code: function columnIndexing(selector, colNames, dtypes) {
    if (isUndefined(selector) || selector === null) {
      return null;
    }
  
    // Handle boolean case if applicable
    var maxLen = colNames.size();
    var boolIdxResult = attemptBoolIndexing(selector, maxLen);
    if (!isUndefined(boolIdxResult)) {
      return boolIdxResult;
    }
  
    // Handle integer index / col name lookup hybrid
    var opts = resolverOpts(RESOLVE_MODE.COL, maxLen,
      colNames._getIndex(), dtypes);
    return resolveSelector(selector, opts);
  }
docs: Column name indexing 'colNames' should be a string vector of column names Returns null if all rows/elements should be selected in order.