signal but does not update the displayed value when the input changes. What is the cause?

Angular

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

@Component({
  selector: 'app-display',
  standalone: true,
  template: `
    <p>Value: {{ value() }}</p>
  `
})
export class DisplayComponent {
  @Input() value = signal(0);

  ngOnChanges() {
    // No code here
  }
}

A The input signal is reassigned instead of updated, breaking reactivity.

B The component does not import 'Signal' type, causing a runtime error.

C The template uses value() which is invalid syntax for signals.

D The component lacks a manual subscription to the input signal.

💡 Hint

Check how the input signal is handled and whether it is replaced or updated.

Reassigning the input signal breaks the reactive connection. The input signal should be passed and used directly without reassignment to keep updates working.

🧠 Conceptual

expert

How do model signals differ from input signals in Angular's reactive system?

Choose the statement that best describes the difference between model signals and input signals in Angular.

A Input signals are mutable inside the component, but model signals are read-only from outside.

B Model signals are owned and updated inside the component, while input signals come from outside and should not be mutated internally.

C Model signals and input signals are identical and interchangeable in Angular components.

D Input signals automatically trigger change detection, but model signals require manual triggering.

💡 Hint

Think about ownership and mutation rights of signals in component design.

Model signals are created and controlled inside the component and can be updated there. Input signals come from parent components and should be treated as read-only to avoid breaking unidirectional data flow.