Subject types (Subject, BehaviorSubject, ReplaySubject) in Angular - Step-by-Step Execution

Concept Flow - Subject types (Subject, BehaviorSubject, ReplaySubject)

Create Subject

↓

Subscribe Observers

↓

Emit Values

↓

Subject: Sends to current subscribers only

↓

BehaviorSubject: Sends last value + new values

↓

ReplaySubject: Sends last N values + new values

This flow shows how different Subject types handle subscriptions and emitted values in Angular.

Execution Sample

Angular

import { Subject } from 'rxjs';
const subject = new Subject<number>();
subject.subscribe(v => console.log('Sub1:', v));
subject.next(1);
subject.subscribe(v => console.log('Sub2:', v));
subject.next(2);

This code creates a Subject, subscribes two observers at different times, and emits values to show how Subject sends values only to current subscribers.

Execution Table

Step	Action	Subject State	Subscribers	Output
1	Create Subject	No values emitted	None	None
2	Subscribe Sub1	No values emitted	Sub1	None
3	Emit value 1	Value 1 emitted	Sub1	Sub1: 1
4	Subscribe Sub2	Value 1 emitted	Sub1, Sub2	None
5	Emit value 2	Value 2 emitted	Sub1, Sub2	Sub1: 2, Sub2: 2

💡 No more emissions; Subject completes or unsubscribes.

Variable Tracker

Variable	Start	After Step 3	After Step 5
subjectValue	undefined	1	2
subscribers	0	1	2

Key Moments - 3 Insights

Why does Sub2 not receive the value 1 when it subscribes after it was emitted?

Because Subject only sends values to subscribers active at the time of emission, as shown in execution_table step 4 where Sub2 subscribes but no output occurs.

How would BehaviorSubject behave differently at step 4?

BehaviorSubject sends the last emitted value immediately to new subscribers, so Sub2 would receive value 1 upon subscribing, unlike Subject.

What does ReplaySubject do differently compared to BehaviorSubject?

ReplaySubject can send multiple past values (not just the last one) to new subscribers, depending on its buffer size.

Visual Quiz - 3 Questions

Look at the execution_table, what output occurs at step 3?

A Sub1: 1

B Sub2: 1

C Sub1: 2

D No output

💡 Hint

Check the Output column at step 3 in execution_table.

At which step does Sub2 start receiving values?

A Step 4

B Step 3

C Step 5

D Step 2

💡 Hint

Look at Subscribers and Output columns for Sub2 in execution_table.

If we replace Subject with BehaviorSubject, what changes at step 4?

A Sub2 receives no values

B Sub2 receives value 1 immediately

C Sub1 stops receiving values

D No change

💡 Hint

Recall BehaviorSubject sends last emitted value to new subscribers.

Concept Snapshot

Subject types in Angular:
- Subject: sends values only to current subscribers
- BehaviorSubject: sends last emitted value immediately to new subscribers
- ReplaySubject: sends last N emitted values to new subscribers
Use Subjects to multicast values to multiple observers
Choose type based on how past values should be replayed

Full Transcript

This lesson shows how Angular's Subject types work. A Subject sends values only to subscribers active when the value is emitted. BehaviorSubject remembers the last value and sends it immediately to new subscribers. ReplaySubject can remember multiple past values and send them to new subscribers. The example code creates a Subject, subscribes one observer, emits a value, then subscribes a second observer who does not get the previous value. When a new value is emitted, both observers receive it. This helps understand how these Subjects behave differently when subscribers join at different times.