Comparing two arrays of objects for matching properties then creating two new arrays

I have the following two arrays:

var oldOrder = [{"Id": "10","Qty": 1}, {"Id": "3","Qty": 2}, {"Id": "9","Qty": 2}]; 
var newOrder = [{"Id": "5","Qty": 2},{"Id": "10","Qty": 3}, {"Id": "9","Qty": 1}];

I need to end up with a newArray containing only one object per Id, but if the ids in an object in the oldOrder array and newOrder array then I need to get the difference in the Qty positive or negative. So for example the above would create this new array:

var newArray = [{"Id": "5","Qty": 2},{"Id": "10","Qty": 2}, {"Id": "9","Qty": -1}];

I would also like to get the dropped orders that are present in the oldOrder Array but not in the newOrder array:

var droppedOrders = [{"Id": "3","Qty": 2}];

I have already got this logic for getting the dropped orders but I am struggling with creating the newArray and tying the logic into the same function if possible?

function comparer(otherArray){
  return function(current){
    var onlyInOld = otherArray.filter(function(other){
      return other.Id == current.Id;
    }).length == 0;
    return onlyInOld;
  }
}

var droppedOrders= oldOrder.filter(comparer(newOrder));
console.log(droppedOrders);

Edited to add that I cannot use ESC6 features like spread or fat arrow etc.

Answer

You can easily achieve the result using Map

1) Using ES6

var oldOrder = [
  { Id: "10", Qty: 1 },
  { Id: "3", Qty: 2 },
  { Id: "9", Qty: 2 },
];
var newOrder = [
  { Id: "5", Qty: 2 },
  { Id: "10", Qty: 3 },
  { Id: "9", Qty: 1 },
];

const oldMap = new Map(oldOrder.map((o) => [o.Id, o]));
const newMap = new Map(newOrder.map((o) => [o.Id, o]));

const result = newOrder.map((o) =>
  oldMap.has(o.Id) ? { ...o, Qty: o.Qty - oldMap.get(o.Id).Qty } : o
);

const droppedOrders = oldOrder.filter(({ Id }) => !newMap.has(Id));

console.log(result);
console.log(droppedOrders);

/* This is not a part of answer. It is just to give the output fill height. So IGNORE IT */

.as-console-wrapper {
  max-height: 100% !important;
  top: 0;
}

2) Using ES5

var oldOrder = [
  { Id: "10", Qty: 1 },
  { Id: "3", Qty: 2 },
  { Id: "9", Qty: 2 },
];
var newOrder = [
  { Id: "5", Qty: 2 },
  { Id: "10", Qty: 3 },
  { Id: "9", Qty: 1 },
];

const oldMap = {};
const newMap = {};
oldOrder.forEach(function (o) {
  oldMap[o.Id] = o;
});
newOrder.forEach(function (o) {
  newMap[o.Id] = o;
});

const result = newOrder.map(function (o) {
  return oldMap[o.Id]
    ? Object.assign({}, o, { Qty: o.Qty - oldMap[o.Id].Qty })
    : o;
});

const droppedOrders = oldOrder.filter(function (o) {
  return !newMap[o.Id];
});

console.log(result);
console.log(droppedOrders);

/* This is not a part of answer. It is just to give the output fill height. So IGNORE IT */
.as-console-wrapper { max-height: 100% !important; top: 0; }