Janko opened the scoring in the fourth minute and, after Luuk de Jong had doubled their advantage six minutes before the break, he added further goals on the stroke of half-time, on the hour mark and 20 minutes from time.
The win moves the champions back to within a point of PSV Eindhoven at the top of the table.
Visitors ADO are currently sitting in ninth place, just four points off the top five, while Heerenveen are two places and one point better off, eyeing up a share of AZ Alkmaar's current league position.
