// A custom property on an array is **not iterable**

const arr = [1,2];
arr[10] = 10; // => [1, 2, undefined*8, 10]
arr['ok'] = 'ok';

// for-in: enumerable properties:
for (const index in arr) console.log(arr[index]); // => 1, 2, 10, 'ok'
// for-of: iterable
for (const value of arr) console.log(value); // => 1, 2, undefined*8, 10
// forEach: strip undefined???
arr.forEach(value => console.log(value)); // => 1, 2, 10