Top BehaviorSubject Interview Questions and Answers for Aspiring Developers

BehaviorSubject is one of the most useful yet often misunderstood reactive streams in RxJS. As a special type of Subject, it enables powerful state management and data sharing capabilities in reactive Angular and React applications. Mastering BehaviorSubject can significantly enhance your skills as a front-end developer.

However, BehaviorSubject has some unique behaviors that set it apart from regular Subjects and Observables. This makes it a popular topic in developer interviews To help you ace your next front-end interview, I have compiled this comprehensive guide on BehaviorSubject interview questions

In this article, we will cover:

  • What is BehaviorSubject and how is it different from other Observables?
  • Real-world use cases of BehaviorSubject
  • How to initialize, subscribe, and unsubscribe from a BehaviorSubject
  • Differences between BehaviorSubject vs ReplaySubject vs AsyncSubject
  • Error handling and backpressure strategies with BehaviorSubject
  • Testing BehaviorSubjects in Jasmine and Jest
  • Implementing state management solutions using BehaviorSubject

Let’s get started!

What is BehaviorSubject? How is it Different from Other Observables?

This is one of the most fundamental BehaviorSubject interview questions you may encounter

A BehaviorSubject is a special type of Observable that requires an initial value and emits the current value to new subscribers. It has an internal cache that stores the latest value emitted to its consumers.

This key difference makes BehaviorSubjects useful compared to:

  • Regular Observables – Which do not store values and only emit values after subscription
  • Subjects – Which act as a bridge but do not have memory of previously emitted values

Some other key behaviors of BehaviorSubjects:

  • New subscribers receive the last cached value immediately on subscription
  • Multicasting – emits values to multiple subscribers
  • Has a getValue() method to synchronously retrieve the last value

When Would You Use BehaviorSubject Over Other Observables?

Understanding real-world use cases is crucial for acing BehaviorSubject interview questions.

Some common scenarios where BehaviorSubject shines over regular Observables:

  • Sharing data between unrelated components – BehaviorSubject makes it easy to share data between components without a direct relationship. For example, sharing user data between a header component and sidebar component.

  • State management – BehaviorSubject can hold UI state or app state and notify subscribers of any change. This streamlines state management in complex applications.

  • Reactivity – Ability to react to current state changes in real-time. BehaviorSubject provides the latest state to all subscribers.

  • Caching HTTP requests – Results of a network call can be stored in a BehaviorSubject so that future subscribers can receive cached data immediately without needing to make additional HTTP requests.

  • Rehydrating state on refresh – On app reload or refresh, BehaviorSubjects can rehydrate UI by emitting the last cached state.

Step-by-Step Usage of BehaviorSubject

In order to properly utilize BehaviorSubjects, you need to understand how to initialize, subscribe, retrieve values, and unsubscribe. Here are some key steps involved:

1. Import BehaviorSubject

import { BehaviorSubject } from 'rxjs';

2. Initialize with an initial value

const subject = new BehaviorSubject(0); 

3. Subscribe to it

subject.subscribe(data => {  console.log(data);});

This will immediately emit the initial value to consumers.

4. Update the stream using next();

5. Retrieve current value using getValue()

let current = subject.getValue();

6. Unsubscribe when done


This covers the key aspects of using BehaviorSubjects in code. Familiarity with initializing, subscribing, emitting values, and unsubscribing is essential.

Key Differences Between BehaviorSubject, ReplaySubject and AsyncSubject

Understanding the nuances between different types of RxJS Subjects is vital for nailing BehaviorSubject interview questions.

The key differences are:

  • BehaviorSubject – Emits the latest value to new subscribers. Requires an initial value.
  • ReplaySubject – Emits a buffer (window) of previous values to new subscribers. Does not need initial value.
  • AsyncSubject – Emits the last value only after completion.

ReplaySubject can be configured to store a certain number of previous emissions using the buffer size parameter. AsyncSubject only caches the latest value but emits it after execution completes.

  • Use BehaviorSubject to get current state.
  • Use ReplaySubject to replay N previous emissions.
  • Use AsyncSubject to emit last value after completion.

Strategies for Error Handling with BehaviorSubject

Since faulty error handling can lead to crashes, interviewers often ask about strategies to handle errors with BehaviorSubjects.

