To clarify @NCperson 's answer, W-2 income is sourced where the work is actually (physically) performed. If your work was actually performed in CT, it is CT-source income, not IL-source income. The employer's location is irrelevant.
Illinois taxes non-residents only on Illinois-sourced income.
Finally, make sure your employer withholds CT state taxes for you, not Illinois state taxes. If your employer does not withhold CT taxes, you'll have to make quarterly estimated tax payments to CT.
https://portal.ct.gov/DRS/Individuals/Resident-Income-Tax/Tax-Information#EstTaxPay
here is the link to who is required to file Illinoit Income Tax; everyoine else isn't required.
https://tax.illinois.gov/individuals/filingrequirements.html
To clarify @NCperson 's answer, W-2 income is sourced where the work is actually (physically) performed. If your work was actually performed in CT, it is CT-source income, not IL-source income. The employer's location is irrelevant.
Illinois taxes non-residents only on Illinois-sourced income.
Finally, make sure your employer withholds CT state taxes for you, not Illinois state taxes. If your employer does not withhold CT taxes, you'll have to make quarterly estimated tax payments to CT.
https://portal.ct.gov/DRS/Individuals/Resident-Income-Tax/Tax-Information#EstTaxPay