Hand made JS generators

async-generator.js

function range(from, to) {
  return { next: nextPromise, [Symbol.asyncIterator]: iterator }

  function next() {
    return from <= to
      ? { done: false, value: from++ }
      : { done: true }
  }

  function iterator() {
    return this
  }

  function nextPromise() {
    return new Promise(fun)
  }

  function fun(resolve, reject) {
    setTimeout(ready, 100)

    function ready() {
      resolve(next())
    }
  }
}

sync-generator.js

function range(from, to) {
  return { next: next, [Symbol.iterator]: iterator }

  function next() {
    return from <= to
      ? { done: false, value: from++ }
      : { done: true }
  }

  function iterator() {
    return this
  }
}