filter.js

filter = function(pred, iterable) {
  return {
    [Symbol.iterator]: () => {
      const it = iterable[Symbol.iterator]();
      return {
        next: () => {
          let next;
          while (next = it.next(), !next.done) {
            if (!pred(next.value)) {
              continue;
            }
            break;
          }
          return next;
        }
      };
    }
  };
};