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

function createDeepSingleMapper(field, emit) {
  var parsedField = parseField(field);
  return function (doc) {
    var value = doc;
    for (var i = 0, len = parsedField.length; i < len; i++) {
      var key = parsedField[i];
      value = value[key];
      if (typeof value === 'undefined') {
        return; // do nothing
      }
    }
    emit(value);
  };
}

return function (doc) {
    var toEmit = [];
    for (var i = 0, iLen = fields.length; i < iLen; i++) {
      var parsedField = parseField(fields[i]);
      var value = doc;
      for (var j = 0, jLen = parsedField.length; j < jLen; j++) {
        var key = parsedField[j];
        value = value[key];
        if (typeof value === 'undefined') {
          return; // don't emit
        }
      }
      toEmit.push(value);
    }
    emit(toEmit);
  };
}

function pick(obj, arr) {
  var res = {};
  for (var i = 0, len = arr.length; i < len; i++) {
    var parsedField = parseField(arr[i]);
    var value = getFieldFromDoc(obj, parsedField);
    if (typeof value !== 'undefined') {
      setFieldInDoc(res, parsedField, value);
    }
  }
  return res;
}