Structural vs attribute directives in Angular - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to use a structural directive that conditionally shows a paragraph.

Angular

<p *[1]="isVisible">This paragraph is conditionally visible.</p>

Drag options to blanks, or click blank then click option'

AngIf

BngClass

CngStyle

DngModel

Attempts:

3 left

💡 Hint

Common Mistakes

Using ngClass or ngStyle which only change appearance, not structure.

Forgetting the asterisk (*) before the directive.

The ngIf directive is a structural directive that adds or removes elements based on a condition.

2fill in blank

medium

Complete the code to apply a CSS class conditionally using an attribute directive.

Angular

<div [[1]]="isActive ? 'active' : ''">Content here</div>

Drag options to blanks, or click blank then click option'

AngIf

BngModel

CngStyle

DngClass

Attempts:

3 left

💡 Hint

Common Mistakes

Using ngIf which removes or adds elements instead of changing classes.

Using ngStyle which changes styles but not classes.

The ngClass directive is an attribute directive that adds or removes CSS classes based on conditions.

3fill in blank

hard

Fix the error in the code to correctly use a structural directive for looping over items.

Angular

<li *ngFor="let item of [1]">{{item}}</li>

Drag options to blanks, or click blank then click option'

AitemsList[]

BitemsList()

CitemsList

DitemsList{}

Attempts:

3 left

💡 Hint

Common Mistakes

Adding parentheses which makes it a function call.

Using brackets or braces which are invalid here.

The ngFor directive expects an iterable variable, not a function call or invalid syntax.

4fill in blank

hard

Fill both blanks to create a structural directive that shows a message only if the user is logged in.

Angular

<div *[1]="[2]">Welcome back!</div>

Drag options to blanks, or click blank then click option'

AngIf

BisLoggedIn

CngFor

DuserLogged

Attempts:

3 left

💡 Hint

Common Mistakes

Using ngFor which is for loops, not conditions.

Using a variable name that does not exist or is incorrect.

The ngIf directive conditionally shows the element when isLoggedIn is true.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps item names to their lengths only if length is greater than 3.

Angular

const lengths = { [1]: [2].length for [1] of items if [2].length > 3 };

Drag options to blanks, or click blank then click option'

Aitem

Citems

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

Using 'length' as a variable name which is a property.

This code uses item as the variable for each element in items. It maps item names to their lengths if the length is greater than 3.

Practice

1. Which of the following best describes a structural directive in Angular?

easy

A. It binds data to an element's property.

B. It changes the style of an element without altering the DOM structure.

C. It changes the layout by adding or removing elements from the DOM.

D. It listens to user events like clicks or key presses.

Solution

1. Step 1: Understand what structural directives do

   Structural directives modify the DOM layout by adding or removing elements dynamically.

2. Step 2: Compare with other directive types

   Attribute directives change appearance or behavior but do not add or remove elements.

3. Final Answer:

   It changes the layout by adding or removing elements from the DOM. -> Option C

4. Quick Check:

   Structural directive = changes DOM structure [OK]

Hint: Structural = add/remove elements; attribute = change style [OK]

Common Mistakes:

• Confusing attribute directives with structural directives
• Thinking attribute directives add or remove elements
• Mixing event listeners with directive types

2. Which of the following is the correct syntax for using the structural directive that repeats elements in Angular?

easy

A. <div *ngFor="let item of items">{{item}}</div>

B. <div [ngFor]="items">{{item}}</div>

C. <div (ngFor)="items">{{item}}</div>

D. <div #ngFor="items">{{item}}</div>

Solution

1. Step 1: Recall the syntax for the structural directive *ngFor

   The correct syntax uses an asterisk (*) before ngFor and a let variable to iterate.

2. Step 2: Check each option's syntax

   Only <div *ngFor="let item of items">{{item}}</div> uses *ngFor="let item of items" which is correct.

3. Final Answer:

   <div *ngFor="let item of items">{{item}}</div> -> Option A

4. Quick Check:

   *ngFor syntax = <div *ngFor="let item of items">{{item}}</div> [OK]

Hint: Structural directives use * before directive name [OK]

Common Mistakes:

• Using square brackets [] instead of * for structural directives
• Using parentheses () which are for events
• Using template reference (#) incorrectly

3. Given the following Angular template code, what will be rendered?

<ul>
  <li *ngIf="showItem">Item 1</li>
  <li>Item 2</li>
</ul>

Assuming showItem is false.

medium

A. No output, error occurs

B. Item 1Item 2

C.

D. Item 2

Solution

1. Step 1: Understand *ngIf behavior with false condition

   *ngIf removes the element from the DOM if the condition is false.

2. Step 2: Apply this to the template

   The first <li> with *ngIf="showItem" will not render because showItem is false. The second <li> renders normally.

3. Final Answer:

   <ul><li>Item 2</li></ul> -> Option D

4. Quick Check:

   *ngIf false removes element [OK]

Hint: *ngIf false means element is removed, not hidden [OK]

Common Mistakes:

• Thinking *ngIf hides element but keeps it in DOM
• Assuming both list items render regardless
• Confusing *ngIf with attribute directives

4. Identify the error in this Angular template snippet:

<button *ngIf="isEnabled" [disabled]="isDisabled">Click Me</button>

Assuming isEnabled and isDisabled are boolean variables.

medium

A. The syntax is correct; no error present.

B. The *ngIf directive must be replaced with [ngIf].

C. The [disabled] attribute should be replaced with *disabled.

D. Using *ngIf and [disabled] together on the same element is invalid.

Solution

1. Step 1: Check compatibility of *ngIf and [disabled]

   *ngIf controls element presence; [disabled] controls attribute binding. Both can coexist.

2. Step 2: Verify syntax correctness

   The syntax uses *ngIf with a boolean and [disabled] with a boolean, which is valid Angular syntax.

3. Final Answer:

   The syntax is correct; no error present. -> Option A

4. Quick Check:

   *ngIf and [disabled] can be used together [OK]

Hint: *ngIf controls element, [disabled] controls attribute independently [OK]

Common Mistakes:

• Thinking *ngIf and attribute bindings cannot be combined
• Confusing *ngIf with [ngIf]
• Replacing attribute bindings with structural directive syntax

5. You want to create a custom directive that highlights text by changing its background color when hovered. Which type of directive should you create and why?

hard

A. Structural directive, because it needs to add or remove elements on hover.

B. Attribute directive, because it changes the element's appearance without altering layout.

C. Structural directive, because it modifies the DOM structure dynamically.

D. Attribute directive, because it listens to user events but does not change styles.

Solution

1. Step 1: Understand the directive's purpose

   The directive changes background color on hover, affecting appearance only.

2. Step 2: Choose directive type based on behavior

   Attribute directives change element style or behavior without adding/removing elements, fitting this case.

3. Final Answer:

   Attribute directive, because it changes the element's appearance without altering layout. -> Option B

4. Quick Check:

   Appearance change = attribute directive [OK]

Hint: Appearance changes use attribute directives, layout changes use structural [OK]

Common Mistakes:

• Choosing structural directive for style changes
• Confusing event listening with directive type
• Thinking attribute directives add/remove elements