RxJS withLatestFrom Operator seed

seedWithLatestFrom.js

// One of the main 'gotchas' of combination operators like combineLatest and withLatestFrom is that they won't emit until all source observables emit at least once.
// In order to make your combination observables start immediately when they're subscribed to, use the startWith operator on each inner observable 

const Rx = require("rxjs");
// emit every 5s
const source = Rx.Observable.interval(5000).startWith('begin');
// emit every 1s
const secondSource = Rx.Observable.interval(1000);
// withLatestFrom slower than source
const example = secondSource.pipe(
  // both sources must emit at least 1 value (5s) before emitting
  Rx.operators.withLatestFrom(source),
  Rx.operators.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));