If I use object.getElementsByTagName(tagName) in a for statement

If I use object.getElementsByTagName(tagName) in a for statement

If I use object.getElementsByTagName(tagName) in a for statement,
for (index = 0; index < object.getElementsByTagName(tagName).length;
index++) {
object.getElementsByTagName(tagName)[index].property = value;
}
Does the browser instantiate a new nodeList object for every pass through
the loop, or does the browser simply refer to a single generated list
every time; or maybe, it instantiates a list, references the object
specified and unloads the list object every pass through the loop?
I've been wondering if its better to store the nodeList object to a
variable and reference it when neened.