Only be strong and very courageous, to observe to do according to all the law, which Moses my servant commanded you: don't turn from it to the right hand or to the left, that you may have good success wherever you go.
"According to all the law  ..." Yes, indeed, according to this testimony, even in the days of Joshua, there was a "Law of Moses," containing all that the Israelites were expected to do!  Efforts to make the Pentateuch itself a late book by dating from the times of the judges, or the monarchy, or the post-exilic priesthood are among the most stupid and impossible tasks the so-called "higher critics" ever assigned themselves!  This is as good a place as any to take a little closer look at what the Word of God says concerning this: