TSP>var arr=['10', '10', '10']
TSP>for( item in arr  ) console.log(parseInt(item))
TSP>

TSP>И получаем 0 1 2

let list = [4, 5, 6];

for (let i in list) {
   console.log(i); // "0", "1", "2",
}

for (let i of list) {
   console.log(i); // "4", "5", "6"
}