RxJS-Subject: Enable Communication Between Components

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RxJS-Subject: Enable Communication Between Components

  • RxJS (Reactive Extensions for JavaScript) is a library for reactive programming using observables that makes it easier to compose asynchronous or callback-based code.
  • A subject in RxJS is used for multicasting data. This means that data can be pushed into a subject and the subject’s subscribers will in turn receive that pushed data.

There are 3 Variants of Subject:


  • BehaviorSubject needs an initial value as it must always return a value on subscription even if it hasn’t received a next().
  • Upon subscription, it returns the last value of the subject. A subscriber will get the latest value upon initial subscription.
  • For instance, an event stream of birthdays is a Subject, but the stream of a person’s age would be a BehaviorSubject.


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


  • The AsyncSubject is a variant where only the last value of the Observable execution is sent to its observers, and only when the execution completes.

In this blog, we’ll be focusing on using the Subject and in follow-up post we’ll show how to use Subject Variants.


  • Angular must be installed with minimum version 6

Setup an Angular Project:

  • Use Angular CLI (Command Line Interface) to create a new project, you can use any of the below command. Go to the directory where you would like to create a project and then run command below command.
    • ng new <project name>


    • ng n <project name>


  • Now go into your project directory through terminal and run ‘ng serve’ command.
    • cd subject-angular
    • ng serve -o

Above command will going to launch the application on http://localhost:4200/

and your project directory will going to look like.

Project Directory -RxJS

After setting up the Angular project perform below steps:

  • Run the command ng g c components/new to generate the new component and  In new.component.html replace the content with :

<input type=”text” placeholder=”Enter message”


  • Now run the command ng g s services/data. This will create service at  src\app\services\data.service.ts


  • Now Import subject and create object of the subject into DataService. The final code shall look like this:

import { Injectable } from ‘@angular/core’;

import { Subject } from ‘rxjs’;


providedIn: ‘root’


export class DataService {

public message = new Subject<string>();

constructor() { }

setMessage(value: string) {




Here message.next() method is used to send messages to an observable which are then sent to all subscribers of that observable.

  • Now, inject this service in new.component.ts and pass an instance of it to the constructor.

Do this for app.component.ts too. The code shall look like this:

import { Component, OnInit } from ‘@angular/core’;

import { DataService} from ‘src/app/service/data.service’;



selector: ‘app-new’,

templateUrl: ‘./new.component.html’,

styleUrls: [‘./new.component.css’]


export class NewComponent implements OnInit {

constructor(public dataService: DataService) { }

ngOnInit() {


sendMessage(event) {




Here we are passing the value to the service function setMessage().

  • Inside app.component.ts , subscribe to the subject to get the data from new.component.ts:

           import { Component } from ‘@angular/core’;

           import { Subscription } from ‘rxjs’;

           import { DataService } from ‘./service/data.service’;


selector: ‘app-root’,

templateUrl: ‘./app.component.html’,

styleUrls: [‘./app.component.css’]


export class AppComponent {

message: string;

subscription: Subscription;

constructor(public dataService: DataService) { }

ngOnInit() {

this.subscription = this.dataService.message.subscribe(

(message) => {

this.message = message;




ngOnDestroy() {




Here message.subscribe() method is used to subscribe to messages that are sent to an observable.






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