The vast majority of bars open to the public do not have written dress codes, but be realistic. For guys, at the bare minimum you should be wearing jeans and a collared shirt. For ladies, stay away from anything particularly revealing or pajama-like.
Whiskey Richard is the founder of nomunication.jp, and has been saying he has lived in Japan for ten years for the past four years. He enjoys drinking basically anything except straight-up Campari.