data = [1, 2, 3, 4, 5, 6];
ogObservable = rxjs.from(data);
filteredObservable = new rxjs.Subject();

isError = (val) => val == 3;

let lastStoredValue = null;
//sub so we can show the values from the result observable
filteredObservable.subscribe(x => console.log(`Got value from resulting observable: ${x}`));

ogObservable.subscribe(x => {
    //console.log(`Got ${x}, last value was ${lastStoredValue}`);
    // seed with the first value
    if (lastStoredValue === null) {
    lastStoredValue = x;
    return;
  }
  // if x is an error state, we need to reset the last stored value
  if (isError(x)) {
    //console.log("error");
    lastStoredValue = null
  }
  // if x is not an error state, then send the last stored value to the subject
    else {
    //console.log(`Not an error, pushing ${lastStoredValue}`);
    filteredObservable.next(lastStoredValue);
    lastStoredValue = x;
  }
});