How can you replace a string in a JavaScript object with a number?

In the code, lat and lng are initialized as number types, but why does it become string when you put it in an object?
I want the coordinate values to be numbers.
What can I do?

ex) lat: “35.87” -> lat: 35.87

let lat_tmp = 0;
let lng_tmp = 0;
const lat_lng = [];
let lat_tmp = 0;
let lng_tmp = 0;
let arr_tmp = [
  {
    lat: 0,
    lng: 0,
  },
];
let idx = 0;
function successFunction(data) {
  var allRows = data.split(/r?n|r/);
  for (var singleRow = 1; singleRow < allRows.length; singleRow++) {

    var rowCells = allRows[singleRow].split(',');
    for (var rowCell = 0; rowCell < rowCells.length; rowCell++) {
      if (rowCell === 3) {
        lat_tmp = rowCells[rowCell];
      } else if (rowCell === 4) {
        lng_tmp = rowCells[rowCell];
      }
    }
    arr_tmp = [{ lat: lat_tmp, lng: lng_tmp }];
    lat_lng[idx] = arr_tmp;
    idx += 1;
  }
  console.log(lat_lng);
}