Async pipe for template subscriptions in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to display the observable value using async pipe in the template.

Angular

<p>{{ dataObservable[1] }}</p>

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

A| map

B| subscribe

C| then

D| async

Attempts:

3 left

💡 Hint

Common Mistakes

Using '| subscribe' which is not a valid Angular template pipe.

Trying to manually subscribe in the template.

The async pipe subscribes to the observable and displays its latest value automatically.

2fill in blank

medium

Complete the code to assign the observable value to a template variable using async pipe.

Angular

<ng-container *ngIf="userObservable[1] as user">
  <p>{{ user.name }}</p>
</ng-container>

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

A| async

B| filter

C| map

D| subscribe

Attempts:

3 left

💡 Hint

Common Mistakes

Using '| subscribe' which is invalid in templates.

Not using 'as' to assign the variable.

The async pipe allows assigning the emitted value to a template variable with as.

3fill in blank

hard

Fix the error in the template to correctly display the observable's emitted value.

Angular

<div>{{ messageObservable[1] }}</div>

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

A| async

B| subscribe

C.subscribe()

D| then

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to call '.subscribe()' in the template.

Using '| subscribe' which is not a valid pipe.

The async pipe is the correct way to unwrap observables in Angular templates. Using '.subscribe()' or '| subscribe' causes errors.

4fill in blank

hard

Fill both blanks to correctly use async pipe with ngIf and assign the value to a variable.

Angular

<div *ngIf="profileObservable[1][2]profile">
  <p>{{ profile.email }}</p>
</div>

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

A| async

B| subscribe

C as

D let

Attempts:

3 left

💡 Hint

Common Mistakes

Using '| subscribe' instead of '| async'.

Using 'let' instead of 'as' in ngIf.

Use | async to unwrap the observable and as to assign the value to a template variable.

5fill in blank

hard

Fill all three blanks to unwrap the observable, filter items with price greater than 20, and transform the price in the template.

Angular

<div *ngIf="itemsObservable[1] as items">
  <ul>
    <ng-container *ngFor="let item of items">
      <li *ngIf="item.price [2] 20">
        {{ item.name }} - {{ item.price [3] 1.2 | number:'1.2-2' }}
      </li>
    </ng-container>
  </ul>
</div>

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

A| async

B>

C*

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '| subscribe' instead of '| async'.

Using '<' instead of '>' in the price filter.

Forgetting to multiply price before formatting.

Use | async to unwrap the observable, filter items with price greater than 20, and multiply price by 1.2 for display.

Practice

1. What is the main benefit of using the async pipe in Angular templates?

easy

A. It automatically subscribes to Observables and unsubscribes when the component is destroyed.

B. It converts Observables into Promises.

C. It requires manual subscription management in the component class.

D. It only works with Promises, not Observables.

Solution

1. Step 1: Understand what async pipe does

   The async pipe subscribes to an Observable or Promise and returns the latest value it has emitted.

2. Step 2: Recognize automatic subscription management

   It also automatically unsubscribes when the component is destroyed, preventing memory leaks.

3. Final Answer:

   It automatically subscribes to Observables and unsubscribes when the component is destroyed. -> Option A

4. Quick Check:

   Async pipe manages subscriptions automatically = C [OK]

Hint: Async pipe handles subscribe/unsubscribe for you [OK]

Common Mistakes:

• Thinking async pipe converts Observables to Promises
• Believing manual subscription is still needed
• Assuming async pipe only works with Promises

2. Which of the following is the correct syntax to use the async pipe in an Angular template to unwrap an Observable named data$?

easy

A. {{ data$ | subscribe }}

B. {{ data$ | async }}

C. {{ data$.async }}

D. {{ async | data$ }}

Solution

1. Step 1: Recall async pipe syntax

   The async pipe is used with the pipe symbol | followed by async in the template interpolation.

2. Step 2: Match correct syntax

   The correct syntax is {{ data$ | async }} to unwrap the Observable data$.

3. Final Answer:

   {{ data$ | async }} -> Option B

4. Quick Check:

   Use pipe symbol then async = D [OK]

Hint: Remember: variable | async in interpolation [OK]

Common Mistakes:

• Placing async before the variable
• Using dot notation like data$.async
• Using subscribe pipe which does not exist

3. Given this component code:

data$ = of('Hello');

What will be displayed in the template?

<div>{{ data$ | async }}</div>

medium

A. Hello

B. Observable

C. data$ | async

D. Error: Cannot unwrap Observable

Solution

1. Step 1: Understand Observable emitting value

   The Observable data$ emits the string 'Hello' immediately.

2. Step 2: Async pipe unwraps emitted value

   The async pipe subscribes and unwraps the emitted value, displaying 'Hello' in the template.

3. Final Answer:

   Hello -> Option A

4. Quick Check:

   Async pipe unwraps 'Hello' from Observable = B [OK]

Hint: Async pipe shows emitted value, not Observable object [OK]

Common Mistakes:

• Expecting the word 'Observable' to display
• Showing the raw template code
• Thinking async pipe causes an error here

4. Identify the error in this Angular template snippet:

<div>{{ data | async() }}</div>

Assuming data is an Observable.

medium

A. Observable must be converted to Promise first.

B. Missing subscription in component code.

C. Parentheses after async are invalid; async is a pipe, not a function.

D. The variable name should end with $ to use async pipe.

Solution

1. Step 1: Check async pipe usage

   The async pipe is used without parentheses in templates.

2. Step 2: Identify syntax error

   Using async() with parentheses is invalid and causes a template error.

3. Final Answer:

   Parentheses after async are invalid; async is a pipe, not a function. -> Option C

4. Quick Check:

   Async pipe never uses parentheses = A [OK]

Hint: Async pipe is a pipe, not a function--no parentheses [OK]

Common Mistakes:

• Adding parentheses after async pipe
• Thinking async requires manual subscription
• Believing variable must have $ suffix

5. You have an Observable user$ that emits user objects. You want to display the user's name in the template and also show a loading message while waiting for data. Which template code correctly uses the async pipe and handles loading state?

hard

A. <div>{{ user$.subscribe(user => user.name) }}</div>
<div>Loading...</div>

B. <div>{{ user$ | async.name }}</div>
<div>Loading...</div>

C. <div *ngIf="user$ | async">{{ user.name }}</div>
<div *ngIf="!user$">Loading...</div>

D. <div *ngIf="user$ | async as user; else loading">{{ user.name }}</div>
<ng-template #loading>Loading...</ng-template>

Solution

1. Step 1: Use async pipe with *ngIf and as syntax

   The syntax *ngIf="user$ | async as user" unwraps the Observable and assigns the emitted value to user.

2. Step 2: Provide loading template with else

   Using else loading shows the loading message while waiting for data.

3. Final Answer:

   <div *ngIf="user$ | async as user; else loading">{{ user.name }}</div> <ng-template #loading>Loading...</ng-template> -> Option D

4. Quick Check:

   Async pipe with *ngIf and else for loading = A [OK]

Hint: Use *ngIf with async as and else for loading [OK]

Common Mistakes:

• Trying to access property directly on async pipe result without as
• Using subscribe inside template interpolation
• Not handling loading state properly