what is the meaning of [] in return statement

Question

Giving a code as below. What is the meaning of [] in return statement? Is it possible to remove [] like sortField: item.sortField

[sortField]: item[sortField]

is it an array or object?

function generatePaginationQuery(query, sort, nextKey) {
  const sortField = sort == null ? null : sort[0];

  function nextKeyFn(items) {
    if (items.length === 0) {
      return null;
    }

    //Get the last item
    const item = items[items.length - 1];

    if (sortField == null) {
      return { _id: item._id };
    }

    return { _id: item._id, [sortField]: item[sortField] };
  }

  if (nextKey == null) {
    return { paginatedQuery: query, nextKeyFn };
  }

  let paginatedQuery = query;

  if (sort == null) {
    paginatedQuery._id = { $gt: nextKey._id };
    return { paginatedQuery, nextKey };
  }

}