Subject types (Subject, BehaviorSubject, ReplaySubject) in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a simple Subject in Angular.

Angular

import { [1] } from 'rxjs';

const subject = new [1]();

Drag options to blanks, or click blank then click option'

AReplaySubject

BBehaviorSubject

CSubject

DObservable

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Observable' instead of 'Subject'.

Confusing 'BehaviorSubject' or 'ReplaySubject' for the basic Subject.

The Subject class is imported from 'rxjs' and instantiated to create a new Subject.

2fill in blank

medium

Complete the code to create a BehaviorSubject with an initial value of 0.

Angular

import { BehaviorSubject } from 'rxjs';

const behaviorSubject = new BehaviorSubject([1]);

Drag options to blanks, or click blank then click option'

Anull

B0

Cundefined

D''

Attempts:

3 left

💡 Hint

Common Mistakes

Passing null or undefined as initial value.

Forgetting to provide an initial value.

A BehaviorSubject requires an initial value. Here, 0 is used as the starting value.

3fill in blank

hard

Fix the error in the code to create a ReplaySubject that buffers the last 3 values.

Angular

import { ReplaySubject } from 'rxjs';

const replaySubject = new ReplaySubject([1]);

Drag options to blanks, or click blank then click option'

Atrue

B'3'

Cundefined

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Passing a string '3' instead of number 3.

Passing a boolean or undefined.

The ReplaySubject constructor takes a number indicating how many previous values to buffer. It must be a number, here 3.

4fill in blank

hard

Fill both blanks to subscribe to a Subject and log emitted values.

Angular

subject.[1](value => {
  console.[2](value);
});

Drag options to blanks, or click blank then click option'

Asubscribe

Bnext

Clog

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' instead of 'subscribe' to listen.

Using 'console.error' instead of 'console.log'.

Use subscribe to listen to Subject emissions, and console.log to print values.

5fill in blank

hard

Fill all three blanks to create a BehaviorSubject, emit a value, and subscribe to log it.

Angular

const behaviorSubject = new [1](0);

behaviorSubject.[2](5);

behaviorSubject.[3](value => console.log(value));

Drag options to blanks, or click blank then click option'

ABehaviorSubject

Bnext

Csubscribe

DSubject

Attempts:

3 left

💡 Hint

Common Mistakes

Using Subject instead of BehaviorSubject for initial value.

Using subscribe to emit values instead of next.

Create a BehaviorSubject with initial 0, emit a new value with next, and listen with subscribe.

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