# RxJS Fundamentals (Angular)

> Observables, observers, subscriptions, subjects, operators basics, cold vs hot observables, unsubscribe strategies

- 22 interview questions
- Mid-Level
- [Interview Questions: Angular](https://sharpskill.dev/en/technologies/angular/interview-questions.md)

## 1. What is an Observable in RxJS?

**Answer**

An Observable is a lazy collection of multiple values over time. It represents a data stream that can emit zero, one, or multiple values synchronously or asynchronously. Unlike Promises which emit a single value, Observables can emit multiple values and support cancellation via unsubscribe.

## 2. What is the main difference between an Observable and a Promise?

**Answer**

Observables can emit multiple values over time and are cancellable with unsubscribe, while Promises emit a single value (resolve or reject) and cannot be cancelled once started. Additionally, Observables are lazy (execute only on subscription) while Promises are eager (execute immediately).

## 3. How to create a simple Observable that emits values 1, 2, 3?

**Answer**

The of() method from RxJS creates an Observable that emits the provided values as arguments synchronously, then completes. It's the simplest creation operator for known values. Alternatives: from() to convert an array/iterable, or new Observable() for full control over the emission flow.

## 19 more questions available

- What happens if you don't unsubscribe from an Observable?
- What is a Subject in RxJS?

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