).toArray()).to.eql([             {'A': 'a', 'variable': 'B', 'value': 0},             {'A': 'b', 'variable': 'B', 'value': 0},             {'A': 'c', 'variable': 'B', 'value': 0}         ]);     });      it('can melt a dataframe with multiple id columns and single value column passed as input', () => {         expect(df.melt(['A', 'B'], 