When preparing for Angular JavaScript interview questions, one of the most frequently asked topics is the difference between Observables and Promises. Since handling asynchronous operations is crucial in Angular, understanding these concepts and articulating them well in an interview can give you a competitive edge.
In this blog, we’ll dive deep into Observables vs. Promises, their differences, use cases, and best ways to explain them in an interview setting.
A Promise in JavaScript represents a single eventual completion (or failure) of an asynchronous operation. Promises are widely used to handle async tasks like API calls, file reading, or database interactions.
Promise Syntax
const myPromise = new Promise((resolve, reject) => {
setTimeout(() => {
resolve("Data fetched successfully!");
}, 2000);
});
myPromise.then(response => console.log(response)).catch(error => console.log(error));
In this example, after 2 seconds, the promise resolves with a success message.
Observables are part of RxJS (Reactive Extensions for JavaScript) and are used heavily in Angular to handle multiple values over time, making them ideal for real-time data streaming.
Observable Syntax
import { Observable } from 'rxjs';
const myObservable = new Observable(observer => {
observer.next("First Value");
observer.next("Second Value");
setTimeout(() => {
observer.next("Third Value");
observer.complete();
}, 2000);
});
myObservable.subscribe(value => console.log(value));
Here, we can see that an Observable emits multiple values over time before completing.
Feature Promises Observables Execution Executes immediately upon creation Executes only when subscribed Multiple Values Handles a single value Handles multiple values Cancellation Cannot be canceled once initiated Can be canceled using unsubscribe()Operators No transformation operators Has powerful RxJS operators like map(), filter(), merge()Use Case Suitable for one-time API calls Ideal for event streams, WebSockets, user inputs
Now that you understand the fundamentals, let’s break down how to answer Angular JavaScript interview questions about Observables vs. Promises effectively.
“A Promise in JavaScript is used to handle a single asynchronous event, while an Observable is a more powerful approach that allows handling multiple asynchronous data streams over time.”
Promises execute immediately when defined.
Observables execute only when subscribed.
Example:
const promise = new Promise(resolve => resolve("Hello, Promise!"));
promise.then(console.log);
const observable = new Observable(observer => observer.next("Hello, Observable!"));
observable.subscribe(console.log);
In this case, the Promise executes immediately, whereas the Observable only executes when subscribed.
Use Promises when dealing with a single API request.
Use Observables for continuous data streams, like:
Live search suggestions
WebSocket connections
User input tracking (e.g., fromEvent(document, 'click'))
One major advantage of Observables over Promises is RxJS operators. Example:
import { of } from 'rxjs';
import { map } from 'rxjs/operators';
of(1, 2, 3)
.pipe(map(num => num * 10))
.subscribe(console.log);
// Output: 10, 20, 30
This shows how Observables can transform data efficiently using operators.
Sometimes, you might need to convert a Promise into an Observable or vice versa.
Convert a Promise to an Observable
import { from } from 'rxjs';
const promise = fetch('https://jsonplaceholder.typicode.com/posts/1').then(res => res.json());
const observable = from(promise);
observable.subscribe(console.log);
Convert an Observable to a Promise
const observable = new Observable(observer => {
observer.next("Hello");
observer.complete();
});
observable.toPromise().then(console.log);
This shows the flexibility of working with both paradigms in Angular applications.
Here are some frequently asked questions in Angular JavaScript interviews:
What is the main difference between Promises and Observables?
When should you use Observables instead of Promises in Angular?
How does the RxJS subscribe method work?
What is the role of unsubscribe() in Observables?
How do you handle errors in Promises vs. Observables?
Can you explain how map(), filter(), and mergeMap() work in RxJS?
How do you use HttpClient with Observables in Angular?
Understanding Observables vs. Promises is a crucial part of preparing for Angular JavaScript interview questions. While Promises are useful for handling one-time asynchronous operations, Observables provide a more flexible and powerful approach for handling multiple values over time.