Does melee spell attack always replace melee damage?

I was wondering if Shocking Grasp would be 18d(+ spell modifier) or 1d6+(attack modifier) as well as the 1d8roll

Shocking Grasp (Range: Touch) Make a melee spell attack against the target. You have advantage on the attack roll if the target is wearing armor made of metal. On a hit, the target takes 1d8 lightning damage, and it can’t take reactions until the start of its next turn.

I am assuming as it’s a cantrip it’s just the 1d8 but I want to check because it’s unclear to me