Subject types (Subject, BehaviorSubject, ReplaySubject) in Angular

Introduction

Subjects let you send and receive messages in Angular. Different types help you control how messages are shared and remembered.

You want to share data between parts of your app without direct connection.

You need to remember the last message so new listeners get it right away.

You want new listeners to get a few past messages when they start listening.

Syntax

Angular

import { Subject, BehaviorSubject, ReplaySubject } from 'rxjs';

const subject = new Subject<Type>();
const behaviorSubject = new BehaviorSubject<Type>(initialValue);
const replaySubject = new ReplaySubject<Type>(bufferSize);

Subject sends messages only to current listeners.

BehaviorSubject needs an initial value and remembers the last message.

Examples

Subject sends messages only to subscribers after they subscribe. The first message (1) is missed by the subscriber.

Angular

const subject = new Subject<number>();
subject.next(1);
subject.subscribe(value => console.log('Subject:', value));
subject.next(2);

BehaviorSubject sends the last message (1) immediately to new subscribers, then future messages.

Angular

const behaviorSubject = new BehaviorSubject<number>(0);
behaviorSubject.next(1);
behaviorSubject.subscribe(value => console.log('BehaviorSubject:', value));
behaviorSubject.next(2);

ReplaySubject remembers the last 2 messages (2 and 3) and sends them to new subscribers, then future messages.

Angular

const replaySubject = new ReplaySubject<number>(2);
replaySubject.next(1);
replaySubject.next(2);
replaySubject.next(3);
replaySubject.subscribe(value => console.log('ReplaySubject:', value));
replaySubject.next(4);

Sample Program

This Angular component shows how each subject type behaves when you subscribe and send messages. Buttons let you subscribe to each subject and see console logs.

Angular

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

@Component({
  selector: 'app-subject-demo',
  template: `
    <h2>Subject Types Demo</h2>
    <button (click)="subscribeToSubject()">Subscribe to Subject</button>
    <button (click)="subscribeToBehaviorSubject()">Subscribe to BehaviorSubject</button>
    <button (click)="subscribeToReplaySubject()">Subscribe to ReplaySubject</button>
  `
})
export class SubjectDemoComponent {
  subject = new Subject<string>();
  behaviorSubject = new BehaviorSubject<string>('Initial');
  replaySubject = new ReplaySubject<string>(2);

  constructor() {
    this.subject.next('Message 1');
    this.behaviorSubject.next('Message 1');
    this.replaySubject.next('Message 1');
    this.replaySubject.next('Message 2');
  }

  subscribeToSubject() {
    this.subject.subscribe(value => console.log('Subject:', value));
    this.subject.next('Message 2');
  }

  subscribeToBehaviorSubject() {
    this.behaviorSubject.subscribe(value => console.log('BehaviorSubject:', value));
    this.behaviorSubject.next('Message 2');
  }

  subscribeToReplaySubject() {
    this.replaySubject.subscribe(value => console.log('ReplaySubject:', value));
    this.replaySubject.next('Message 3');
  }
}

Important Notes

Subjects are both observers and observables, so you can send and listen to messages.

BehaviorSubject always needs an initial value.

ReplaySubject keeps a buffer of past messages to replay to new subscribers.

Summary

Subject sends messages only to current subscribers.

BehaviorSubject remembers the last message and sends it immediately to new subscribers.

ReplaySubject replays a set number of past messages to new subscribers.

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