withLatestFrom

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

Also provide the last value from another observable.

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

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));

Related Recipes

• Progress Bar
• Game Loop

Additional Resources

• withLatestFrom - Official docs
• Combination operator: withLatestFrom - André Staltz

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