withLatestFrom

연산자(operator) 정의: 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

( jsBin | jsFiddle )

//emit every 5s
const source = Rx.Observable.interval(5000);
//emit every 1s
const secondSource = Rx.Observable.interval(1000);
const example = source
  .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

( jsBin | jsFiddle )

//emit every 5s
const source = Rx.Observable.interval(5000);
//emit every 1s
const secondSource = Rx.Observable.interval(1000);
//withLatestFrom slower than source
const example = secondSource
  //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

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

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