NJ will calculate what your tax would be on all your income as if you were a resident.  Then they will prorate the amount of NJ tax to the percentage of income you actually earn in NJ. 

So if your income from all sources is $10,000 but your NJ source income is $8,000, NJ will calculate the NJ state tax on $10,000 then multiply the total state taxes generated by 80% ($8,000/$10,000). So even though you all your income is being reported on your NJ state tax return, you are only being taxed on the NJ source portion of your total income.