A function to generate keyset paginated queries for mongodb

generate-pagination-query.js

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

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

    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 };
  }

  const sortOperator = sort[1] === 1 ? "$gt" : "$lt";

  const paginationQuery = [
    { [sortField]: { [sortOperator]: nextKey[sortField] } },
    {
      $and: [
        { [sortField]: nextKey[sortField] },
        { _id: { [sortOperator]: nextKey._id } }
      ]
    }
  ];

  if (paginatedQuery.$or == null) {
    paginatedQuery.$or = paginationQuery;
  } else {
    paginatedQuery = { $and: [query, { $or: paginationQuery }] };
  }

  return { paginatedQuery, nextKeyFn };
}

That works 👏🏻