Andrew Kaspick wrote:
I would have to think that $('abc').getElementsByTagName('span') would
run faster than making use of $$.
$$ would have to evaulate the entire document. Starting at a known
parent should be much much faster.
On 7/3/06, Maninder, Singh
$$('div#abc span').each(function(el) {
//do something
});
Actually there should not be any noticeable performance difference. The
only real difference between:
$$('div#abc span')
and
$('abc').getElementsByTagName('span')
is parsing the CSS selector and building a match expression. But the
cost of that is fairly low (two regular expression and few statements
that are only executed once).
Otherwise it will execute the statements above basically the same (find
then "abc" element using $() then find all "span" tags that are below
that element in the DOM hierarchy by using document.getElementsByTagName).
It is informative to take a look at
http://dev.rubyonrails.org/browser/spinoffs/prototype/src/selector.js?format=txt
Look at the "findElements" method and you can see it does the following:
1. Tries to find the element using $() if an id is given
2. Gets a list of all elements in scope (determined by the previous CSS
selector fragment) and filters that list down by tag name if the tag
name is given.
3. Once it has filtered the list down as much as possible with standard
DOM search methods, only then will it do a linear search of the elements
to ensure that the id, class and tag name match the CSS selector.
It is quite efficient although there is still room for improvement when
finding elements that match a class name. I would only use something
other than $$() if I really had some performance constraint.
Eric
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