As a example of jQuery code (https://coderwall.com/p/7uchvg), I read that the expression $('#foo a'); behaves like this:
Find every a in the page and then filter a inside #foo.
And it does not look efficient.
Is that correct? And if yes, how should we do that in a better way?
That is correct - Sizzle (jQuery's selector engine) behaves the same way as CSS selectors. CSS and Sizzle selectors are evaluated right-to-left, and so #foo a will find all a nodes, then filter those by nodes that descend from #foo.
You improve this by ensuring that your leaf selectors have a high specificity, usually by giving them a class or ID.
how should we do that in a better way?
Use the context parameter from jQuery.
$('a', '#foo');
Now jQuery will search all anchors within the context of the element with id: foo.
In your query the context is defaulted to document when omitted:
$('#foo a'); == $('#foo a', document);
In this case, your query is indeed not efficient.
You might take a look at this article.
While it is true that Sizzle is a right-to-left engine (which is the same way css is interpreted), it is not true that the specific selector in your example would select all anchor elements on the page and then filter their parents to match the id of "foo". Sizzle actually optimizes any selector that starts with an ID and uses that as the context for the entire selection, rather than using the document. In other words, the selector you've chosen basically translates to:
document.getElementById("foo").getElementsByTagName("a")
Really, that's not a bad selector at all.
However, given the other things jQuery needs to do (which includes looping over the elements to merge them onto the jQuery instance), jQuery("#foo").find("a") will always be the fastest because jQuery implements a jQuery object creation shortcut for id-only selectors, and then it does the find rooted from #foo.
In other words, Sizzle itself is not much different when doing Sizzle("#foo a") and Sizzle("a", document.getElementById("foo")), but jQuery("#foo").find... will be faster because of jQuery's own ID shortcut.
By the way, my remarks on Sizzle is assuming querySelectorAll is not being used. If it is, Sizzle just passes it on to qsa, which still isn't as fast as using jQuery's ID shortcut.
You can use find() for more granular control on your selector order:
$('#foo').find('a');
This will of course be more impressive with more complex selectors, where you can chain find() and filter().
For the record $('#foo').find('a') === $('a','#foo')
[Update] ok, I realized later that it's exactly what your link says...
The jQuery selector engine (Sizzle) has been refactored last year, you'll find detailed explanations here:
http://www.wordsbyf.at/2011/11/23/selectors-selectoring/
Instead of filtering with a inside #foo elements, simply attach a class to a elements and get a elements with class like $("a.class");. This would be more efficient.
Yet another "try it for yourself":
jsperf for various selectors on 10000 elements
jsperf for various selectors on 300 elements
jsperf for various selectors on a "more representative DOM"
Doesn't seem to be much difference with a "flat" DOM (1 & 2), but the performance varies much more with a nested DOM.
Also note that some of the test cases aren't selecting the correct elements (i.e. $('.a') vs $('.a', context)), but I left them from the original tests just for comparison.
This example will retrieve the all anchors elements a in an element called foo, to Find every a in the page and then filter a inside #foo as you want u should select a #foo
$("a #foo");
this will retrieve all the foo elements inside a elements.
Related
I'm using SVG.js select() function which uses querySelector() function.
Currently, the command I use is: select("[id='1']") (1 could be replaced by some other number)
What I'd like to do is to select the first inner element inside this element. Alternatively, I could select it by tag name.
How to do it?
I tried select("[id='1']:first") but received an error.
By the way, the reason I select it like that is that apparently querySelector has a problem with id's which are numbers.
:first is a jQuery thing. For what you're doing, you can use :first-child, which is a CSS thing:
select("[id='1'] > :first-child");
That selector matches all elements that are the first child of elements with id="1", but if select is using querySelector under the covers, you'll get the first such element.
Note that the > in that is the child combinator: It means we're looking for :first-child within [id='1']. (An earlier version of this answer used [id='1'] :first-child, which uses a descendant combinator [just whitespace]. It would matter for selecting a list of elements, but not if selecting only on the first match.) (You need one or the other, since without any combinator ([id='1']:first-child) it would b elooking for the first [id='1'] that was also a :first-child.)
"I'm using SVG.js select() function which uses querySelector() function."
But your comment under TJ's answer suggests it uses querySelectorAll(). There's a difference.
"What I'd like to do is to select the first inner element inside this element."
If it does use querySelector, then use this selector:
"[id='1'] > *"
That'll give you the first child element inside the [id='1'] element.
But if it actually uses querySelectorAll, then using TJ's :first-child selector will work, but as he noted, you need to be aware that it will return all elements that are the first child of their parent.
