startWith

signature: startWith(an: Values): Observable

Emit given value first.

πŸ’‘ A BehaviorSubject can also start with an initial value!

​​Ultimate RxJS​​

Examples

( example tests )

Example 1: startWith on number sequence

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { startWith } from 'rxjs/operators';
import { of } from 'rxjs';
​
//emit (1,2,3)
const source = of(1, 2, 3);
//start with 0
const example = source.pipe(startWith(0));
//output: 0,1,2,3
const subscribe = example.subscribe(val => console.log(val));

Example 2: startWith for initial scan value

( StackBlitz | | jsBin | jsFiddle )

// RxJS v6+
import { startWith, scan } from 'rxjs/operators';
import { of } from 'rxjs';
​
//emit ('World!', 'Goodbye', 'World!')
const source = of('World!', 'Goodbye', 'World!');
//start with 'Hello', concat current string to previous
const example = source.pipe(
startWith('Hello'),
scan((acc, curr) => `${acc} ${curr}`)
);
/*
output:
"Hello"
"Hello World!"
"Hello World! Goodbye"
"Hello World! Goodbye World!"
*/
const subscribe = example.subscribe(val => console.log(val));

Example 3: startWith multiple values

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { startWith } from 'rxjs/operators';
import { interval } from 'rxjs';
​
//emit values in sequence every 1s
const source = interval(1000);
//start with -3, -2, -1
const example = source.pipe(startWith(-3, -2, -1));
//output: -3, -2, -1, 0, 1, 2....
const subscribe = example.subscribe(val => console.log(val));

Additional Resources

πŸ“ Source Code: https://github.com/ReactiveX/rxjs/blob/master/src/internal/operators/startWith.ts​