If your rival carries that equipment, if you go back or withdrawal he might throw it on you. If you getting closer, he might break it over your head. What is the best solution offered for such a threat?
You can only try to shorten the distance to the attacker as soon and quick as possible as the chair, baseball bat or any other object he is potentially throwing has the strongest impact on the end of the motion or object. By that I mean, that the stick/bat/chair will be moving fastest at the point furthest from the attackers arm (the hit will be strongest here), so getting inside that point on the bat/blunt object is crucial. If you can get inside the objects long range, the weapon for the most part is neutralized.
You could use another object, back pack as a shield 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 doesn't want to hit his friend ;-))
If, for instance, someone has a baseball bat, you want to get closer to the side he's swinging it from. Even if you get hit around the body, you can brace for impact with your arm like a shield and it will not be debilitating if you're closer than his focal point of impact. His power is cut in half if you're 45 degrees or less from his frontal angle and thus cannot reach the focal point of impact. The ideal for stopping a baseball bat is to cut off that angle where he will hit you, which is right in front of him. Use angles and move diagonally into the bat (or chair). You see, he is relying on you to be right in front of him, like a baseball! See, his center-line is facing the home plate. Ideally, the bat is an extension of that center-line hitting the baseball. If you're not where he wants you to be, STRIKE!! Batter (a-hole): "Hey Stand Still!!"... Pitcher (you): Yeah Right!!
Here is an example of a technique against a baseball bat as taught by the Krav Maga Association "Krav Maga Global". Techniques against chairs and other larger objects are also in curriculum and work similarly. In fact they are often much easier to defend against, since they are much heavier and make the attacker more immobile. The baseball bat is the most dangerous from this family of weapons, which can be called club-like objects. Most of the techniques are based on what @mitro said in his excellent answer, namely shortening the distance as quickly as possible.
Another obvious, but often forgotten technique (in fact the most important one in all of self-defence) is very good in the particular scenario due to the large objects slowing the attacker: running away. Of course since this might not always be feasible actual techniques are required.
I would back away from them until I felt I was credibly out of the range of their thrown object, and then walk/run away as necessary to deescalate the situation.
If I was unable to move away from the armed aggressor because of circumstances (e.g. trapped in an enclosed space), I would attempt to feel out their speed/reflexes as I closed distance and then time a rush inside their optimal striking distance and take them down hard (but with the least amount of force necessary).