scan.js

export function* scan(iterable, reducer, initialValue) {
  const it = iterable[Symbol.iterator]();
  let accumulator = initialValue;
  let index = 0;
  if (arguments.length < 3) {
    const step = it.next();
    if (step.done) {
      return;
    }

    accumulator = step.value;
    ++index;
  }

  for (const value of it) {
    accumulator = reducer(accumulator, value, index++);
    yield accumulator;
  }
}