I have a parent element with a real lot of child elements (1000s). I am looking for the fastest possible way to get a handle to the last child element. The options I've found are:
$('.parent .child').last()
and
$('.parent .child:last')
Any opinions on which one is reliably faster across browsers?
EDIT
I wrote a test in jsfiddle to measure this out and it turns out the difference is pretty much negligible. Though .last() was performing better, the difference is negligible. So i think even with the :last selector, it is actually getting the whole list of elements and then returning the last element? Unbelievable.
Fiddle: http://jsfiddle.net/techfoobar/GFb9f/8/
Many modern browsers support document.querySelectorAll(), so $('.parent .child').last() should be faster, as the selector string can be passed as is, and then the last matched item popped off.
In the latter, the :last is not a standard pseudo selector, and Sizzle has to start chunking the selector string to start matching.
Overall though, I would use what you believe is the most readable. To begin optimising this, first ensure that your application has performance issues and you have identified this selector as the bottleneck.
You have to see this performance test!
UPDATE: There are already good answers on this related question.
I just ran this jsPerf test in Chrome 18 and found that .class performs significantly better than tag.class despite the myriad of resources that say otherwise. I know this questions was recently asked on SO but without a definitive / satisfactory answer other than "don't use jQuery". Can someone shed some light on the latest best practices?
NOTE: Assume clarity and semantics are taken care of (e.g. I have a class on an anchor tag called item-link it's going to come off just as clear with a.item-link as it will .item-link).
You're asking about best practices, so I would lean more toward the way that is more readable vs. one that can possibly shave a microsecond off your script runtime.
The latest browsers have a better implementation of getElementsByClassName and hence when you use jquerys class selector only it immediately fallsback to this method and hence the performance difference and gain.
When you precede it with a tag selector firstly jquery needs to process your selector and then break it down to the different tokens and then it goes about searching for the control, not sure what order is followed here, whether all elements with the className is retrieved first and then the additional selector is applied or vice versa but I am sure you can research it by looking at the jquery source code..
And regarding the debate readable or not, I am not sure what you stand to gain by knowing that the selector is on an what control, if that is what you need then well name your classes better for gods sake..What next precede ids with tagname?? :/
I'm trying to improve a websites rendering speed.
Both CSS and JS files mostly reference elements like this:
Javascript:
$('.some_element').doSth()
CSS:
.some_element { /* do something */ }
Just curious - is this the optimal way of referencing elements in terms of javascript parsing and website rendering? Wouldn't it be better to do something like div.some_element?
Thanks for some infos!
If speed is a priority you might want to switch to vanilla javascript as much as you can. Native javascript is faster than jQuery.
If you want to keep your jQuery selector use parent context to make the search for the element more efficient. Example $('#parent').find(child)
You can find more tips on javascript an jquery optimization on the web:
http://engineeredweb.com/blog/10/12/3-tips-make-your-jquery-selectors-faster/
div.some_element will be different than .some_element, if you don't just have divs that use the some_element class.
Maybe compare render times using Chrome's built-in developer tools (or an alternative) to see if it helps you, but I doubt it'll be significant.
The fastest way to find a single element is usually with an id, not a class:
document.getElementById("whatever")
If you have to use jQuery (which is not as fast as plain javascript), then you would use:
$("#whatever")
If speed is really important, you can resolve the DOM element once when the page loads and just save the direct DOM reference so you don't have to find it when your code actually executes later.
As with all questions of performance, the only real way to answer a performance question is to benchmark a couple of implementation options and actually test which is faster.
I have noticed while monitoring/attempting to answer common jQuery questions, that there are certain practices using javascript, instead of jQuery, that actually enable you to write less and do ... well the same amount. And may also yield performance benefits.
