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

function checkFieldsLogicallySound(indexFields, selector) {
  var firstField = indexFields[0];
  var matcher = selector[firstField];

  if (typeof matcher === 'undefined') {
    /* istanbul ignore next */
    return true;
  }

  var isInvalidNe = Object.keys(matcher).length === 1 &&
    getKey(matcher) === '$ne';

  return !isInvalidNe;
}

function getSingleFieldCoreQueryPlan(selector, index) {
  var field = getKey(index.def.fields[0]);
  //ignoring this because the test to exercise the branch is skipped at the moment
  /* istanbul ignore next */
  var matcher = selector[field] || {};
  var inMemoryFields = [];

  var userOperators = Object.keys(matcher);

  var combinedOpts;

  userOperators.forEach(function (userOperator) {

    if (isNonLogicalMatcher(userOperator)) {
      inMemoryFields.push(field);
    }

    var userValue = matcher[userOperator];

function getKeyFromDoc(doc, index) {
  var res = [];
  for (var i = 0; i < index.def.fields.length; i++) {
    var field = getKey(index.def.fields[i]);
    res.push(doc[field]);
  }
  return res;
}

function userOperatorLosesPrecision(selector, field) {
  var matcher = selector[field];
  var userOperator = getKey(matcher);

  return userOperator !== '$eq';
}