Does warwicks passive heal for what damage is dealt or regardless?

Wanted to know if the new Warwick's passive heals only for dmg dealt or if it is there regardless of dmg dealt.

Ex: ww hits for an extra 10 magic dmg but it's reduced by .2 for 8 magic dmg. Would he still heal for 10?

Because if it is my first thought then each champ in the game has about 20% reduction against magic dmg right off the bat, making his passive very insignificant damage wise. Then if you add reduced healing due to it then it kinda destroys Warwick's ability to be a front liner. Being that without his e, he will explode on the 2ND rotation of spells or from the adc.