Adam Clarke on Jonah 3:10

And Gods saw their works - They repented, and brought forth fruits meet for repentance; works which showed that they did most earnestly repent. He therefore changed his purpose, and the city was saved. The purpose was: If the Ninevites do not return from their evil ways, and the violence that is in their hands, within forty days, I will destroy the city. The Ninevites did return, etc., and therefore escaped the threatened judgment. Thus we see that the threatening was conditional.