startWith() Operator in Angular

Introduction

The startWith() operator in Angular, part of the RxJS library, is used to prepend an initial value or sequence of values to the beginning of an observable sequence. This operator ensures that these initial values are emitted immediately upon subscription, before the observable begins emitting its usual sequence of values.

Installation

Ensure you have RxJS installed in your Angular project. If it's not already installed, you can add it using npm:

npm install rxjs

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Usage

Import the startWith operator from RxJS in the component or service where you want to use it:

import { startWith } from 'rxjs/operators';

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Example

Let's illustrate how startWith() works with a practical example.

Service Definition

Assume you have a service that provides an observable stream of numbers:

import { Injectable } from '@angular/core';
import { Observable, interval } from 'rxjs';

@Injectable({
  providedIn: 'root'
})
export class NumberService {

  getNumbers(): Observable<number> {
    return interval(1000); // Emits incremental numbers every second
  }
}

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Component Implementation

In your component, you can consume this service and use the startWith() operator to prepend an initial value before the stream of numbers starts emitting.

import { Component, OnInit } from '@angular/core';
import { NumberService } from './number.service';
import { startWith } from 'rxjs/operators';

@Component({
  selector: 'app-number-display',
  template: `
    <h2>Number Stream</h2>
    <ul>
      <li *ngFor="let number of numberStream$ | async">{{ number }}</li>
    </ul>
  `
})
export class NumberDisplayComponent implements OnInit {

  numberStream$: Observable<number>;

  constructor(private numberService: NumberService) { }

  ngOnInit(): void {
    this.numberStream$ = this.numberService.getNumbers().pipe(
      startWith(0) // Prepend 0 as the initial value
    );
  }

}

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In this example

• this.numberService.getNumbers() returns an observable that emits incremental numbers every second using interval(1000).
• startWith(0) is used within the pipe() method to prepend the initial value 0 to the beginning of the observable sequence. This means that 0 will be emitted immediately upon subscription, before the interval starts emitting numbers.

Template Usage

In the component's template (number-display.component.html), subscribe to numberStream$ using the async pipe to display the emitted numbers:

<h2>Number Stream</h2>
<ul>
  <li *ngFor="let number of numberStream$ | async">{{ number }}</li>
</ul>

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Summary

The startWith() operator is useful when you need to ensure that certain initial values are emitted before the observable's regular sequence starts. It's commonly used to initialize the state of UI components or provide default values in reactive programming scenarios within Angular applications