Though God gave the victory, yet it is said Joshua discovered Amalek, because Joshua was a type of Christ, and of the same name, and in him it is that we are more than conquerors. The Lord said, Write this for a memorial This is the first mention of writing we find in Scripture.
Related Commentaries of Exodus 17:13
Exodus 17:13-14
13 And Joshua discomfited Amalek and his people with the edge of the sword.
14 And the LORD said unto Moses, Write this for a memorial in a book, and rehearse it in the ears of Joshua: for I will utterly put out the remembrance of Amalek from under heaven.