BehaviorSubject as simple store in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a BehaviorSubject with initial value 0.

Angular

import { BehaviorSubject } from 'rxjs';

const count$ = new BehaviorSubject([1]);

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

A0

Bnull

Cundefined

D''

Attempts:

3 left

💡 Hint

Common Mistakes

Using null or undefined as initial value causes errors.

Leaving the initial value empty.

The BehaviorSubject needs an initial value. Here, 0 is the starting number.

2fill in blank

medium

Complete the code to subscribe and log the current value from the BehaviorSubject.

Angular

count$.[1](value => console.log(value));

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

Aemit

Bsubscribe

Clisten

Dobserve

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent methods like emit or listen.

Trying to access value directly without subscribing.

To get updates from a BehaviorSubject, use subscribe.

3fill in blank

hard

Fix the error in updating the BehaviorSubject's value.

Angular

count$.[1](count$.value + 1);

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

Aset

Bchange

Cupdate

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using set or update which do not exist on BehaviorSubject.

Trying to assign value directly.

To update a BehaviorSubject's value, use the next method.

4fill in blank

hard

Fill both blanks to create a simple store with BehaviorSubject and expose it as an observable.

Angular

private countSubject = new BehaviorSubject([1]);

public count$ = this.countSubject.[2]();

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

A0

BasObservable

Csubscribe

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Exposing the BehaviorSubject directly instead of asObservable.

Using subscribe instead of asObservable.

The store starts with 0 and exposes the observable using asObservable() to prevent external updates.

5fill in blank

hard

Fill all three blanks to implement an increment method that updates the BehaviorSubject's value.

Angular

increment() {
  const current = this.countSubject.[1];
  this.countSubject.[2](current [3] 1);
}

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

Avalue

Bnext

C+

Dsubscribe

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to subscribe inside increment method.

Using wrong operators or methods to update value.

Get the current value with value, then call next with current plus one.

Practice

1. What is the main purpose of using BehaviorSubject in Angular as a simple store?

easy

A. To hold and share the latest value with all subscribers immediately

B. To perform HTTP requests automatically

C. To create Angular components dynamically

D. To manage routing between pages

Solution

1. Step 1: Understand BehaviorSubject role

   BehaviorSubject holds a current value and shares it with subscribers immediately when they subscribe.

2. Step 2: Compare with other options

   The other options describe unrelated Angular features like HTTP, components, and routing.

3. Final Answer:

   To hold and share the latest value with all subscribers immediately -> Option A

4. Quick Check:

   BehaviorSubject shares latest value immediately [OK]

Hint: BehaviorSubject always gives current value to new subscribers [OK]

Common Mistakes:

• Confusing BehaviorSubject with HTTP or routing
• Thinking it delays value delivery
• Assuming it creates components

2. Which of the following is the correct way to update the value stored in a BehaviorSubject named store$?

easy

A. store$.update(newValue);

B. store$.next(newValue);

C. store$.setValue(newValue);

D. store$.emit(newValue);

Solution

1. Step 1: Recall BehaviorSubject update method

   The method to update a BehaviorSubject's value is next().

2. Step 2: Check other method names

   Methods like update(), setValue(), and emit() do not exist on BehaviorSubject.

3. Final Answer:

   store$.next(newValue); -> Option B

4. Quick Check:

   Use next() to update BehaviorSubject [OK]

Hint: Use next() to push new values to BehaviorSubject [OK]

Common Mistakes:

• Using update() or setValue() instead of next()
• Confusing EventEmitter with BehaviorSubject
• Trying to assign value directly

3. Given this Angular code snippet, what will be logged to the console?

const store$ = new BehaviorSubject(0);
store$.subscribe(value => console.log('Subscriber 1:', value));
store$.next(5);
store$.subscribe(value => console.log('Subscriber 2:', value));
store$.next(10);

medium

A. Subscriber 1: 0
Subscriber 2: 0
Subscriber 1: 5
Subscriber 2: 5
Subscriber 1: 10
Subscriber 2: 10

B. Subscriber 1: 0
Subscriber 1: 5
Subscriber 2: 0
Subscriber 1: 10
Subscriber 2: 10

C. Subscriber 1: 0
Subscriber 1: 5
Subscriber 2: 5
Subscriber 1: 10
Subscriber 2: 10

D. Subscriber 1: 5
Subscriber 2: 5
Subscriber 1: 10
Subscriber 2: 10

Solution

1. Step 1: Trace first subscription

   Subscriber 1 subscribes first and immediately receives initial value 0, then receives 5 after next(5).

2. Step 2: Trace second subscription

   Subscriber 2 subscribes after next(5), so it immediately receives current value 5.

3. Step 3: Trace next(10) call

   Both subscribers receive 10 after next(10).

4. Final Answer:

   Subscriber 1: 0 Subscriber 1: 5 Subscriber 2: 5 Subscriber 1: 10 Subscriber 2: 10 -> Option C

5. Quick Check:

   BehaviorSubject sends current value on subscribe [OK]

Hint: New subscribers get latest value immediately [OK]

Common Mistakes:

• Assuming second subscriber gets initial 0 instead of 5
• Missing initial value emission on subscribe
• Confusing order of console logs

4. Identify the error in this Angular code using BehaviorSubject as a simple store:

const store$ = new BehaviorSubject();
store$.subscribe(value => console.log(value));
store$.next(42);

medium

A. subscribe() must be called with an object, not a function

B. next() cannot be called after subscribe()

C. BehaviorSubject cannot emit numbers

D. BehaviorSubject requires an initial value when created

Solution

1. Step 1: Check BehaviorSubject constructor

   BehaviorSubject requires an initial value passed to its constructor; here it is missing.

2. Step 2: Validate other statements

   Calling next() after subscribe() is valid; subscribe() accepts a function; BehaviorSubject can emit numbers.

3. Final Answer:

   BehaviorSubject requires an initial value when created -> Option D

4. Quick Check:

   BehaviorSubject must have initial value [OK]

Hint: Always provide initial value to BehaviorSubject constructor [OK]

Common Mistakes:

• Omitting initial value in constructor
• Thinking next() can't be called after subscribe()
• Confusing subscribe() argument types

5. You want to create a simple store using BehaviorSubject to hold a user's profile object and update it safely. Which approach correctly updates only the user's name without losing other profile data?

const profile$ = new BehaviorSubject({ name: 'Alice', age: 30 });
// Update name to 'Bob' here

hard

A. profile$.next({ ...profile$.value, name: 'Bob' });

B. profile$.next({ name: 'Bob' });

C. profile$.value.name = 'Bob';

D. profile$.update({ name: 'Bob' });

Solution

1. Step 1: Understand BehaviorSubject value update

   Directly assigning to value property does not notify subscribers; next() must be called with full updated object.

2. Step 2: Preserve existing data while updating name

   Use spread operator to copy existing profile and override name, then call next() with new object.

3. Step 3: Check other options

   profile$.next({ name: 'Bob' }); loses age property; profile$.value.name = 'Bob'; mutates value without notification; profile$.update({ name: 'Bob' }); uses non-existent update() method.

4. Final Answer:

   profile$.next({ ...profile$.value, name: 'Bob' }); -> Option A

5. Quick Check:

   Use next() with spread to update partial data [OK]

Hint: Use spread operator with next() to update partial store data [OK]

Common Mistakes:

• Overwriting entire object losing other properties
• Mutating value directly without next()
• Using non-existent update() method