A specific example
$(this) vs this
Inside a click event referencing the clicked objects id
jQuery
$(this).attr("id");
Javascript
this.id;
Are there any other common practices like this? Where certain Javascript operations could be accomplished easier, without bringing jQuery into the mix. Or is this a rare case? (of a jQuery "shortcut" actually requiring more code)
EDIT : While I appreciate the answers regarding jQuery vs. plain javascript performance, I am actually looking for much more quantitative answers. While using jQuery, instances where one would actually be better off (readability/compactness) to use plain javascript instead of using $(). In addition to the example I gave in my original question.
this.id (as you know)
this.value (on most input types. only issues I know are IE when a <select> doesn't have value properties set on its <option> elements, or radio inputs in Safari.)
this.className to get or set an entire "class" property
this.selectedIndex against a <select> to get the selected index
this.options against a <select> to get a list of <option> elements
this.text against an <option> to get its text content
this.rows against a <table> to get a collection of <tr> elements
this.cells against a <tr> to get its cells (td & th)
this.parentNode to get a direct parent
this.checked to get the checked state of a checkbox Thanks #Tim Down
this.selected to get the selected state of an option Thanks #Tim Down
this.disabled to get the disabled state of an input Thanks #Tim Down
this.readOnly to get the readOnly state of an input Thanks #Tim Down
this.href against an <a> element to get its href
this.hostname against an <a> element to get the domain of its href
this.pathname against an <a> element to get the path of its href
this.search against an <a> element to get the querystring of its href
this.src against an element where it is valid to have a src
...I think you get the idea.
There will be times when performance is crucial. Like if you're performing something in a loop many times over, you may want to ditch jQuery.
In general you can replace:
$(el).attr('someName');
with:
Above was poorly worded. getAttribute is not a replacement, but it does retrieve the value of an attribute sent from the server, and its corresponding setAttribute will set it. Necessary in some cases.
The sentences below sort of covered it. See this answer for a better treatment.
el.getAttribute('someName');
...in order to access an attribute directly. Note that attributes are not the same as properties (though they mirror each other sometimes). Of course there's setAttribute too.
Say you had a situation where received a page where you need to unwrap all tags of a certain type. It is short and easy with jQuery:
$('span').unwrap(); // unwrap all span elements
But if there are many, you may want to do a little native DOM API:
var spans = document.getElementsByTagName('span');
while( spans[0] ) {
var parent = spans[0].parentNode;
while( spans[0].firstChild ) {
parent.insertBefore( spans[0].firstChild, spans[0]);
}
parent.removeChild( spans[0] );
}
This code is pretty short, it performs better than the jQuery version, and can easily be made into a reusable function in your personal library.
It may seem like I have an infinite loop with the outer while because of while(spans[0]), but because we're dealing with a "live list" it gets updated when we do the parent.removeChild(span[0]);. This is a pretty nifty feature that we miss out on when working with an Array (or Array-like object) instead.
The correct answer is that you'll always take a performance penalty when using jQuery instead of 'plain old' native JavaScript. That's because jQuery is a JavaScript Library. It is not some fancy new version of JavaScript.
The reason that jQuery is powerful is that it makes some things which are overly tedious in a cross-browser situation (AJAX is one of the best examples) and smooths over the inconsistencies between the myriad of available browsers and provides a consistent API. It also easily facilitates concepts like chaining, implied iteration, etc, to simplify working on groups of elements together.
Learning jQuery is no substitute for learning JavaScript. You should have a firm basis in the latter so that you fully appreciate what knowing the former is making easier for you.
-- Edited to encompass comments --
As the comments are quick to point out (and I agree with 100%) the statements above refer to benchmarking code. A 'native' JavaScript solution (assuming it is well written) will outperform a jQuery solution that accomplishes the same thing in nearly every case (I'd love to see an example otherwise). jQuery does speed up development time, which is a significant benefit which I do not mean to downplay. It facilitates easy to read, easy to follow code, which is more than some developers are capable of creating on their own.
In my opinion then, the answer depends on what you're attempting to achieve. If, as I presumed based on your reference to performance benefits, you're after the best possible speed out of your application, then using jQuery introduces overhead every time you call $(). If you're going for readability, consistency, cross browser compatibility, etc, then there are certainly reasons to favor jQuery over 'native' JavaScript.
There's a framework called... oh guess what? Vanilla JS. Hope you get the joke... :D It sacrifices code legibility for performance... Comparing it to jQuery bellow you can see that retrieving a DOM element by ID is almost 35X faster. :)
So if you want performance you'd better try Vanilla JS and draw your own conclusions. Maybe you won't experience JavaScript hanging the browser's GUI/locking up the UI thread during intensive code like inside a for loop.
Vanilla JS is a fast, lightweight, cross-platform framework for
building incredible, powerful JavaScript applications.
