The NodeList interface provides the abstraction of an ordered
collection of nodes, without defining or constraining how this collection
is implemented. NodeList objects in the DOM are live.
The items in the NodeList are accessible via an integral
index, starting from 0.
Returns the indexth item in the collection. If
index is greater than or equal to the number of nodes in
the list, this returns null.
Parameters: index - Index into the collection. The node at the indexth position in the NodeList, or null if that is not a valid index.