Component selector usage in Angular - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What will be rendered by this Angular component usage?

Given the Angular component with selector app-greeting and template <p>Hello, Angular!</p>, what will be the output when used as <app-greeting></app-greeting> in another component's template?

Angular

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

@Component({
  selector: 'app-greeting',
  template: '<p>Hello, Angular!</p>'
})
export class GreetingComponent {}

A No output, component not rendered

B <p>Hello, Angular!</p>

C <app-greeting></app-greeting>

D Error: Unknown element 'app-greeting'

💡 Hint

Think about how Angular replaces component selectors with their templates.

Angular replaces the app-greeting tag with the component's template content, so the output is the paragraph with 'Hello, Angular!'.

📝 Syntax

intermediate

Which selector syntax is conventional for an Angular component?

Which of the following selector values is conventional for an Angular component?

A '.app-header'

B '[app-header]'

C '#app-header'

D 'app-header'

💡 Hint

Angular component selectors are usually element selectors.

Angular component selectors are typically element selectors like 'app-header'. Class selectors ('.app-header'), ID selectors ('#app-header'), and attribute selectors ('[app-header]') are valid but more commonly used for directives.

🔧 Debug

advanced

Why does this Angular component not render when used as <app-footer></app-footer>?

Given this component code, why does using <app-footer></app-footer> in a template not render anything?

Angular

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

@Component({
  selector: '[app-footer]',
  template: '<footer>Footer content</footer>'
})
export class FooterComponent {}

A The component is missing the @NgModule declaration.

B The template syntax is invalid and causes a runtime error.

C The selector is an attribute selector, so <app-footer> won't match it.

D Angular requires selectors to be class selectors for components.

💡 Hint

Check the selector type and how it matches HTML elements.

The selector '[app-footer]' means the component matches elements with the attribute 'app-footer'. Using element does not have this attribute, so Angular does not render the component there.

❓ state_output

advanced

What is the output when using a component with a selector that matches multiple elements?

Consider this Angular component with selector div[app-highlight]. What will be rendered when used as below?

<div app-highlight>Text 1</div>
<span app-highlight>Text 2</span>

Angular

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

@Component({
  selector: 'div[app-highlight]',
  template: '<div style="color: red;">Highlighted</div>'
})
export class HighlightComponent {}

A

<div style="color: red;">Highlighted</div>
<span app-highlight>Text 2</span>

B

<div style="color: red;">Highlighted</div>
<span style="color: red;">Text 2</span>

C

Text 1
<span app-highlight>Text 2</span>

D Error: Multiple elements matched by selector

💡 Hint

Think about how Angular matches selectors and replaces elements.

The selector 'div[app-highlight]' matches only

elements with the attribute 'app-highlight'. So only the first div is replaced by the component template. The span with the attribute is not matched and remains unchanged.

🧠 Conceptual

expert

Why should Angular component selectors avoid common HTML tag names?

Why is it recommended to use custom prefixes like app- in Angular component selectors instead of common HTML tags like header or footer?

A To prevent conflicts with native HTML elements and ensure Angular renders the component correctly.

B Because Angular does not allow selectors that match native HTML tags.

C To improve performance by reducing the number of DOM elements.

D Because browsers block custom elements without prefixes.

💡 Hint

Think about how browsers and Angular handle native HTML tags versus custom elements.

Using common HTML tags as selectors can cause conflicts because browsers already recognize those tags natively. Using prefixes like 'app-' creates unique custom elements that Angular can safely replace without conflicts.

Practice

1. What does the selector property in an Angular component define?

easy

A. The HTML tag or attribute used to place the component in templates

B. The CSS styles applied to the component

C. The TypeScript class name of the component

D. The module where the component is declared

Solution

1. Step 1: Understand the role of selector in Angular components

   The selector tells Angular what HTML tag or attribute to look for to insert the component's template.

2. Step 2: Differentiate selector from other properties

   CSS styles and class names are unrelated to selector; selector is specifically for placement in HTML.

