custom operator filterChanges to filter only increasing numbers

index.js

const { rxObserver } = require('api/v0.3');
const { timer, from, of, EMPTY, pipe } = require('rxjs');
const { zip, startWith, pairwise, switchMap, filter } = require('rxjs/operators');


// create a source using values
const values$ = from([ 5, 10, 20, 10, 5, 20, 0, 5 ]);
const source$ = timer(0, 10).pipe(
  zip(values$, (_,x)=>x)
);

// it will take a predicate to compare values
// by default it will behave as distinctUntilChanged()
const filterChanges = (predicate = ((a,b) => a!==b)) => {
  // store previous value
  let prevValue = void 0;
  return pipe(
    filter((value, index)=>{
      // pass through the first value
      if (index === 0) {
        prevValue = value;
        return value;
      }

      // compare current with prev
      const result = predicate(value, prevValue);
      prevValue = value;
      return result;
    })
  );
};

// compare values pairwise
const result$ = source$.pipe(
  filterChanges((a, b) => a > b)
);

source$.subscribe(rxObserver('source$'));
result$.subscribe(rxObserver('result$'));