generators + requestIdleCallback

gen-ric.js

// this is a generic runner for iterators 
// for every yield, it checks to see if any time is remaining 
// on the idle loop, and executes more work if so. 
// else, it queues up for the next idle period 

function go(it, callback){
  requestIdleCallback(deadline => {
    let val = it.next()
    while(!val.done){
      if(deadline.timeRemaining() <= 0){
        go(it, callback)
        return 
      }  
      val = it.next()
    }
    callback(val.value)
  })
}

// this is a function that sums all numbers up to a given number 
// written as a generator, it yields every 1000 steps, 
// but you can adjust that as you please 
// yielding more often introduces overhead, 
// while less often might block thread 
function* summation(x){
  let i = x, ret = 1
  while(i>1){    
    ret += i
    i--
    if(i%1000 === 0) yield 
  }
  return ret 
}

// delay for a second before starting to get clean stats 
setTimeout(function(){
  console.profile()
  go(summation(1000000), result => {
    console.log(result)
    console.profileEnd()
  })
}, 1000)

Hmmmm nice idea, had fun going a bit further :)

https://jsfiddle.net/4L6n0qwy/4/