‘ And YHWH spoke to Joshua, saying, “Command the priests who bear the Ark of the testimony, that they come up out of Jordan.” '
Having described the exit from Jordan of the Ark in verse 11 the writer now amplifies what happened and adds to it the words of Joshua about the event. This repetition and moving onwards is typical of ancient narratives. It occurs regularly throughout the Pentateuch (and is misrepresented by modern commentators as the evidence of sources as though ancient men wrote in the same way as moderns). There were of course possibly sources (such as the Book of Jasher) but not as suggested by the Documentary Theory.
The Ark is here called ‘the Ark of the Testimony', a regular description only otherwise found in Exodus, although ‘the Ark of Testimony' is used in Numbers 4:5; Numbers 7:89. It then became ‘the Ark of the covenant of YHWH' (Numbers 10:33; Numbers 14:44; Deuteronomy 10:8; Deuteronomy 31:9; Deuteronomy 31:25 see also Deuteronomy 31:26). This description was thus of great significance. The Ark of the Testimony was the initial description of the Ark which resulted from the revelation of God at Sinai and the giving of His ‘testimony', His covenant in Exodus 20:1-17. Thus here the description emphasises that YHWH's spoken ‘testimony' to Israel was renewed as they entered the land. Jordan had become a new Sinai, where the revelation of YHWH's power had been revealed, and from which His people would move in the strength of a renewed covenant. This will immediately be followed by the circumcision of the people (Joshua 5:3).