*ngIf for conditional rendering in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to show the paragraph only if isVisible is true.

Angular

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

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

AngFor

BngShow

CngSwitch

DngIf

Attempts:

3 left

💡 Hint

Common Mistakes

Using *ngShow which is not an Angular directive.

Using *ngFor which is for loops, not conditions.

The *ngIf directive in Angular conditionally includes or excludes an element based on the expression. Here, it shows the paragraph only if isVisible is true.

2fill in blank

medium

Complete the code to display the message only when user.isLoggedIn is true.

Angular

<div *[1]="user.isLoggedIn">Welcome back!</div>

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

AngFor

BngClass

CngIf

DngSwitchCase

Attempts:

3 left

💡 Hint

Common Mistakes

Using *ngFor which is for repeating elements.

Using *ngClass which is for styling, not rendering.

*ngIf checks the condition and renders the element only if user.isLoggedIn is true.

3fill in blank

hard

Fix the error in the code to conditionally show the button when showButton is true.

Angular

<button *[1]="showButton">Click me</button>

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

AngIf

BngShow

CngRepeat

DngSwitch

Attempts:

3 left

💡 Hint

Common Mistakes

Using *ngShow which is from other frameworks.

Using *ngRepeat which is not Angular.

The correct directive for conditional rendering is *ngIf. *ngShow and *ngRepeat are not valid Angular directives.

4fill in blank

hard

Fill both blanks to show the div only if isAdmin is true and add a CSS class highlight.

Angular

<div *[1]="isAdmin" [class.[2]]="isAdmin">Admin Panel</div>

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

AngIf

BngClass

Chighlight

DngShow

Attempts:

3 left

💡 Hint

Common Mistakes

Using *ngShow which is not Angular.

Confusing directive names and class names.

*ngIf conditionally renders the div. The [class.highlight] adds the CSS class highlight when isAdmin is true.

5fill in blank

hard

Fill all three blanks to display a message only if user.isActive is true, show the username, and add a CSS class active-user.

Angular

<p *[1]="user.isActive" [class.[2]]="user.isActive">Hello, [3]</p>

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

AngIf

Bactive-user

Cuser.name

DngShow

Attempts:

3 left

💡 Hint

Common Mistakes

Using *ngShow which is not Angular.

Forgetting to use interpolation for the username.

*ngIf conditionally renders the paragraph. The [class.active-user] adds the CSS class when user.isActive is true. The interpolation {{user.name}} shows the username.

Practice

1. What does the Angular directive *ngIf do in a template?

easy

A. It loops over a list and renders each item.

B. It styles an element dynamically.

C. It binds a variable to an input field.

D. It conditionally includes or removes an element from the DOM based on a boolean expression.

Solution

1. Step 1: Understand the purpose of *ngIf

   *ngIf is used to conditionally render elements in Angular templates.

2. Step 2: Identify what *ngIf controls

   It controls whether an element is included or removed from the DOM based on a true or false condition.

3. Final Answer:

   It conditionally includes or removes an element from the DOM based on a boolean expression. -> Option D

4. Quick Check:

   *ngIf = conditional rendering [OK]

Hint: Remember *ngIf shows or hides elements by condition [OK]

Common Mistakes:

• Confusing *ngIf with *ngFor for looping
• Thinking *ngIf changes styles only
• Assuming *ngIf binds data to inputs

2. Which of the following is the correct syntax to conditionally display a paragraph only if isVisible is true?

easy

A. <p *ngIf="isVisible">Hello</p>

B. <p ngIf="isVisible">Hello</p>

C. <p *ngFor="isVisible">Hello</p>

D. <p *ngShow="isVisible">Hello</p>

Solution

1. Step 1: Identify the correct Angular directive syntax

   Angular uses *ngIf with an asterisk to indicate a structural directive.

2. Step 2: Check the attribute and expression format

   The expression should be inside quotes after *ngIf=, like *ngIf="isVisible".

