Virtual scrolling for large lists in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the Angular CDK virtual scrolling module.

Angular

import { [1] } from '@angular/cdk/scrolling';

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

AScrollingModule

BScrollModule

CVirtualScrollModule

DVirtualModule

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-existent module like VirtualScrollModule.

Confusing with ScrollModule which does not exist.

The correct module to import for virtual scrolling is ScrollingModule from Angular CDK.

2fill in blank

medium

Complete the template code to enable virtual scrolling with a fixed item size.

Angular

<cdk-virtual-scroll-viewport itemSize=[1]>
  <div *cdkVirtualFor="let item of items">{{item}}</div>
</cdk-virtual-scroll-viewport>

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

A100%

B'50px'

Cauto

D50

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a string with units like '50px' instead of a number.

Using 'auto' or '100%' which are invalid for itemSize.

The itemSize input expects a number representing the pixel height of each item, so 50 is correct.

3fill in blank

hard

Fix the error in the component code to correctly provide the list for virtual scrolling.

Angular

items = Array.from({length: 1000}, (_, i) => [1]);

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

Ai + 1

B'item ' + i

Ci * 2

Di

Attempts:

3 left

💡 Hint

Common Mistakes

Using just the index number which may not be descriptive.

Using arithmetic expressions that don't create strings.

The virtual scroll list should be an array of strings like 'item 0', 'item 1', etc. So 'item ' + i is correct.

4fill in blank

hard

Fill both blanks to set the viewport height and enable virtual scrolling in the template.

Angular

<cdk-virtual-scroll-viewport [1]="200px" itemSize=[2]>
  <div *cdkVirtualFor="let item of items">{{item}}</div>
</cdk-virtual-scroll-viewport>

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

Astyle.height

B50

Cstyle.width

Dheight

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'height' attribute directly which is invalid.

Confusing width with height.

Passing a string like '50px' for itemSize.

The viewport height is set using the style.height="200px" attribute, and itemSize=50 provides the fixed pixel height for each item.

5fill in blank

hard

Fill all three blanks to create a virtual scroll viewport with a fixed height, item size, and track the rendered range.

Angular

<cdk-virtual-scroll-viewport [1]="300px" itemSize=[2] (scrolledIndexChange)="[3] = $event">
  <div *cdkVirtualFor="let item of items">{{item}}</div>
</cdk-virtual-scroll-viewport>

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

Astyle.height

B50

CstartIndex

DscrollIndex

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'width' instead of 'height' for style.

Passing itemSize as a string with units.

Using an incorrect variable name for the event binding.

Use style.height to set viewport height, 50 as item size in pixels, and startIndex to track the scrolled index.

Practice

1. What is the main benefit of using cdk-virtual-scroll-viewport in Angular for large lists?

easy

A. It renders only visible items to improve performance.

B. It automatically sorts the list items alphabetically.

C. It adds animations to list items when scrolling.

D. It duplicates list items for faster loading.

Solution

1. Step 1: Understand virtual scrolling purpose

   Virtual scrolling aims to improve performance by rendering only the visible items in a large list.

2. Step 2: Identify cdk-virtual-scroll-viewport role

   This Angular component implements virtual scrolling by showing only the items visible in the viewport, reducing DOM load.

3. Final Answer:

   It renders only visible items to improve performance. -> Option A

4. Quick Check:

   Virtual scrolling = render visible items only [OK]

Hint: Virtual scroll shows visible items only to boost speed [OK]

Common Mistakes:

• Thinking it sorts or animates items
• Assuming it duplicates items for speed
• Confusing with pagination

2. Which of the following is the correct way to use virtual scrolling in Angular template?

easy

A. <cdk-virtual-scroll-viewport><div *cdkVirtualFor="let item in items">{{item}}</div></cdk-virtual-scroll-viewport>

B. <cdk-virtual-scroll-viewport itemSize="50"><div *ngFor="let item of items">{{item}}</div></cdk-virtual-scroll-viewport>

C. <cdk-virtual-scroll-viewport itemSize="50"><div *cdkVirtualFor="let item of items">{{item}}</div></cdk-virtual-scroll-viewport>

D. <cdk-virtual-scroll-viewport itemSize="50"><div *ngFor="let item in items">{{item}}</div></cdk-virtual-scroll-viewport>

Solution

1. Step 1: Identify correct directive for virtual scroll

   Angular CDK virtual scroll requires *cdkVirtualFor instead of *ngFor to render visible items only.

