How to use the pouchdb-selector-core.getValue function in pouchdb-selector-core

var opts = Object.assign({
      include_docs: true,
      reduce: false
    }, queryPlan.queryOpts);

    if ('startkey' in opts && 'endkey' in opts &&
        collate(opts.startkey, opts.endkey) > 0) {
      // can't possibly return any results, startkey > endkey
      /* istanbul ignore next */
      return {docs: []};
    }

    var isDescending = requestDef.sort &&
      typeof requestDef.sort[0] !== 'string' &&
      getValue(requestDef.sort[0]) === 'desc';

    if (isDescending) {
      // either all descending or all ascending
      opts.descending = true;
      opts = reverseOptions(opts);
    }

    if (!queryPlan.inMemoryFields.length) {
      // no in-memory filtering necessary, so we can let the
      // database do the limit/skip for us
      if ('limit' in requestDef) {
        opts.limit = requestDef.limit;
      }
      if ('skip' in requestDef) {
        opts.skip = requestDef.skip;
      }

var ascFields = index.fields.filter(function (field) {
    return getValue(field) === 'asc';
  });
  if (ascFields.length !== 0 && ascFields.length !== index.fields.length) {