Subject types (Subject, BehaviorSubject, ReplaySubject) in Angular - Mini Project: Build & Apply

Using Subject Types in Angular: Subject, BehaviorSubject, ReplaySubject

📖 Scenario: You are building a simple Angular app that shares messages between components using different types of Subjects.This helps you understand how Subject, BehaviorSubject, and ReplaySubject work differently in real apps.

🎯 Goal: Create an Angular standalone component that sets up three different Subjects: Subject, BehaviorSubject, and ReplaySubject.Send messages through each and subscribe to show how they behave differently.

📋 What You'll Learn

Create a Subject called simpleSubject.

Create a BehaviorSubject called behaviorSubject with initial value 'Initial'.

Create a ReplaySubject called replaySubject with buffer size 2.

Subscribe to each subject and store the latest message in separate variables.

Send at least two messages to each subject.

Display the latest messages from each subject in the component template.

💡 Why This Matters

🌍 Real World

Sharing data reactively between components in Angular apps, such as user input, notifications, or live updates.

💼 Career

Understanding Subjects is essential for Angular developers to manage asynchronous data streams and component communication effectively.

1

Set up Subjects in the component

In the Angular component class, import Subject, BehaviorSubject, and ReplaySubject from rxjs. Then create three properties: simpleSubject as a new Subject<string>(), behaviorSubject as a new BehaviorSubject<string>('Initial'), and replaySubject as a new ReplaySubject<string>(2).

Angular

import { Component } from '@angular/core';
import { Subject, BehaviorSubject, ReplaySubject } from 'rxjs';

@Component({
  selector: 'app-subject-demo',
  standalone: true,
  template: ``,
})
export class SubjectDemoComponent {
  // Create simpleSubject, behaviorSubject, and replaySubject here
}

Hint

Remember to import Subject, BehaviorSubject, and ReplaySubject from rxjs and create each as a class property.

2

Add variables to hold latest messages

Add three string properties to the component class: simpleMessage, behaviorMessage, and replayMessage. Initialize each with an empty string ''.

Angular

import { Component } from '@angular/core';
import { Subject, BehaviorSubject, ReplaySubject } from 'rxjs';

@Component({
  selector: 'app-subject-demo',
  standalone: true,
  template: ``,
})
export class SubjectDemoComponent {
  simpleSubject = new Subject();
  behaviorSubject = new BehaviorSubject('Initial');
  replaySubject = new ReplaySubject(2);

  // Add simpleMessage, behaviorMessage, replayMessage variables here
}

Hint

These variables will store the latest message received from each subject.

3

Subscribe to subjects and send messages

In the component constructor, subscribe to simpleSubject, behaviorSubject, and replaySubject. For each subscription, update the corresponding message variable (simpleMessage, behaviorMessage, replayMessage) with the received value. Then send two messages to each subject: 'First' and 'Second'.

Angular

import { Component } from '@angular/core';
import { Subject, BehaviorSubject, ReplaySubject } from 'rxjs';

@Component({
  selector: 'app-subject-demo',
  standalone: true,
  template: ``,
})
export class SubjectDemoComponent {
  simpleSubject = new Subject();
  behaviorSubject = new BehaviorSubject('Initial');
  replaySubject = new ReplaySubject(2);

  simpleMessage = '';
  behaviorMessage = '';
  replayMessage = '';

  constructor() {
    // Subscribe to subjects and send messages here
  }
}

Hint

Subscribe to each subject and update the message variables inside the subscription callback. Then send two messages to each subject using next().

4

Display latest messages in the template

In the component template, add three paragraphs showing the latest messages from each subject. Use interpolation to display simpleMessage, behaviorMessage, and replayMessage with labels: Simple Subject:, Behavior Subject:, and Replay Subject: respectively.

Angular

import { Component } from '@angular/core';
import { Subject, BehaviorSubject, ReplaySubject } from 'rxjs';

@Component({
  selector: 'app-subject-demo',
  standalone: true,
  template: `
    
  `,
})
export class SubjectDemoComponent {
  simpleSubject = new Subject();
  behaviorSubject = new BehaviorSubject('Initial');
  replaySubject = new ReplaySubject(2);

  simpleMessage = '';
  behaviorMessage = '';
  replayMessage = '';

  constructor() {
    this.simpleSubject.subscribe(value => this.simpleMessage = value);
    this.behaviorSubject.subscribe(value => this.behaviorMessage = value);
    this.replaySubject.subscribe(value => this.replayMessage = value);

    this.simpleSubject.next('First');
    this.simpleSubject.next('Second');

    this.behaviorSubject.next('First');
    this.behaviorSubject.next('Second');

    this.replaySubject.next('First');
    this.replaySubject.next('Second');
  }
}

Hint

Use interpolation {{ variable }} inside paragraph tags to show each message with a label.

Practice

1. Which type of Subject in Angular immediately sends the last emitted value to new subscribers?

easy

A. AsyncSubject

B. BehaviorSubject

C. ReplaySubject

D. Subject

Solution

1. Step 1: Understand BehaviorSubject behavior

   BehaviorSubject stores the last emitted value and sends it immediately to any new subscriber.

2. Step 2: Compare with other Subjects

   Subject sends only to current subscribers without remembering past values. ReplaySubject replays multiple past values, not just the last one.