3. Final Answer:

   <p *ngIf="isVisible">Hello</p> -> Option A

4. Quick Check:

   Use *ngIf with asterisk and quotes [OK]

Hint: Look for *ngIf with asterisk and quotes [OK]

Common Mistakes:

• Omitting the asterisk before ngIf
• Using ngIf without * (like ngIf="isVisible")
• Confusing *ngIf with *ngFor or *ngShow

3. Given this Angular template snippet:

<div *ngIf="showMessage">Welcome!</div>
<button (click)="showMessage = !showMessage">Toggle</button>

What happens when the button is clicked twice starting with showMessage = true?

medium

A. The message 'Welcome!' disappears after first click and reappears after second click.

B. The message 'Welcome!' stays visible all the time.

C. The message 'Welcome!' disappears and never comes back.

D. The button click causes an error because showMessage is not defined.

Solution

1. Step 1: Understand initial state and button action

   Initially, showMessage is true, so the <div> with 'Welcome!' is shown. Clicking the button toggles showMessage between true and false.

2. Step 2: Trace two clicks

   First click sets showMessage to false, hiding the message. Second click sets it back to true, showing the message again.

3. Final Answer:

   The message 'Welcome!' disappears after first click and reappears after second click. -> Option A

4. Quick Check:

   Toggle true/false shows/hides message [OK]

Hint: Toggle flips boolean, so message appears/disappears alternately [OK]

Common Mistakes:

• Assuming message never reappears after hiding
• Thinking button click causes error
• Believing message is always visible

4. Consider this Angular template:

<div *ngIf="user.isLoggedIn">Hello, {{user.name}}!</div>

But the app crashes with an error: Cannot read property 'isLoggedIn' of undefined. What is the best fix?

medium

A. Remove the *ngIf directive completely.

B. Change to *ngIf="user?.isLoggedIn" to safely check property.

C. Initialize user as an empty object in the component.

D. Use *ngIf="user.isLoggedIn === true" instead.

Solution

1. Step 1: Identify cause of error

   The error occurs because user is undefined when Angular tries to read user.isLoggedIn.

2. Step 2: Use safe navigation operator

   Using user?.isLoggedIn prevents error by checking if user exists before accessing isLoggedIn.

3. Final Answer:

   Change to *ngIf="user?.isLoggedIn" to safely check property. -> Option B

4. Quick Check:

   Use ? to avoid errors on undefined objects [OK]

Hint: Use ?. to safely access nested properties [OK]

Common Mistakes:

• Removing *ngIf loses conditional rendering
• Initializing user empty but not fixing undefined timing
• Checking equality without safe navigation still errors

5. You want to display a message only if a user has a non-empty email property and is verified (isVerified true). Which *ngIf expression correctly handles this?

hard

A. *ngIf="user.email == '' && user.isVerified"

B. *ngIf="user.email || user.isVerified"

C. *ngIf="user.email && user.isVerified"

D. *ngIf="user.email.length > 0 || user.isVerified"

Solution

1. Step 1: Understand the condition requirements

   The message should show only if user.email is not empty (truthy) AND user.isVerified is true.

2. Step 2: Analyze each option

   *ngIf="user.email && user.isVerified" uses logical AND to require both conditions true. *ngIf="user.email || user.isVerified" uses OR, which is incorrect. *ngIf="user.email == '' && user.isVerified" checks for empty string which is opposite of requirement. *ngIf="user.email.length > 0 || user.isVerified" uses OR and length check but allows message if only one condition is true.

3. Final Answer:

   *ngIf="user.email && user.isVerified" -> Option C

4. Quick Check:

   Use AND (&&) to require both conditions true [OK]

Hint: Use && to require both conditions true [OK]

Common Mistakes:

• Using || instead of && allowing partial matches
• Checking for empty string instead of truthy value
• Using length without checking for undefined user