Yes you can leave, we are still a free country but here is the catch. You most likely have to be reintroduced again to our friends at TSA and Customs who might be interested why you chose to take this unortharized extra trip and shakedown when returning might be a lot more intense. Remember and be happy that there just doing there jobs protecting us from the bad guys of the world. And there suspicions are needed to keep us safe.
Yes you can.
I agree. It's best to check with airport security and to allow enough time to reenter security. There are trains to take you into the city.
Before check with the airport staffing they are the best ones to answer your question as things change rapidly because of the ungoing pandemic.
During my 8hr layover to India flight ,I had a short walk outside the airport. You can check with airport security or the help desk once you land but ensure you reach within 2.5hrs before your flight because of the huge crowd for security Check. I would suggest to plan the exit within 7hrs timeframe. (You have metro train outside JFK airport try to see if you can utilise it)
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