for..in, which should not be confused with for..of, can be used to iterate over the enumerable properties of an object:

const dog = { name: 'Roger', color: 'gray' }

for (let property in dog) {
  console.log(property) // 'name' in the first iteration
                        // 'color' in the second

  console.log(dog[property]) // 'Roger' in the first iteration
                             // 'gray' in the second
}

Since arrays are a special kind of object, we can iterate over the items in an array too:

const dogs = ['Roger', 'Vanille']

for (let index in dogs) {
  console.log(dogs[index])
}

Lessons this unit:

0:	Introduction
1:	The `for` loop
2:	The `do-while` loop
3:	The `while` loop
4:	The `for-of` loop
5:	▶︎ The `for-in` loop
6:	Other kinds of loops