On their homepage there's some perf comparisons:
There's already an accepted answer but I believe no answer typed directly here can be comprehensive in its list of native javascript methods/attributes that has practically guaranteed cross-browser support. For that may I redirect you to quirksmode:
http://www.quirksmode.org/compatibility.html
It is perhaps the most comprehensive list of what works and what doesn't work on what browser anywhere. Pay particular attention to the DOM section. It is a lot to read but the point is not to read it all but to use it as a reference.
When I started seriously writing web apps I printed out all the DOM tables and hung them on the wall so that I know at a glance what is safe to use and what requires hacks. These days I just google something like quirksmode parentNode compatibility when I have doubts.
Like anything else, judgement is mostly a matter of experience. I wouldn't really recommend you to read the entire site and memorize all the issues to figure out when to use jQuery and when to use plain JS. Just be aware of the list. It's easy enough to search. With time you will develop an instinct of when plain JS is preferable.
PS: PPK (the author of the site) also has a very nice book that I do recommend reading
When:
you know that there is unflinching cross-browser support for what you are doing, and
it is not significantly more code to type, and
it is not significantly less readable, and
you are reasonably confident that jQuery will not choose different implementations based on the browser to achieve better performance, then:
use JavaScript. Otherwise use jQuery (if you can).
Edit: This answer applies both when choosing to use jQuery overall versus leaving it out, as well as choosing whether to to use vanilla JS inside jQuery. Choosing between attr('id') and .id leans in favor of JS, while choosing between removeClass('foo') versus .className = .className.replace( new Regexp("(?:^|\\s+)"+foo+"(?:\\s+|$)",'g'), '' ) leans in favor of jQuery.
Others' answers have focused on the broad question of "jQuery vs. plain JS." Judging from your OP, I think you were simply wondering when it's better to use vanilla JS if you've already chosen to use jQuery. Your example is a perfect example of when you should use vanilla JS:
$(this).attr('id');
Is both slower and (in my opinion) less readable than:
this.id.
It's slower because you have to spin up a new JS object just to retrieve the attribute the jQuery way. Now, if you're going to be using $(this) to perform other operations, then by all means, store that jQuery object in a variable and operate with that. However, I've run into many situations where I just need an attribute from the element (like id or src).
Are there any other common practices
like this? Where certain Javascript
operations could be accomplished
easier, without bringing jQuery into
the mix. Or is this a rare case? (of a
jQuery "shortcut" actually requiring
more code)
I think the most common case is the one you describe in your post; people wrapping $(this) in a jQuery object unnecessarily. I see this most often with id and value (instead using $(this).val()).
Edit: Here's an article that explains why using jQuery in the attr() case is slower. Confession: stole it from the tag wiki, but I think it's worth mentioning for the question.
Edit again: Given the readability/performance implications of just accessing attributes directly, I'd say a good rule of thumb is probably to try to to use this.<attributename> when possible. There are probably some instances where this won't work because of browser inconsistencies, but it's probably better to try this first and fall back on jQuery if it doesn't work.
If you are mostly concerned about performance, your main example hits the nail on the head. Invoking jQuery unnecessarily or redundantly is, IMHO, the second main cause of slow performance (the first being poor DOM traversal).
It's not really an example of what you're looking for, but I see this so often that it bears mentioning: One of the best ways to speed up performance of your jQuery scripts is to cache jQuery objects, and/or use chaining:
// poor
$(this).animate({'opacity':'0'}, function() { $(this).remove(); });
// excellent
var element = $(this);
element.animate({'opacity':'0'}, function() { element.remove(); });
// poor
$('.something').load('url');
$('.something').show();
// excellent
var something = $('#container').children('p.something');
something.load('url').show();
I've found there is certainly overlap between JS and JQ. The code you've shown is a good example of that. Frankly, the best reason to use JQ over JS is simply browser compatibility. I always lean toward JQ, even if I can accomplish something in JS.
This is my personal view, but as jQuery is JavaScript anyway, I think theoretically it cannot perform better than vanilla JS ever.
But practically it may perform better than hand-written JS, as one's hand-written code may be not as efficient as jQuery.
Bottom-line - for smaller stuff I tend to use vanilla JS, for JS intensive projects I like to use jQuery and not reinvent the wheel - it's also more productive.
The first answer's live properties list of this as a DOM element is quite complete.
