get columns(): Immutable.Seq {
    return this._get_axis(1);
  }
Returns the indexed Immutable.Seq of columns

pandas equivalent: [DataFrame.columns](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.columns.html)

@returns {Seq.Indexed<string>}

@example
const df = new DataFrame([{x: 1, y: 2}, {x: 2, y: 3}, {x: 3, y: 4}]);

// Returns Seq ['x', 'y']
df.columns;