sync-producer.js

function * Producer() {
  let number = 0;
  
  while (true) {
    yield number++;
  }
}

function * takeWhile(iterator, predicate) {
  let finished = false;
  
  while (!finished) {
    const { value, done } = iterator.next();
    
    if (done) {
      finished = true;
    }
    
    if (predicate(value)) {
      yield value;
    } else {
      finished = true;
    }
  }
}

const lessThan = (number) => takeWhile(
  Producer(), 
  (n) => n < number
);

function * head(iterable) {
  const iterator = iterable[Symbol.iterator]();
  
  const { value, done } = iterator.next();
  
  yield value;
}

function * tail(iterable) {
  const iterator = iterable[Symbol.iterator]();
  
  iterator.next();
  yield * iterator;
}

function * map(iterable, fn, context) {
  const iterator = iterable[Symbol.iterator]();
  
  for (let value of iterator) {
    yield fn.call(context, value);
  }
}

function * filter(iterable, fn, context) {
  const iterator = iterable[Symbol.iterator]();
  
  for (let value of iterator) {
    if (fn.call(context, value)) {
      yield value;
    }
  }
}

function * reduce(iterable, fn, initial, context) {
  const iterator = iterable[Symbol.iterator]();
  let accumulator = initial;
  
  if (!accumulator) {
    const { value } = iterator.next();
    
    accumulator = value;
  }
  
  for (let value of iterator) {
    yield accumulator = fn.call(context, accumulator, value);
  }
}

const arr = [1, 2, 3, 4, 5];

const double = map(arr, (n) => n * 2);
const even = filter(arr, (n) => n % 2 === 0)
const sum = reduce(arr, (prev, curr) => prev + curr);