Michael D. Coogan, The Old Testament: A Historical and Literary Introduction to the Hebrew Scriptures, third edition (OUP, 2014), 55-56.
A description of the process by which the separate sources or documents were combined is also hypothetical, but a possible scenario is as follows. Using some earlier traditions, and reflecting their own perspectives as well, J [Yahwist] and E [Elohist] were written independently, the former in Judah, probably during the tenth century BCE, and the latter in the northern kingdom of Israel in the ninth, or perhaps a century or so later. When the norther kingdom fell to the Assyrians in 722 BCE, refugees from there brought with them to Jerusalem the E source, which was combined with the J in Jerusalem, but in such a way that while J remained intact, E was used as a kind of supplement; this accounts for its fragmentary nature. Finally, in the sixth century BCE, P [Priestly] shaped these sources along with its own material and D [Deuteronomistic] into what became the Pentateuch.
The Amateur Exegete 