Point must only contain numeric elements, instead got type string

I'm working with node (using the MongoClient library) and mongodb, trying to add a geospatial index to my data

The records in my collection have a longitude and latitude field. I understand that in order to create a geospatial index, you need a location field that corresponds to the geoJson point format (https://medium.com/@AbbasPlusPlus/geospatial-data-in-mongodb-storage-indexing-and-queries-1c6db21b7970) . Therefore after inserting the data I run:

async function addLocation() {
  const res = collection.updateMany(
    { location: { $exists: false } }, // filter
    {
      $set: {
        location: {
          type: "Point",
          coordinates: ["$longitude", "$latitude"],
        },
      },
    } 
  );
}

Next, I tried to add a geospatial index with:

async function createIndex() {

  try {

    await collection.createIndex({ location: "2dsphere" });
    console.log('Index created');

  } catch (err) {
    console.error(err);
  }
}

This gives the error in the title

Looking at the records I saw the screenshot. I assume the long, lat fields should have numbers instead of operators.

How can I fix addLocation, so that it puts correct values in the long, lat fields?

to update fields based on values of other fields or if you want to use complex expressions(ex- $map etc) you'll have to use the bracket notation with updates
see aggregation pipeline with updates

  const res = collection.updateMany(
    { location: { $exists: false } },
    [
     {
      $set: {
        location: {
          type: "Point",
          coordinates: ["$longitude", "$latitude"],
        },
      },
     }
    ] 
  );