Some best practices are:

  • Use catchError operator to gracefully handle errors in the stream.
  • Leverage onErrorResumeNext to fallback to a backup stream if errors occur.
  • Handle errors in subscribe block using error() callback.
  • Use throwError() to raise errors and catch() to handle them.
  • Tap into RxJS built-ins like of, empty and throw for handling different error scenarios.

Robust error handling ensures your application does not crash when an unexpected error occurs while using BehaviorSubjects.

How Does BehaviorSubject Handle Backpressure?

Backpressure occurs when consuming Observables cannot keep up with the rate of emissions from producing Observables. BehaviorSubjects have built-in backpressure handling capabilities:

  • It buffers all incoming items without dropping any values
  • Subscribers receive all the buffered values on subscription
  • This ensures no loss of data due to backpressure
  • However, buffering can cause memory issues if source emits too frequently
  • Use onBackpressureBuffer or onBackpressureDrop to control behavior

Testing BehaviorSubjects with Jasmine and Jest

Testing knowledge is highly valued during technical interviews nowadays. To test BehaviorSubjects:

  • Use the subscribe method to get notified when BehaviorSubject emits
  • Assert expected values are received using matchers like toEqual
  • Test BehaviorSubject emits initial value properly
  • Test updates using
  • Mock dependencies using spies like jasmine.createSpy
  • Ensure proper cleanup by unsubscribing during ngOnDestroy

Here is a sample Jasmine test for a BehaviorSubject that caches a username:


let username; let subject;beforeEach(() => {  subject = new BehaviorSubject('John'); });it('emits initial value on subscription', () => {  subject.subscribe(val => username = val);  expect(username).toEqual('John'); });it('emits new value on next()', () => {'Mary');  expect(username).toEqual('Mary');});

This covers the key aspects of unit testing BehaviorSubjects.

Implementing State Management with BehaviorSubject

BehaviorSubject is commonly used for state management in Angular and React:

  • Keep state as private BehaviorSubjects in services
  • Expose Observable wrappers to components
  • Components subscribe to Observables to get state updates
  • Service methods update state by calling

For example:


// cart.service.tsexport class CartService {  private cart = new BehaviorSubject<CartItem[]>([]);  cart$ = this.cart.asObservable();  addToCart(item: CartItem) {    // Logic to add item;   }}// cart.component.tsexport class CartComponent {    items = [];    constructor(private cartService: CartService) {    this.cartService.cart$.subscribe(data => {      this.items = data;    });  }}

This covers the fundamentals of managing state with BehaviorSubjects.

Key Takeaways from BehaviorSubject Interview Questions

To summarize, here are some key takeaways:

  • BehaviorSubject requires initial value and emits current value to new subscribers
  • Useful for sharing state across components and state management
  • Different from ReplaySubject and AsyncSubject in caching behavior
  • Handles errors using catchError and onErrorResumeNext
  • Manages backpressure by buffering values
  • Can be tested using subscriptions and spies
  • Enables simple state management solutions

Most upvoted answer is plainly wrong claiming that:

“A ReplaySubject acts exactly like a BehaviorSubject if you set its buffer size to 1 at the beginning.”

This is not totally true; check this great blog post on differences between those two. For example if you subscribe to a completed BehaviorSubject, you won’t receive the last value but for a ReplaySubject(1) you will receive the last value.

This is am important difference that should not be overlooked:

Check this code example here which comes from another great blog post on the topic.

From: Randall Koutniks book “Build Reactive Websites with RxJS.” :

A Subject is an object that’s a turbocharged observable. Subjects work a lot like regular observables, but each subscription is connected to the same source. Also, subjects can be observers, and they have next, error, and done methods that let them send data to all subscribers at once. Subjects are observers, so they can be sent straight into a subscribe call. All events from the original observable will be sent to its subscribers through the subject.

We can use the ReplaySubject to track history. A ReplaySubject records the last n events and plays them back to every new subscriber. For example in a chat applications. We can use it for tracking the record of previous chat history.

A BehaviorSubject is a simplified version of the ReplaySubject. The ReplaySubject stored an arbitrary number of events, the BehaviorSubject only records the value of the latest event. When a BehaviorSubject receives a new subscription, it sends the most recent value to the subscriber along with any new values that are sent. The BehaviorSubject is useful when dealing with single units of state, such as configuration options.

The accepted answer is wrong because BehaviorSubject!= ReplaySubject(1), as some posts have said. This isn’t just a matter of style; it’s the right answer.

