I was under the impression that when a browser (generally) searches for an element that has a class it is a depth first search.
Recently I was asked to put some code together for a colleague, and asked to identify forms on a page with the substring of 'webform' in the class. I knew there was a form on a page I tested and used the following JS:
document.querySelector("[class*=webform]")
However, this returned the body element of the page whose class attribute had the substring of 'webform' in it. Generally (this question being browser dependant) is the searching in the DOM for elements containing a certain class depth first? Is it totally implementation or browser dependant (as in querySelector will use one method and another function will use a different method)?
Many thanks.
#hungerstar is right. Apologies for the brain fart, seems I need to brush up on my trees a little!
So in conclusion, it proved that it is depth-first. Great!
Related
I was locating the elements for selenium through the inspect function of my actual browser (chrome) and never had any issues. Now i had a case where the located element couldn't be found and i figured out that in my selenium chrome instance, the element has another ID then in my "normal" one and that's why i can't locate it.
This is the case with some other elements too and i can't spot the pattern.
In "Ikognito"-mode of chrome, i get the same values as in my normal browser, and the ids that my selenium browser gets are the same aswell everytime i launch the programm.
Does anybody have an explanation for this ?
Is this common practise upon web developers and whats the way to go about this issue on future projects ?
Do i always have to run my selenium browser first and then extract the elements IDs out of there ?
Although locating elements by id is the preferred way, it is not the only one, the are way more options like:
name attribute
link text for a elements
partial link text for a elements
HTML tag name
class attribute
CSS selector
XPath selector
The latter one - XPath is the most powerful as it's almost a programming language. Unlike other selector strategies XPath selectors have full awareness of DOM page, can lookup all attributes, text, parent/child objects, traverse axes and if it's not enough you can go for functions and operators to precisely select whatever element is needed.
With regards to dynamic IDs - it's quite a common practice when the page is not deterministic and the content is dynamic. Theoretically you can ask your application developers to come up with a custom HTML attribute which will be used for automation and maybe user tracking, but if for some reason it is not possible - you will have to define another way of locating the element.s
I want to validate a custom polymer element. To do this, I want in javascript to access all my nested polymer elements to see if they are valids.
I can't find an easy way to do this.
this.querySelectorAll does not find my inputs that are nested in other polymer elements. It seems I can't use "/deep/" in these selectors.
Is there an easy way to do this ? Or do I have to do a recursive javascript methods that will call a querySelectorAll in all elements with shadow roots ?? (I guess performances will get ugly...)
Thanks for your help.
If there is no fast solution, I will probably try the other way around (have my inputs register to the parent)
Answer:
element.querySelectorAll() will find some elements when using /deep/, however, it only goes so far (1 shadow dom level). This would indeed necessitate recursive calls from each ElementNode.
Note:
This type of behavior largely goes against the core tenets of HTML (i.e. that the web page works no matter how well-formed the content is). In other words, all elements are valid no matter their placement.
As an example, I have made a custom element that only renders specific child elements and hides all others. This still keeps in line with the above tenet, as an element's base rendering is controlled by the element/agent, but allows for the developer/designer to customize its presentation aside from the standard presentation.
I recently asked How to extend multiple elements with Polymer and it turned out, in fact, you can't really. The idea was to create a web component that can be applied to different elements to "decorate" them.
Addy Osmani answered this question with a few approaches to handle that use case.
One of them was:
The alternative (if you strictly want to do this all in one custom
element, which imo, makes this less clean) is to do something like
checking against the type of element being extended, which could
either be done in the manner you linked to or by checking as part of
your element registration process.
Despite from fact that this approach might be less clean, my question is:
How I can find out what element is going to be extended while I'm registering my own?
As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.
Closed 11 years ago.
What is the preferred method when dealing with choosing a class vs. an ID?
For instance, you can have a bunch of elements that might be styled identically and could all use the same class. However, for readability purposes, it's sometimes nice to have a unique ID for each element instead.
Obviously you don't want to go ridiculously overboard where every element has an ID. However, where do you guys draw the line and does using all IDs where you could be using classes slow things down noticeably? If so... when?
How to stop obliterating semantic HTML.