3. Final Answer:

   BehaviorSubject -> Option B

4. Quick Check:

   Last value sent immediately = BehaviorSubject [OK]

Hint: BehaviorSubject always sends last value to new subscribers [OK]

Common Mistakes:

• Confusing ReplaySubject with BehaviorSubject
• Thinking Subject remembers past values
• Assuming AsyncSubject behaves like BehaviorSubject

2. Which of the following is the correct way to create a ReplaySubject that stores the last 3 values?

easy

A. new ReplaySubject(1)

B. new ReplaySubject()

C. new ReplaySubject(3)

D. new ReplaySubject('3')

Solution

1. Step 1: Check ReplaySubject constructor

   ReplaySubject takes a number argument specifying how many past values to store and replay.

2. Step 2: Validate options

   new ReplaySubject(3) correctly passes number 3. new ReplaySubject() uses no argument (defaults to unlimited or 1 depending on implementation). new ReplaySubject(1) stores only 1 value. new ReplaySubject('3') passes a string, which is invalid.

3. Final Answer:

   new ReplaySubject(3) -> Option C

4. Quick Check:

   ReplaySubject(3) stores last 3 values [OK]

Hint: ReplaySubject(n) stores last n values for new subscribers [OK]

Common Mistakes:

• Passing string instead of number to ReplaySubject
• Omitting the buffer size argument
• Confusing ReplaySubject with BehaviorSubject constructor

3. Consider this code snippet:

const subject = new BehaviorSubject('start');
subject.next('first');
subject.subscribe(value => console.log('Subscriber 1:', value));
subject.next('second');
subject.subscribe(value => console.log('Subscriber 2:', value));

What will be printed to the console?

medium

A. Subscriber 1: startSubscriber 1: firstSubscriber 1: secondSubscriber 2: second

B. Subscriber 1: firstSubscriber 1: secondSubscriber 2: firstSubscriber 2: second

C. Subscriber 1: firstSubscriber 1: secondSubscriber 2: second

D. Subscriber 1: startSubscriber 1: secondSubscriber 2: startSubscriber 2: second

Solution

1. Step 1: Trace BehaviorSubject emissions

   BehaviorSubject starts with 'start', then next emits 'first'. Subscriber 1 subscribes after 'first', so it receives the last value 'first' immediately, then 'second' when emitted.

2. Step 2: Analyze Subscriber 2 subscription

   Subscriber 2 subscribes after 'second' is emitted, so it immediately receives 'second'.

3. Final Answer:

   Subscriber 1: start
   Subscriber 1: first
   Subscriber 1: second
   Subscriber 2: second -> Option A

4. Quick Check:

   BehaviorSubject sends last value on subscribe [OK]

Hint: BehaviorSubject sends last value immediately on subscribe [OK]

Common Mistakes:

• Assuming Subscriber 2 gets 'first' value
• Ignoring initial 'start' emission to Subscriber 1
• Ignoring immediate emission on subscription

4. What is wrong with this code snippet?

const subject = new Subject();
subject.next('hello');
subject.subscribe(value => console.log(value));

medium

A. The subscription will not receive 'hello' because Subject does not replay past values.

B. The code will throw a syntax error because subscribe is called after next.

C. The Subject should be replaced with BehaviorSubject to avoid errors.

D. The next() method cannot be called before any subscribers exist.

Solution

1. Step 1: Understand Subject behavior

   Subject sends values only to current subscribers. Since next('hello') is called before subscribe, no subscriber receives it.

2. Step 2: Check for errors

   There is no syntax or runtime error. The code runs but subscriber misses the first value.

3. Final Answer:

   The subscription will not receive 'hello' because Subject does not replay past values. -> Option A

4. Quick Check:

   Subject sends only to current subscribers [OK]

Hint: Subject does not replay values to late subscribers [OK]

Common Mistakes:

• Expecting subscriber to get values emitted before subscription
• Thinking next() before subscribe causes error
• Confusing Subject with BehaviorSubject or ReplaySubject

5. You want to create a message service that stores the last 2 messages and sends them to any new subscriber immediately. Which Subject type and setup should you use?

hard

A. Use ReplaySubject(1) and call next() twice to replay last 2 messages.

B. Use BehaviorSubject with initial value and call next() twice to store last 2 messages.

C. Use Subject and manually store last 2 messages in an array to send on subscribe.

D. Use ReplaySubject(2) to store and replay last 2 messages to new subscribers.

Solution

1. Step 1: Identify requirement for storing multiple past messages

   BehaviorSubject stores only the last single value, so it cannot replay two messages automatically.

2. Step 2: Choose Subject type that replays multiple past values

   ReplaySubject with buffer size 2 stores and replays last 2 messages automatically to new subscribers.

3. Step 3: Evaluate other options

   Subject does not store past messages. ReplaySubject(1) stores only one message, not two.

4. Final Answer:

   Use ReplaySubject(2) to store and replay last 2 messages to new subscribers. -> Option D

5. Quick Check:

   ReplaySubject(n) replays last n messages [OK]

Hint: ReplaySubject(n) replays last n messages automatically [OK]

Common Mistakes:

• Using BehaviorSubject to store multiple past messages
• Assuming Subject stores past messages
• Setting ReplaySubject buffer size too low