You can only try to shorten the distance to the attacker as soon and quick as possible as the chair, baseballbat or any other object he is potentially trying to throw has the strongest impact on the end of the motion or object.
You could use another object, back pack as a schield to protect you, if available.
If there are multiple attackers, try to get one of the attackers between you and the threat (having him as shield) and as close as possible to him. (Hoping that the original attacker is a good friend of the guy and dont want to hit his friend ;-))