Can Janna be played to base at reaction speed, and will her ability work there?

Ruling: Janna can be played to base at reaction speed (anywhere you could play a unit whenever you can play reactions), but her ability will not trigger because it requires a valid target for the "return an enemy unit from here to base" effect, and when played to base there are no valid targets.

Sequence:

• Janna can be played to base or any battlefield you control at reaction speed
• Her ability requires a valid target to be put on the chain
• When played to base, "here" refers to your base, where there are no enemy units to target
• Without a valid target, the ability cannot be put on the chain at all
• Therefore, neither the heal nor the return effect will occur

Nuances:

• If the ability were already on the chain and the target became invalid later, the heal would still resolve
• The card may receive an errata to make the target optional (similar to what happened with Salvage)