Limit concurency count on javascript

limit_worker_sample.js

var list = []
for(i = 0; i < 200; i++) {
  list.push(i)
}
var semaphore = [true, true, true, true, true, true]
var uploadImage = function(num, callback) {
  setTimeout(function() {
    console.log('upload', num)
    callback()
  }, 2000)
}
var startWorker = function() {
  var timerId = setInterval(function() {
    if(list.length <= 0) {
      console.log('end')
      clearInterval(timerId)
      return;
    }
    if(semaphore.length >= 1) {
      semaphore.shift()

      var num = list.shift()
      uploadImage(num, function() {
        semaphore.push(true)
      })
    } else {
      console.log('waiting....')
    }
  }, 1000)
}
for(i in semaphore) {
  startWorker();
}