How can I group by month without losing the order by year?

Trying to sort data for display in a graph by the year and the month category. Sorting first by year and then by month goes great, but then as soon as I try to group each item together based on month, the year sorting goes out the window.

Let's say that I have an array of objects as follows

const arr = [
        {month: 1, year: 2021},
        {month: 2, year: 2021},
        {month: 3, year: 2021},
        {month: 3, year: 2021},
        {month: 4, year: 2021},
        {month: 9, year: 2020},
        {month: 9, year: 2020},
        {month: 10, year: 2020},
        {month: 11, year: 2020},
        {month: 12, year: 2020},
        {month: 12, year: 2020},
      ];

I then sort based on year and then month so I get this resulting arr:

[
  {month: 9, year: 2020},
  {month: 9, year: 2020},
  {month: 10, year: 2020},
  {month: 11, year: 2020},
  {month: 12, year: 2020},
  {month: 12, year: 2020},
  {month: 1, year: 2021},
  {month: 2, year: 2021},
  {month: 3, year: 2021},
  {month: 3, year: 2021},
  {month: 4, year: 2021}
]

How can I then group the objects into an array of each month while not losing this order? Trying to end up with something like the following Object. Every time I try to group by month it ends up sorting it based on month and so Month: 1 ends up coming first which screws up the graph.

const groupedUp = {
  9: [{month: 9, year: 2020},{month: 9, year: 2020}],
  10: [{month: 10, year: 2020}],
  11: [{month: 11, year: 2020}],
  12: [{month: 12, year: 2020}, {month: 12, year: 2020}],
  1: [{month: 1, year: 2021}],
  2: [{month: 2, year: 2021}],
  3: [{month: 3, year: 2021},{month: 3, year: 2021}],
  4: [{month: 4, year: 2021}]
}

In an object, numeric properties are always sorted in ascending order. You can use a Map instead to maintain insertion order when grouping.

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let sorted = [
  {month: 9, year: 2020},
  {month: 9, year: 2020},
  {month: 10, year: 2020},
  {month: 11, year: 2020},
  {month: 12, year: 2020},
  {month: 12, year: 2020},
  {month: 1, year: 2021},
  {month: 2, year: 2021},
  {month: 3, year: 2021},
  {month: 3, year: 2021},
  {month: 4, year: 2021}
];
let res = sorted.reduce((map, curr) => {
  if (!map.has(curr.month)) map.set(curr.month, []);
  map.get(curr.month).push(curr);
  return map;
}, new Map);
res.forEach((v, k) => console.log(k, '=', JSON.stringify(v)));

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