God saw their works, that they turned from their evil way. God repented of the evil which he said he would do to them, and he didn't do it.
Before leaving this verse which has the record of God's sparing Nineveh, it should be remembered that the punishment was merely deferred, not cancelled, and that, in time, after the people had turned again to terror and violence, God indeed executed his wrath upon them.  Keil summed up that point thus: