Top RxJS Interview Questions (2020) | CodeUsingJava








Most frequently asked RxJS Interview Questions



  1. What is RxJS?
  2. What is Stream in RxJS?
  3. What is Observable?
  4. What is the benefits of RxJS Observables on promises?
  5. What is the difference between Cold and Hot Observables?
  6. What is the difference between React and RxJS?
  7. What are RxJS Operators?
  8. What are observers?
  9. What are the advantages of Observable over Promise?
  10. What is Subscriptions?
  11. What is Subject?

What is RxJS?

RxJS is a library for reactive Streams which can be used to deal with asynchronous data streams and events called "Reactive Extensions for JavaScript" (RxJS).

  • RxJS is a library for reactive programming using Observables, to make it easier to compose asynchronous or callback-based code.
  • It is a powerful tool that turns complicated series of actions into concise code that is easy to manipulate.


What is Stream in RxJS?

  • A stream is a sequence of data elements made available over time.
  • A stream can be thought of as items on a conveyor belt being processed one at a time rather than in large batches.
  • The reason for it being called a stream is that you should think of the data as continuous and not really having an end, unless you explicitly define an end.


What is Observable?

  • Observable represents the idea of an invokable collection of future values or events.
  • In RxJS, we model asynchronous data streams using observable sequences or just called observables, too.


What is the benefits of RxJS Observables on promises?

  • Observables provide a feed, so that you can receive data from the same object multiple times.
  • So observables work like streams of data.
Cold and Hot Observables

What is the difference between Cold and Hot Observables ?

Cold Observables:
  • Cold observables start to run upon subscription, so observable sequence only starts pushing values to observers when subscribe is called.
Hot Observables:
  • Hot observables produce values even before subscriptions is made.
  • Hot observables such as mousemove events, stock pickers or WebSocket connections, are already produced in values even before subscription is active


What is the difference between React and RxJS ?

  • React is an open-source JavaScript library providing a view for data rendered as hyper text markup language.
  • RxJS stands for Reactive Extensions for JavaScript. It is a library for composing asynchronous programming in web development and event-based programs using observable sequences and LINQ-style query operators.


React RxJS

What are RxJS Operators ?

  • An operator is a method that acts on an Observable and changes the stream in some way.
  • The purpose of operator is to modify or filter the originally emitted values in a way that we normally need for the project tasks.


What are observers ?

  • Observer is a set of callbacks that are informative to listen to the values of the Observable.
  • Observers may listen or subscribe to the data being observed.

What are the advantages of Observable over Promise ?

  • An Observable is like a Stream. It enables to pass zero or more events where the callback is called for each event.
  • Observable is preferred over Promise because it provides the features of Promise and more.
  • With Observable we can handle a single or multiple events.
  • Observable also has the advantage over Promise to be cancelable.
  • Observable allows lazy initialization.
  • Observable allows to format data
  • Observable provides operators like map, forEach, reduce etc

What is Subscriptions ?

  • Subscriptions are objects returned when an Observable is subscribed.
  • Listening to the stream is called subscribing.


What is Subject ?

  • Subjects are unique types of Observers, that enables you to subscribe to other Observables and also listen to published data
  • Listening to the stream is called subscribing.