The pair have made a big impact at the League One club.
Rotherham boss Steve Evans said: "It's fair to say that both lads have helped raise the standard in training and in games.
"And I believe there's still much more to come from both of them.
"James has really hit the ground running and he's been a key component in the recent good results we've had, and Haris is very quickly acclimatising to a level where it's about finding space to allow him to unleash the outstanding natural talent that he possesses which makes him a Premier League player."
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