generators-demo.ts

// Returns an iterator of all positive integers
function* allPositiveIntegers(): IterableIterator<number> {
  let i = 1;
  while (true) {
    yield i++;
  }
}

// Returns an iterator which passes through selected items
// from another iterator, selected by a predicate
function* filter<T>(iterator: IterableIterator<T>, predicate: (value: T) => boolean): IterableIterator<T> {
  for (const x of iterator) {
    if (predicate(x)) {
      yield x;
    }
  }
}

// Returns an iterator which selects the first 'n' values
// from another iterator
function* take<T>(iterator: IterableIterator<T>, n: number): IterableIterator<T> {
  for (const x of iterator) {
    if (!n--) {
      return;
    }

    yield x;
  }
}

const isEven = x => (x % 2) == 0;

// Log out the first 5 even positive integers
for (const x of take(filter(allPositiveIntegers(), isEven), 5)) {
  console.log(x);
}