Subject types (Subject, BehaviorSubject, ReplaySubject) in Angular - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an Angular Subject?

A Subject is a special type of Observable that allows values to be multicasted to many Observers. It acts like a bridge to send data to multiple subscribers.

intermediate

How does a BehaviorSubject differ from a regular Subject?

BehaviorSubject stores the latest value emitted and immediately sends it to new subscribers. It requires an initial value when created.

intermediate

What is the main feature of a ReplaySubject?

ReplaySubject records a specified number of previous values and replays them to new subscribers, even if they subscribe late.

beginner

Which Subject type would you use to always get the latest value upon subscription?

Use BehaviorSubject because it holds the current value and sends it immediately to new subscribers.

beginner

Can a regular Subject replay old values to new subscribers?

No, a regular Subject does not store or replay old values. New subscribers only receive future emissions.

Which Subject type requires an initial value when created?

A ReplaySubject

B Subject

C BehaviorSubject

D AsyncSubject

BehaviorSubject needs an initial value to store and emit immediately to new subscribers.

What does a ReplaySubject do when a new subscriber joins?

A Sends only future values

B Sends the latest value only

C Does not send any values

D Replays a set number of past values

ReplaySubject replays a specified number of past values to new subscribers.

Which Subject type does NOT store any previous values?

A Subject

B ReplaySubject

C BehaviorSubject

D AsyncSubject

A regular Subject does not store or replay any previous values.

If you want subscribers to always get the last emitted value immediately, which Subject should you use?

A BehaviorSubject

B Subject

C ReplaySubject

D None of the above

BehaviorSubject holds the latest value and sends it immediately to new subscribers.

Which Subject type can replay multiple past values to late subscribers?

A Subject

B ReplaySubject

C BehaviorSubject

D EventEmitter

ReplaySubject can replay multiple past values to subscribers who join late.

Explain the differences between Subject, BehaviorSubject, and ReplaySubject in Angular.

Think about how each Subject handles new subscribers and stored values.

Describe a scenario where using a BehaviorSubject is better than a regular Subject.

Consider when you want new subscribers to get the current state right away.

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