Generator + Promises === Shared Async Completion

async_completion.js

/**
All of the intervals which call the pollDate() function
after different time intervals will resolve with similar 
qualifying times thanks to managed state in the *fetchDate() 
generator function.
*/

function* fetchDate() {
  var isFetching = false
  var date = 0
  while (true) {
    const diff = new Date().getTime() - date
    if (!isFetching && diff > 500) {
      isFetching = true
      setTimeout(() => {
        isFetching = false
        date = new Date().getTime()
      }, 250)
    }
    yield isFetching ? -1 : date
  }
}

var gen = fetchDate()

function getDate(name) {
  return new Promise((resolve, reject) => {
    const date = gen.next().value
    if (date > 0) {
      resolve(date)
    } else {
      setTimeout(() => resolve(gen.next().value), 100)
    }
  })
}

const pollDate = async function(name) {
  const date = await getDate(name)
  return date > 0 ? date : pollDate(name)
}

const intervals = [0, 500, 501, 750, 1000, 2500]
intervals.map(interval =>
  setTimeout(async () => {
    const date = await pollDate(`After ${interval}`)
    console.log(`"After ${interval}" Task Finished: ${date}`)
  }, interval)
)

/**
Example console output:

> "After 0" Task Finished: 1499970482871
> "After 500" Task Finished: 1499970482871
> "After 501" Task Finished: 1499970482871
> "After 750" Task Finished: 1499970482871
> "After 1000" Task Finished: 1499970483735
> "After 2500" Task Finished: 1499970485236

*/