A lot of people talk about “guards” in the comments, which is also where I most often found a use case for the Replay subjects. That is, if you have a situation like take(1) and don’t just want to take the first value

Check for example the following:

with the result:

In those cases clearly youd want a ReplaySubject! Working code:

  • Subject: A subscriber will not get published values until after they have subscribed.
  • Behavior: When a new subscriber signs up, they get either the most recent published value OR the first published value right away.

Another difference is you can use the value getter of BehaviorSubject to get the current value. This is very useful when you need just current value in certain circumstances. For example, when a user clicks something and you need the value only once. In this case, you dont need to subscribe and then unsubscribe suddenly. The only need is:

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8 Answers 8 Sorted by:

It really comes down to behavior and semantics. With a

  • Subject: Someone who has subscribed will only get published values that were sent out after the subscription. Do you really want that? Does the subscriber need to know anything about past values? If not, this is the right choice. If not, pick another one. For example, with component-to-component communication. Let’s say you have a part that sends events to other parts when a button is clicked. You can use a service with a subject to communicate.
  • BehaviorSubject – the last value is cached. A subscriber will get the latest value upon initial subscription. This subject is meant to stand for a value that changes over time. For example a logged in user. The initial user might be an anonymous user. The new value, however, is the authenticated user state once a user logs in. The BehaviorSubject is initialized with an initial value. This is sometimes important to coding preference. Say for instance you initialize it with a null. After that, you need to do a null check in your subscription. Maybe OK, or maybe annoying.
  • ReplaySubject—it can store up to a certain number of emissions Any subscribers will get all the cached values upon subscription. When would you need this behavior? To be honest, I haven’t needed it except in this case: if you launch a ReplaySubject with a buffer size of 1, it acts exactly like a BehaviorSubject. The last value is always stored, so it looks like it changes over time. There is no need for a null check like there was when the BehaviorSubject was set up with a null value. In this case, the subscriber doesn’t get any value until the first publication.

So it really comes down to the behavior you are expecting (as for which one to use). A BehaviorSubject is probably what you should use most of the time because what you really want to show is “value over time.” But I personally dont see anything wrong with the substitution of ReplaySubject initialized with 1.

The last thing you want to do is use the default Subject when you need some caching behavior. Take for example you are writing a routing guard or a resolve. You fetch some data in that guard and set it in a service Subject. On the routed part, you subscribe to the service subject to try to get the value that was sent out by the guard. OOPs. Wheres the value? It was already emitted, DUH. Use a “caching” subject!.

  • Subject: When it subscribes, it always gets the data that is pushed after it subscribes. e. previously pushed values are not received.

With this example, here’s the result that’ll be printed in the console:

  • Replay subjects: can be useful because they store a buffer of past values that will be sent to new subscriptions.

Here’s an example of how to use replay subjects that keep a buffer of two previous values and send them out on new subscriptions:

Here’s what that gives us at the console:

  • Behavior subjects are like replay subjects, but they will only send out the last value that was sent out, or a default value if no value was sent out before.

And the result:

A handy summary of the different observable types, non intuitive naming i know lol.

  • Subject: A subscriber will not get published values until after they have subscribed.
  • Behavior: When a new subscriber signs up, they get either the most recent published value OR the first published value right away.
  • When a new subscriber signs up, they get all previously published value(s) right away.

Subject vs ReplaySubject vs BehaviorSubject: Differences for Interviews | Angular Interview Concepts

How do you answer Behavioral Interview questions?

Use the STAR method. The STAR method stands for Situation, Task, Action, Result. It’s a formula worth memorizing because it can help you structure your responses to behavioral interview questions. Situation: Start by establishing the situation and sharing any important details. Task: Recount your specific task or responsibility.

Why do interviewers ask behavioral questions?

Learn why interviewers ask them, how to answer them using the STAR method, and discover most common behavioral interview questions (with sample answers). Behavioral job interview questions assess your skills based on how you behaved in certain professional scenarios in the past — in order to gauge how you’d handle similar challenges in the future.

What are some examples of behavioral job interview questions?

Here are a few other popular examples of behavioral job interview questions: Give us an example of a goal you failed to meet, and how you handled the situation. Tell us about a time when you solved a problem at your job that wasn’t part of your job description. Tell us of a time when you took a risky decision and it didn’t pay off.

What is a behavioral interview?

They are focused on your past behavior in different situations, and that’s precisely why they’re called behavioral. The idea is that your past behavior is a reliable indicator of your future performance — and these questions are a way for interviewers to get to know you, your strengths and weaknesses included.

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