Subscribing to observables in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to subscribe to the observable and log the data.

Angular

this.dataService.getData().[1](data => console.log(data));

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

Amap

Bsubscribe

Cfilter

Dpipe

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'map' instead of 'subscribe' which only transforms data but does not listen.

Using 'pipe' without subscribing does not trigger the observable.

The subscribe method is used to listen to the observable and receive data.

2fill in blank

medium

Complete the code to handle errors when subscribing to the observable.

Angular

this.apiService.fetchItems().subscribe({ next: items => this.items = items, error: [1] });

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

Aconsole.log()

B() => alert('Done')

Citems => this.items = items

Derr => console.error(err)

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a non-function value to error handler.

Confusing error handler with next handler.

The error property expects a function to handle errors. Here, it logs the error.

3fill in blank

hard

Fix the error in the subscription to properly unsubscribe on component destroy.

Angular

subscription = this.service.getUpdates().[1](data => this.update(data));

ngOnDestroy() {
  this.subscription.unsubscribe();
}

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

Apipe

Bmap

Csubscribe

Dfilter

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pipe' which returns an observable, not a subscription.

Trying to unsubscribe something that is not a subscription.

Only subscribe returns a subscription object that can be unsubscribed.

4fill in blank

hard

Fill both blanks to subscribe to an observable and handle completion.

Angular

this.observable.[1]({ next: val => console.log(val), [2]: () => console.log('Complete') });

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

Asubscribe

Bcomplete

Cerror

Dpipe

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pipe' instead of 'subscribe'.

Confusing 'error' and 'complete' handlers.

The subscribe method accepts an object with next, error, and complete handlers.

5fill in blank

hard

Fill all three blanks to subscribe with next, error, and complete handlers.

Angular

this.dataStream.[1]({ [2]: data => this.process(data), [3]: err => this.handleError(err), complete: () => this.finish() });

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

Asubscribe

Bnext

Cerror

Dpipe

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pipe' instead of 'subscribe'.

Mixing up 'next' and 'error' keys.

Use subscribe with an object containing next, error, and complete callbacks.

Practice

1. What does subscribing to an observable in Angular allow you to do?

easy

A. React to data or events over time

B. Create new components automatically

C. Change the Angular version

D. Compile TypeScript to JavaScript

Solution

1. Step 1: Understand the purpose of observables

   Observables emit data or events over time, like a stream of values.

2. Step 2: Role of subscribing

   Subscribing lets you listen and react to these emitted values as they arrive.

3. Final Answer:

   React to data or events over time -> Option A

4. Quick Check:

   Subscribing = Reacting to data/events [OK]

Hint: Subscribing means listening to data changes over time [OK]

Common Mistakes:

• Thinking subscribing creates components
• Confusing subscribing with compilation
• Believing subscribing changes Angular version

2. Which is the correct syntax to subscribe to an observable named data$ in Angular?

easy

A. data$.subscribe(value => console.log(value));

B. data$.subscribe = value => console.log(value);

C. subscribe(data$ => console.log(data$));

D. data$.subscribe(console.log());

Solution

1. Step 1: Identify correct subscribe method usage

   The subscribe method is called on the observable with a callback function.

2. Step 2: Check syntax correctness

   data$.subscribe(value => console.log(value)); correctly calls data$.subscribe with an arrow function receiving the emitted value.

3. Final Answer:

   data$.subscribe(value => console.log(value)); -> Option A

4. Quick Check:

   Correct subscribe syntax = data$.subscribe(value => console.log(value)); [OK]

Hint: Use observable.subscribe(callback) to listen [OK]

Common Mistakes:

• Assigning subscribe instead of calling it
• Calling subscribe as a standalone function
• Calling console.log() prematurely

3. Given this code snippet:

const obs$ = of(1, 2, 3);
obs$.subscribe({
  next: x => console.log(x),
  error: err => console.error(err),
  complete: () => console.log('Done')
});

What will be the console output?

medium

A. Done 1 2 3

B. 1 2 3 Done

C. 1 2 3

D. Error

Solution

1. Step 1: Understand observable emissions

   The observable emits values 1, 2, 3 in sequence.

2. Step 2: Analyze subscription callbacks

   The next callback logs each value, then complete logs 'Done'.

3. Final Answer:

   1 2 3 Done -> Option B

4. Quick Check:

   Values then complete message = 1 2 3 Done [OK]

Hint: next logs values, complete logs 'Done' after all [OK]

Common Mistakes:

• Assuming complete logs before values
• Ignoring complete callback
• Expecting error without error emission

4. What is wrong with this subscription code?

this.data$.subscribe(data => {
  this.value = data;
});
// No unsubscribe called anywhere

medium

A. It will unsubscribe automatically

B. It will throw a syntax error

C. It will cause a memory leak if component is destroyed

D. It will not receive any data

Solution

1. Step 1: Identify subscription management

   The code subscribes but never unsubscribes, risking resource leaks.

2. Step 2: Understand Angular component lifecycle

   Without unsubscribing, the subscription stays active after component destruction, causing memory leaks.

3. Final Answer:

   It will cause a memory leak if component is destroyed -> Option C

4. Quick Check:

   Missing unsubscribe = memory leak risk [OK]

Hint: Always unsubscribe to avoid memory leaks [OK]

Common Mistakes:

• Assuming Angular unsubscribes automatically
• Thinking no data will be received
• Confusing syntax errors with runtime issues

5. You want to subscribe to two observables obs1$ and obs2$ and combine their latest values whenever either emits. Which RxJS operator should you use before subscribing?

hard

A. filter

B. map

C. catchError

D. combineLatest

Solution

1. Step 1: Understand the need to combine latest values

   You want to react to the latest values from both observables whenever either emits.

2. Step 2: Identify the correct RxJS operator

   combineLatest emits an array of latest values from all observables whenever any emits.

3. Final Answer:

   combineLatest -> Option D

4. Quick Check:

   Combine latest values = combineLatest operator [OK]

Hint: Use combineLatest to merge latest values from multiple observables [OK]

Common Mistakes:

• Using map which transforms but doesn't combine
• Using filter which only filters emissions
• Using catchError which handles errors, not combining