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In this post, we will see common uses of the RxJS (Reactive Extensions for JavaScript) library for reactive programming using observables that makes it easier to compose asynchronous or callback-based code.

It provides many operators that are functions that build on the observables foundation to enable sophisticated manipulation of collections. Operators take configuration options, and return a function that takes a source observable. Consequently, when executing this returned function, the operator observes the source observable emitted values, transforms them, and returns a new observable of those transformed values.

Now we will see some operators and their use with examples.

  1. from: From converts various other objects and data types into observables.
  2. groupBy: It groups the repeated object from the list
  3. mergeMap: mergeMap converts all repeated object to an array
  4. pipe: It’s a method on observables that is used for composing operators.
const data = [
{id: 1, Name: "-2" },
{id: 2, Name: "-2" },
{id: 1, Name: "-2" },
{id: 3, Name: "-2" },
{id: 4, Name: "-2" },
{id: 2, Name: "-2" },
{id: 3, Name: "-2" },
{id: 5, Name: "-1"}
const source = from(data)
const values = source.pipe(
groupBy(val =>,
mergeMap(group => group.pipe(toArray()))

here, mergeMap uses toArray() merge data into array.

const source = of(1, 2, 3, 4);
const example = source.pipe(reduce((acc, val) => acc + val));
const subscribe = example.subscribe(val => console.log('Sum:', val));
//output: Sum: 10'

Reduce passes the result of this callback (acc) from one array element to the other.


Delay delay’s the emitted values by a specified time.

const message = merge(
example.pipe(mapTo('World!'), delay(1000)),
example.pipe(mapTo('Goodbye'), delay(2000)),
example.pipe(mapTo('World!'), delay(3000))
// Output: 'Hello'...'World!'...'Goodbye'...'World!'

Two service calls and two responses are returned when the API is called twice.

const ob$ = this.httpClient.get('')
ob$.subscribe((data) => {
ob$.subscribe((data) => {

shareReplay avoids the two service calls and yet gets two responses.

const ob$ = this.httpClient.get('').pipe(shareReplay())
ob$.subscribe((data) => {
ob$.subscribe((data) => {

shareReplay reduces duplicate service calls. In upcoming blogs we’ll discuss more more rxjs operators.

About the author

Sushil Joshi

Sushil is a UI and UX developer at Synerzip. He has been developing interfaces for more than five years and has extensive knowledge of Bootstrap Framework, CSS3 and SCSS (CSS preprocessor). Sushil holds a Masters in Computer Applications from Pune University.

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