How to write inside a DIV box with javascript

How to write inside a DIV box with javascript

I'm making a mini-game where a player attacks a npc and in the center is a
white box (div) that I call (log), because when the player damages/attacks
the npc, I want to be able for that open white space to log what happened.
I'm using the getElementById(log) and then adding something along the
lines of "document.write("You attacked X npc"), but its not working.
Any idea on how I can get text INSIDE the box and not outside it? Thanks