Is 当做 and 看做 a synonym or not a synonym?
In traditional Chinese used in Taiwan, 當作 can be used when you use something or treat something as if it's something else. For example:

他把你當作奴隸在使喚。
He ordered you around as if you are slave.
他把球棒當作武器用。
He used a bat as a weapon.

Whereas 看作 can be used when you recognize something as something else.