Before becoming Israel's leader, Joshua son of Nun was whose assistant?

Answer: Moses

After Moses died, the LORD spoke to Joshua son of Nun, Moses' assistant, and told him to lead the people across the Jordan.

“Now after the death of His servant Moses, the LORD spoke to Joshua son of Nun, Moses’ assistant, saying, “Moses My servant is dead. Now therefore arise, you and all these people, and cross over the Jordan into the land that I am giving to the children of Israel.”— Joshua 1:1-2 (BSB)