[рони]

orloff,

<script>
const item = [
    {id: 1, name: 'test', address: {lat: 5}},
    {id: 2, name: 'test', address: {lat: 5}},
    {id: 3, name: 'test', address: {lat: 7}},
    {id: 4, name: 'test', address: {lat: 7}},
    {id: 5, name: 'test', address: {lat: 7}},
    {id: 6, name: 'test', address: {lat: 9}},
    {id: 7, name: 'test', address: {lat: 1}},
    {id: 8, name: 'test', address: {lat: 3}},
    {id: 9, name: 'test', address: {lat: 8}},
    {id: 10, name: 'test', address: {lat: 9}},
  ]
let ratioOffset = {};
function find() {
    item.forEach((el, i) => { // Перебор всех точек
     let lat = el.address.lat;
     el.ratioOffset = ratioOffset[lat] = (ratioOffset[lat]|| 0) +1
   });
    console.log(item);
  }
find();
document.write(`<pre>${JSON.stringify(item, "", 1)}</pre>`)
  </script>