In Mark implementation, care has been taken so that indexed contents are not enumerable. So e.g. when you run a for ... in loop on a Mark object, you'll only se

In Mark implementation, care has been taken so that indexed contents are not enumerable. So e.g. when you run a for ... in loop on a Mark object, you'll only see properties, not the contents. This is one of the difference between Mark object and an array. Array contents are enumerable by default.