In 2018 I was a resident of New Jersey the whole year, but my spouse was a part-year resident: he permanently moved to Germany on August 1st and established his domicile there. After that date, he earned wages and paid (high!) taxes in Germany.
We filed joint federal income tax. For the federal income tax, we reported his German earnings but avoided double taxation by using a foreign tax exclusion.
For the NJ income tax, my understanding is that he should not have to report his German earnings, since he was no longer a resident of NJ.
But is it possible indeed to file a joint NJ state income tax return, with myself being a full-year resident, and him being a part-year resident? I don't see any reason why not but I didn't find the documentation clear on that point: https://www.state.nj.us/treasury/taxation/pdf/pubs/tgi-ee/git6.pdf