BehaviorSubject as simple store in Angular - Mini Project: Build & Apply

BehaviorSubject as simple store

📖 Scenario: You are building a simple Angular service to hold and share a user's login status across components.

🎯 Goal: Create a simple store using BehaviorSubject to keep track of whether the user is logged in or not.

📋 What You'll Learn

Create a BehaviorSubject to hold a boolean login status

Create a public observable to expose the login status

Add a method to update the login status

Use Angular service with proper imports

💡 Why This Matters

🌍 Real World

Many Angular apps need to share user login status or other state across components. BehaviorSubject is a simple way to build a reactive store for this.

💼 Career

Understanding BehaviorSubject and reactive state management is important for Angular developers working on real-world apps with shared state.

1

Create initial BehaviorSubject

Create an Angular service class called AuthService. Inside it, create a private BehaviorSubject named loggedIn initialized with false. Import BehaviorSubject from rxjs.

Angular

import { Injectable } from '@angular/core';
import { BehaviorSubject } from 'rxjs';

@Injectable({ providedIn: 'root' })
export class AuthService {
  // Your code here
}

Hint

Use new BehaviorSubject<boolean>(false) to create the initial store.

2

Add public observable for login status

Add a public readonly observable called isLoggedIn$ that exposes the loggedIn BehaviorSubject as an observable using the asObservable() method.

Angular

import { Injectable } from '@angular/core';
import { BehaviorSubject } from 'rxjs';

@Injectable({ providedIn: 'root' })
export class AuthService {
  private loggedIn = new BehaviorSubject(false);
  // Add public observable isLoggedIn$ here
}

Hint

Use this.loggedIn.asObservable() to expose the BehaviorSubject as observable.

3

Add method to update login status

Add a public method called setLoginStatus that takes a boolean parameter status and calls next(status) on the loggedIn BehaviorSubject to update the login status.

Angular

import { Injectable } from '@angular/core';
import { BehaviorSubject } from 'rxjs';

@Injectable({ providedIn: 'root' })
export class AuthService {
  private loggedIn = new BehaviorSubject(false);
  public readonly isLoggedIn$ = this.loggedIn.asObservable();

  // Add setLoginStatus method here
}

Hint

Define setLoginStatus(status: boolean) and call this.loggedIn.next(status) inside it.

4

Complete the Angular service

Ensure the AuthService class is decorated with @Injectable({ providedIn: 'root' }) and all imports are present. The service should now act as a simple store for login status using BehaviorSubject.

Angular

import { Injectable } from '@angular/core';
import { BehaviorSubject } from 'rxjs';

// Verify the service decorator and imports
@Injectable({ providedIn: 'root' })
export class AuthService {
  private loggedIn = new BehaviorSubject(false);
  public readonly isLoggedIn$ = this.loggedIn.asObservable();

  public setLoginStatus(status: boolean): void {
    this.loggedIn.next(status);
  }
}

Hint

Check that @Injectable and imports are correctly added at the top.

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