3. Final Answer:

   The HTML tag or attribute used to place the component in templates -> Option A

4. Quick Check:

   Selector = HTML tag/attribute [OK]

Hint: Selector defines how to use the component in HTML [OK]

Common Mistakes:

• Confusing selector with styles or class names
• Thinking selector is the component's class name
• Assuming selector defines module info

2. Which of the following is a valid selector syntax for an Angular component?

easy

A. [my-component]

B. my-component

C. .my-component

D. myComponent

Solution

1. Step 1: Recognize selector types in Angular

   Angular selectors can be tag names (like 'my-component'), attribute selectors ('[my-component]'), or class selectors ('.my-component').

2. Step 2: Identify valid selector syntax

   The option 'my-component' is a valid tag selector string used in the selector property. The other options represent attribute or class selectors.

3. Final Answer:

   my-component -> Option B

4. Quick Check:

   Tag selector syntax = tag name string [OK]

Hint: Tag selectors are simple tag names without brackets or dots [OK]

Common Mistakes:

• Using selector syntax directly instead of HTML usage
• Confusing attribute selector brackets with HTML tags
• Using camelCase instead of kebab-case for tags

3. Given this component selector: selector: '[appHighlight]', how would you use it in HTML?

medium

A.

B.

C.

D.

Solution

1. Step 1: Understand attribute selector syntax

   The selector '[appHighlight]' means the component is used as an attribute on an HTML element.

2. Step 2: Match usage to selector

   Using
   applies the component to the div element as an attribute, matching the selector.

3. Final Answer:

   <div appHighlight></div> -> Option C

4. Quick Check:

   Attribute selector = element with attribute [OK]

Hint: Attribute selectors used as attributes on elements [OK]

Common Mistakes:

• Using tag syntax for attribute selectors
• Using class or id instead of attribute
• Forgetting brackets mean attribute selector

4. Why does this Angular component not render?

@Component({ selector: 'my-widget', template: '
Hi
' }) export class MyWidgetComponent {}

Used in HTML as: <my-widget></my-widget>

medium

A. Component is missing @NgModule declarations

B. Selector must be prefixed with 'app-' by Angular rules

C. Selector is correct; problem is elsewhere

D. Selector should be in square brackets for attribute usage

Solution

1. Step 1: Check selector syntax

   The selector 'my-widget' is valid as a tag selector and matches usage <my-widget></my-widget>.

2. Step 2: Verify component registration

   Angular requires components to be declared in an NgModule or standalone to render. Missing declaration causes no rendering.

3. Final Answer:

   Component is missing @NgModule declarations -> Option A

4. Quick Check:

   Missing declaration = no render [OK]

Hint: Component must be declared or standalone to render [OK]

Common Mistakes:

• Thinking selector must have 'app-' prefix
• Confusing attribute selector syntax with tag selector
• Ignoring module declarations

5. You want to create a component that can be used as a CSS class selector. Which selector should you use and how do you apply it in HTML?

hard

A. Use selector: '#fancy-button' and apply as <button id="fancy-button">

B. Use selector: 'fancy-button' and apply as <fancy-button>

C. Use selector: '[fancy-button]' and apply as <button fancy-button>

D. Use selector: '.fancy-button' and apply as <button class="fancy-button">

Solution

1. Step 1: Understand class selector syntax

   To use a component as a CSS class selector, prefix the selector with a dot (.) like '.fancy-button'.

2. Step 2: Apply component in HTML

   Use the component by adding the class attribute with the selector name: <button class="fancy-button">.

3. Final Answer:

   Use selector '.fancy-button' and apply as <button class="fancy-button"> -> Option D

4. Quick Check:

   Class selector = dot prefix + class attribute [OK]

Hint: Class selectors start with dot and used as class attribute [OK]

Common Mistakes:

• Using id selector (#) instead of class selector (.)
• Confusing attribute selector brackets with class selector
• Applying class selector as tag or attribute