Node lists are often implemented as node iterators with a filter. This means that getting a property like length is O(n), and iterating over the list by re-checking the length will be O(n^2).
var paragraphs = document.getElementsByTagName('p');
for (var i = 0; i < paragraphs.length; i++) {
doSomething(paragraphs[i]);
}
It is better to do this instead:
var paragraphs = document.getElementsByTagName('p');
for (var i = 0, paragraph; paragraph = paragraphs[i]; i++) {
doSomething(paragraph);
}
This works well for all collections and arrays as long as the array does not contain things that are treated as boolean false.
In cases where you are iterating over the childNodes you can also use the firstChild and nextSibling properties.
var parentNode = document.getElementById('foo');
for (var child = parentNode.firstChild; child; child = child.nextSibling) {
doSomething(child);
}