You can use the > child selector to ensure just one.
"[id='1'] > :first-child"
"Alternatively, I could select it by tag name. How to do it?"
I don't know which element you're referring to, but in general, include the tag name if the selector is selecting on attribute or position. That'll greatly help the engine to narrow down the set of elements.
// querySelector // querySelectorAll
"div[id='1'] > p" ... "div[id='1'] > :first-child"
"I tried select("[id='1']:first") but received an error."
As TJ noted, that's an invalid selector. jQuery's selector engine is non-conforming to the standards in several different ways. Keep your selectors pure as much as possible so that you don't get hooked on needless dependencies.
"By the way, the reason I select it like that is that apparently querySelector has a problem with id's which are numbers."
You can select by numbers if you escape the leading number.
"#\\1 > *"
I am using jQuery to change the width of a sidebar, and within the working code on CSS it shows this:
#wrapper.MenuDisplay #my-sidebar-wrapper {
width: 200px;
}
But after looking through w3schools and mozilla etc, I have not found any selector which looks like #foo.bar.
The reason I am trying to figure this out is that I can't understand why this CSS isn't applied straight away, and how a jQuery toggleClass is able to get this width to be applied (which it successfully does) on a button click.
Does anyone know what is going on here? What the selector is and why it works after clicking on a jQuery button but not straight away on page load?
Many thanks
That means element with id foo and class bar, shortened to #foo.bar
[id][class]
#wrapper is id of an element, and .MenuDisplay is class. Element must have that id and class to get that CSS work.
#foo.bar
matches an element that has both id = "foo" and class = "bar".
It appears you are asking what a sequence of selectors means.
According to current W3C standard, it is...
...a chain of simple selectors that are not separated by a combinator. It always begins with a type selector or a universal selector. No other type selector or universal selector is allowed in the sequence.
Not everyone considers the second sentence of the definition true, because, for example, #this or .that appear not to begin with either a type selector (a, div, span, etc...) or a universal selector (*). However, one should bear in mind that...
...whenever a sequence of selectors does not begin with a type or universal selector, it "defaults" to starting with an omissible universal selector.
So, #this is a shorthand for *#this, .that is a shorthand for *.that. And #this.that is a shorthand for *#this.that.
In fact, if you read the entire universal selector paragraph, you'll notice a note at its end recommending it not to be omitted.
However, almost nobody respects this recommendation and, in practice, the only time you'll see a universal selector is when someone wants to give a CSS property to every single element in the page.
Like in * {box-sizing: border-box;}
As a side note, not omitting the universal selector in your CSS is supposed to make it faster. This is not difficult to test and appears to be true, but the differences are negligible.
Well As far as I can understand from your question,
jQuery Selectors allow you to select and manipulate HTML element(s) or select HTML elements based on their name, id, classes, types, attributes, values of attributes and much more. It's based on the existing CSS Selectors, and in addition, it has some own custom selectors.
In this case:
#foo.bar points to an element in the DOM that has an id="foo" and a class="bar".
jQuery Selector Tester to demonstrate the different selectors:
jQuery Selectors Test Page
Hope I could help!
#foo.bar refers to the element with together foo id and bar class.
$('#foo.bar').fadeOut('slow');
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div id='foo' class='bar'>box</div>
I came across some year old code written by a good developer (yes, I knew him personally) to access all elements having the same id.
$("#choice,#choice")
It returns all elements having the id. But if we use the below
$("#choice")
It returns only the first match, as expected.
After searching for some time, I'm unable to figure out any official links pointing to his technique, as to how it selected all elements with duplicate id.
Can anyone please explain how is this working ?
UPDATE
Please see the question is not about what alternative to use. I'm aware of classSelectors and attributeSelectors and know having duplicate IDs is not recommended, but sometimes you just have to live with years old code the way it is (if you know what I mean).
http://jsbin.com/zodeyexigo/1/edit?html,js,output
If you look at the code of sizzle.js that jQuery uses for selecting elements based on selector you will understand why this happens. It uses following regex to match simple ID, TAG or class selector:
// Easily-parseable/retrievable ID or TAG or CLASS selectors
rquickExpr = /^(?:#([\w-]+)|(\w+)|\.([\w-]+))$/,
but as the selector in question is $("#ID,#ID") it does not match with the selector and uses querySelectorAll (line no 270 in ref link), which replaces selector to "[id='" + nid + "'] " (line no 297 in ref link) which selects all the elements with matching ID.
However, I agree with the other people in this thread, that it is not good idea to have same ID for multiple elements.
Having 2 elements with the same ID is not valid html according to the W3C specification.