2. Step 2: Check syntax and itemSize usage

   The itemSize attribute is required on cdk-virtual-scroll-viewport to set fixed item height for scrolling calculations.

3. Final Answer:

   <cdk-virtual-scroll-viewport itemSize="50"><div *cdkVirtualFor="let item of items">{{item}}</div></cdk-virtual-scroll-viewport> -> Option C

4. Quick Check:

   Use *cdkVirtualFor with itemSize on viewport [OK]

Hint: Use *cdkVirtualFor, not *ngFor, inside cdk-virtual-scroll-viewport [OK]

Common Mistakes:

• Using *ngFor instead of *cdkVirtualFor
• Using 'let item in items' instead of 'of'
• Omitting itemSize attribute

3. Given this Angular template snippet:

<cdk-virtual-scroll-viewport itemSize="40" style="height: 120px;">
  <div *cdkVirtualFor="let item of items">{{item}}</div>
</cdk-virtual-scroll-viewport>

and items = ['A', 'B', 'C', 'D', 'E'], how many items will be visible at once?

medium

A. 3 items

B. 5 items

C. 2 items

D. 4 items

Solution

1. Step 1: Calculate viewport height in pixels

   The viewport height is set to 120px.

2. Step 2: Divide viewport height by itemSize

   Each item is 40px tall, so 120px / 40px = 3 items visible at once.

3. Final Answer:

   3 items -> Option A

4. Quick Check:

   Visible items = viewport height / itemSize = 3 [OK]

Hint: Divide viewport height by itemSize to find visible items [OK]

Common Mistakes:

• Counting all items instead of visible ones
• Ignoring style height on viewport
• Using wrong division operator

4. What is wrong with this Angular virtual scroll code?

<cdk-virtual-scroll-viewport itemSize="50">
  <div *cdkVirtualFor="let item in items">{{item}}</div>
</cdk-virtual-scroll-viewport>

medium

A. The *cdkVirtualFor directive must be on the viewport, not the div.

B. Missing style height on viewport causes no scroll.

C. itemSize must be a number, not a string.

D. Using *cdkVirtualFor with 'in' instead of 'of' causes an error.

Solution

1. Step 1: Check syntax of *cdkVirtualFor directive

   The correct syntax uses 'of' to iterate over items, not 'in'. Using 'in' causes a template error.

2. Step 2: Verify other attributes

   itemSize can be a string representing a number, and style height is recommended but not syntax error. The directive must be on the repeated element, not viewport.

3. Final Answer:

   Using *cdkVirtualFor with 'in' instead of 'of' causes an error. -> Option D

4. Quick Check:

   *cdkVirtualFor uses 'of', not 'in' [OK]

Hint: Use 'of' with *cdkVirtualFor, not 'in' [OK]

Common Mistakes:

• Using 'in' instead of 'of' in *cdkVirtualFor
• Forgetting to set viewport height
• Placing directive on wrong element

5. You want to display a list of 10,000 items with virtual scrolling. Which combination of settings ensures smooth scrolling and correct rendering?

hard

A. Use *ngFor inside a scrollable div without cdk-virtual-scroll-viewport.

B. Use cdk-virtual-scroll-viewport with fixed itemSize, set viewport height, and use *cdkVirtualFor.

C. Use cdk-virtual-scroll-viewport with variable itemSize and *cdkVirtualFor.

D. Use cdk-virtual-scroll-viewport without itemSize, and *ngFor for rendering.

Solution

1. Step 1: Understand virtual scroll requirements

   Virtual scrolling requires a fixed itemSize to calculate visible items and smooth scrolling.

2. Step 2: Use correct directives and styles

   Use cdk-virtual-scroll-viewport with a fixed height and *cdkVirtualFor to render only visible items efficiently.

3. Step 3: Avoid common pitfalls

   Not setting itemSize or using *ngFor causes performance issues or errors.

4. Final Answer:

   Use cdk-virtual-scroll-viewport with fixed itemSize, set viewport height, and use *cdkVirtualFor. -> Option B

5. Quick Check:

   Fixed itemSize + *cdkVirtualFor + viewport height = smooth scroll [OK]

Hint: Fixed itemSize and *cdkVirtualFor are key for smooth virtual scroll [OK]

Common Mistakes:

• Omitting itemSize or viewport height
• Using *ngFor instead of *cdkVirtualFor
• Trying variable item sizes without special handling