You may find also interesting to know some others.
When this is the document :
this.forms to get an HTMLCollection of the current document forms,
this.anchors to get an HTMLCollection of all the HTMLAnchorElements with name being set,
this.links to get an HTMLCollection of all the HTMLAnchorElements with href being set,
this.images to get an HTMLCollection of all the HTMLImageElements
and the same with the deprecated applets as this.applets
When you work with document.forms, document.forms[formNameOrId] gets the so named or identified form.
When this is a form :
this[inputNameOrId] to get the so named or identified field
When this is form field:
this.type to get the field type
When learning jQuery selectors, we often skip learning already existing HTML elements properties, which are so fast to access.
As usual I'm coming late to this party.
It wasn't the extra functionality that made me decide to use jQuery, as attractive as that was. After all nothing stops you from writing your own functions.
It was the fact that there were so many tricks to learn when modifying the DOM to avoid memory leaks (I'm talking about you IE). To have one central resource that managed all those sort of issues for me, written by people who were a whole lot better JS coders than I ever will be, that was being continually reviewed, revised and tested was god send.
I guess this sort of falls under the cross browser support/abstraction argument.
And of course jQuery does not preclude the use of straight JS when you needed it. I always felt the two seemed to work seamlessly together.
Of course if your browser is not supported by jQuery or you are supporting a low end environment (older phone?) then a large .js file might be a problem. Remember when jQuery used to be tiny?
But normally the performance difference is not an issue of concern. It only has to be fast enough. With Gigahertz of CPU cycles going to waste every second, I'm more concerned with the performance of my coders, the only development resources that doesn't double in power every 18 months.
That said I'm currently looking into accessibility issues and apparently .innerHTML is a bit of a no no with that. jQuery of course depends on .innerHTML, so now I'm looking for a framework that will depend on the somewhat tedious methods that are allowed. And I can imagine such a framework will run slower than jQuery, but as long as it performs well enough, I'll be happy.
Here's a non-technical answer - many jobs may not allow certain libraries, such as jQuery.
In fact, In fact, Google doesn't allow jQuery in any of their code (nor React, because it's owned by Facebook), which you might not have known until the interviewer says "Sorry, but you cant use jQuery, it's not on the approved list at XYZ Corporation". Vanilla JavaScript works absolutely everywhere, every time, and will never give you this problem. If you rely on a library yes you get speed and ease, but you lose universality.
Also, speaking of interviewing, the other downside is that if you say you need to use a library to solve a JavaScript problem during a code quiz, it comes across like you don't actually understand the problem, which looks kinda bad. Whereas if you solve it in raw vanilla JavaScript it demonstrates that you actually understand and can solve every part of whatever problem they throw in front of you.
$(this) is different to this :
By using $(this) you are ensuring the jQuery prototype is being passed onto the object.
I use jQuery.
I have been reading a lot about selector performance and optimizing our AJAX app. I am looking to improve my selector performance. I know all the jquery performance tips. I haven't found an exact answer to a question I have. I am using almost every current jquery performance tip and yet my app still seems to lag quite a bit.
So, to optimize I am starting with selectors.
My question is: Is descending from a context to target an id faster than just targeting the id? I can't tell much of a difference.
Example:
Is
$('#childId', $higherElm);
faster than just
$('#childId');
????
Thanks in advance.
As seen in the jQuery source, $('#id') does simply document.getElementById, while $('#id', context) does $(context).find('#id'). So the first is faster.
According to this article, it's faster to have fewer, more direct selectors. #id is better than #id #child, at least in css...
Selecting an ID is the absolute fastest selection you can do.
Adding anything else will just slow it down.
When you're selecting by "id", context doesn't matter much because the engine's going to call getElementById() anyway. Context semantically matters of course, but that check should be pretty fast. (I suppose that in that light, having the context should be slightly slower, but you can't stop doing that if it's got actual meaning for your pages.)
Not sure if the syntax you describe above would be beneficial, but on http://www.artzstudio.com/2009/04/jquery-performance-rules/ it does state in rule #5 that using sub-queries is faster (which makes sense)... they demonstrate it using the $higherElm.find() syntax, though.
In your example - since it maps directly to getElementById which is a native function call - I don't think you'll see much of an improvement. However, selectors that target sets of elements (hence looping), would probably see some, or major, benefit.