wrapper over a state with plain old callbacks

async-add-and-placeholder-object.js

function addAsync (getX, getY) {
  var x, y, consumer;
  
  getX(function (xVal) {
    if (y !== undefined && consumer !== undefined) {
      consumer (xVal + y) 
    }.else {
      x = xVal
    }
  })
  
  getY(function (yVal) {
    if (x !== undefined && consumer !== undefined) {
      consumer (x + yVal) 
    } else {
      y = yVal
    }
  })
  
  return function (doSomethingWithSum) {
    if (x !== undefined && y !== undefined) {
      doSomethingWithSum(x + y);
    } else {
      consumer = doSomethingWithSum;
    }
  }
}

var printSum = function (sum) {
  console.log(sum)
}

var sumPlaceholder = addAsync(fetchX, fetchY);
sumPlaceholder(printSum);