withLatestFrom

signature: withLatestFrom(other: Observable, project: Function): Observable

Also provide the last value from another observable.

πŸ’‘ If you want the last emission any time a variable number of observables emits, try combinelatest!

​​Ultimate RxJS​​

Examples

Example 1: Latest value from quicker second source

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { withLatestFrom, map } from 'rxjs/operators';
import { interval } from 'rxjs';
​
//emit every 5s
const source = interval(5000);
//emit every 1s
const secondSource = interval(1000);
const example = source.pipe(
withLatestFrom(secondSource),
map(([first, second]) => {
return `First Source (5s): ${first} Second Source (1s): ${second}`;
})
);
/*
"First Source (5s): 0 Second Source (1s): 4"
"First Source (5s): 1 Second Source (1s): 9"
"First Source (5s): 2 Second Source (1s): 14"
...
*/
const subscribe = example.subscribe(val => console.log(val));

Example 2: Slower second source

( StackBlitz | jsBin | jsFiddle )

// RxJS v6+
import { withLatestFrom, map } from 'rxjs/operators';
import { interval } from 'rxjs';
​
//emit every 5s
const source = interval(5000);
//emit every 1s
const secondSource = interval(1000);
//withLatestFrom slower than source
const example = secondSource.pipe(
//both sources must emit at least 1 value (5s) before emitting
withLatestFrom(source),
map(([first, second]) => {
return `Source (1s): ${first} Latest From (5s): ${second}`;
})
);
/*
"Source (1s): 4 Latest From (5s): 0"
"Source (1s): 5 Latest From (5s): 0"
"Source (1s): 6 Latest From (5s): 0"
...
*/
const subscribe = example.subscribe(val => console.log(val));

Additional Resources

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