When your CSS selector only has an ID selector (and is not used on a specific context), jQuery uses the native document.getElementById method, which returns only the first element with that ID.
However, in the other two instances, jQuery relies on the Sizzle selector engine (or querySelectorAll, if available), which apparently selects both elements. Results may vary on a per browser basis.
However, you should never have two elements on the same page with the same ID. If you need it for your CSS, use a class instead.
If you absolutely must select by duplicate ID, use an attribute selector:
$('[id="a"]');
Take a look at the fiddle: http://jsfiddle.net/P2j3f/2/
Note: if possible, you should qualify that selector with a tag selector, like this:
$('span[id="a"]');
Having duplicated id on the page making your html not valid . ID is unique identifier for one element on the page (spec). Using classes, that are classify similar elements that's your case and $('.choice') will return set of elements
So in JS Fiddle i have shown an example of what jQuery is doing.
https://jsfiddle.net/akp3a7La/
When you have a
$('#choice,#choice');
It is actually getting all the instances of the objects #choice twice, and then filtering out any duplicates.
in my example i show you how it does that also when you have something like this
$("#choice,li");
Where items are actually your #choice items.
In the Jquery Documentation
https://api.jquery.com/multiple-selector/
it talks about multiple Selectors, which is what i think is happening here, your developer friend is selecting the same ID twice, and it would be returning it twice. as you can only have one input with the same ID once on a page (good html syntax)
I want to get CSS selector of jQuery items set.
By jQuery items set in this case I mean jQuery set created basing on some css selector and extended by some jQuery functions like .children etc like $(".basic-selector").children();
It's easy to get for simple scenario like direct selection:
$("div.someclaass").selector //works ok - gives div.someclass
But I also need to get css selector for more complex scenarios (and here jQuery is not returning valid css selector)
$("div.someclass").children("p").selector //schould give div.someclass > p
And for more comples scenarios like
$("ul, ol").children("li").selector //should give ul > li, ol >li
Is it possible?
Requirements:
It must be valid css selector
I cant change js code of creating sets
It should work for all this kind of jQuery functions that can have
css selector - like children, find, next, nextAll, .filter (with css param) etc.
There are some functions like .prev or .parent that will not have css selector - I'm aware of that.
Why do I need that? - I've got my plugin. It takes some jQuery set that was created basing on selector and executes some function on those items. But also - function needs to be executed in the future on new items added later that would match the set if they would exisit in the time of set creation code execution. For good understanding please check plugin page.
The .selector property is deprecated and can only be used when the jQuery element set was created by passing a selector string directly. As you have seen, .selector stops representing a valid selector altogether once you start chaining traversal or filtering methods from the original set. Judging from the sort of values that .selector can have it would seem that it was simply not intended for anything other than internal use.
But the real problem here is that not all element sets can be represented with a valid CSS selector (not without enumerating all possible :nth-child() permutations anyway), not the least of which is because many of jQuery's non-standard selectors have no standard equivalents. Even if you limit yourself to standard selectors and traversal methods that can be represented with a selector, if the original element set was obtained through some other means, or if it was obtained normally and then tampered with, you won't be able to detect this, not even using .selector which as I mentioned is deprecated anyway.
If you really need a selector string to match your selection, the easiest way is to just write it manually and store it in a separate variable.
This question is related to performance.
If I use a selector like the following
$('#myID a') // Does this find #myID and filter by a?
Or should I write the statement like this?
$('#myID').find('a')
I'm not sure if jQuery is smart enough to execute this statement using the ID first or if it operates exactly like CSS and reads right to left. It's not such a big deal using tags but when you run something like
$('#myID .myClass')
It makes a HUGE difference in performance.
From a NetTuts article: http://net.tutsplus.com/tutorials/javascript-ajax/quick-tip-think-right-to-left-with-jquery/
As an example, if Sizzle comes across a selector like $('#box p'),
it’s true that it works right-to-left, but there’s also a quick regex
optimization that will first determine whether the first section of
the selector is an id. If so, it’ll use that as the context, when
searching for the paragraph tags.
Relevant comment from SizzleJS:
// Take a shortcut and set the context if the root selector is an ID
// (but not if it'll be faster if the inner selector is an ID)
When an Id is in the selector. jQuery will first execute document.getElementById then begin filtering for child elements.
basically this is why it is never a great idea to use just attribute or class selectors $('.someclass') or $('[name=myname]') without being more specific. Because it causes the code to traverse the DOM and look at every element to find that class.
By just adding a tagname to the same selector $('div.someclass') or $('div.[name=myname]') you improve efficiency becuase it will first run. document.getElementsByTagName narrowing the number of elements to search.