Unlike most countries around the World, the USA requires all air passengers to ENTER the country before getting a connecting flight onto their  final destination outside the USA  You need to complete a visa as if you were staying in the USA . Pass thru customs and passport control and be checked on numerous occasions.Then you will need to stand in line to get thru security to depart the USA .This is a very time consuming exercise and you should be aware that an onward flight should not be scheduled close to your arrival time into the USA