Most people learn HTML from looking at source code and of HTML and tinkering with it, learning how <tag>foo</tag> looks and running along with it. They don't really gain a deep understand of it, but they go on to do things that require a deep understanding, the side effect is the problem you and thousands of others have every day -- they're doing things and they don't know fully how these tools work, because it looks so simple on the surface and the powerful uses are are "hidden" in the funny manual that nobody feels the need to read. Everything is plainly explained and been written down for a long time.
What IDs are for (directly from the HTML4 spec, with my notes)
The id attribute assigns a unique identifier to an element (it only happens ONCE, never TWICE or more, I'm tired of seeing people come on this site and dropping in their code with the same ID in twenty elements)
The id attribute has several roles in HTML:
As a style sheet selector. (This means, you can use it to describe CSS styles)
As a target anchor for hypertext links.(When you can jump to a section of a page)
As a means to reference a particular element from a script.(document.getElementById("whatever"))
As the name of a declared OBJECT element.
For general purpose processing by user agents (e.g. for identifying fields when extracting data from HTML pages into a database, translating HTML documents into other formats, etc.).
What Classes are for (directly from the HTML4 spec, with my notes)
The class attribute [...] assigns one or more class names to an element (this one gets to be re-used to your heart's content) ; the element may be said to belong to these classes. A class name may be shared by several element instances. The class attribute has several roles in HTML:
As a style sheet selector (when an author wishes to assign style information to a set of elements).
For general purpose processing by user agents. (Basically, it's just another part of an element)
What? I don't get it.
IDs: It's the fingerprint of something, there's only one, you only use each fingerprint once in the entire document. You only use it when you need to give something an ID. You probably don't want to have hundreds of these, or even tens of these. You rarely if ever need to start making these. The specific uses are for target anchors, improving selector speed in rare edge-cases. Generally you never describe your CSS based on IDs, you might have some edge-cases such as #HEADER .body h1, which may be different from your #BODY, I'd still advise against making them IDs for no real reason.
Classes: Nothing to do with unique fingerprints or linking to sections of a page, classes don't uniquely identify something. Classes describe a group of things that belong together or should behave the same way. If you're part of the class called coffee you should exhibit classes as one might expect from coffee, if you're a class of cellphone, then look like a cellphone (don't provide coffee).
But how the heck am I supposed to access the 4th cell in the 6th column of some table, or group of divs or that 20th list item?
This is where people who don't know what HTML is throw their hands up in the air and decide to assign IDs to all the elements. This is a total side-effect of nobody properly explaining to you how HTML works. That's a nice way of saying you didn't RFTM or ask questions early on (user1066982 in this case, did, which is amazing and makes me happy, I'm writing this to point other people to in the future who fail at HTML).
You need to start learning right now. Stop pretending you understand this stuff.
HTML is not a string of text such as <foo><bar>baz</bar>blah<ding/></foo>, sure that's how you write HTML but if that's what you believe it is you do not understand HTML in the browser.
HTML is a document that is structured like XML. HTML documents have a model, that means they aren't flat text. The text-representation of that document is a way your browser can take flat text and turn it into a tree structure. Trees are like arrays, except they aren't just flat elements in an array one-after-another, but rather they nest so one element may point to several other elements.
This below isn't a diagram (stolen from the w3c's spec on the Document Object Model) of how to write HTML text, this is a diagram of how your browser stores it in memory:
Since it's in memory like that, it doesn't mean "Oh crap! I have no way to access the first TD in the second TR of the table body in the table!", it means you simply and plainly explain to your code that there is a child element inside of the table.
JavaScript provides a full DOM API that allows you to access every single node in that DOM tree.
PHP provides a full DOM API that allows you to access every single node in that DOM tree.
C++ has a full DOM API that allows you to access every single node in that DOM tree.
ASP provides a full DOM API that allows you to access every single node in that DOM tree.
EVERYTHING that touches the DOM provides a full DOM API that allows you to access every single node in that DOM tree, with the exception of sub-standard software that throws regular expressions around in a futile attempt at parsing HTML.
Use the API for the DOM to access those nodes based on semantic HTML. Semantic HTML means you have a structure to your HTML that makes sense. Paragraphs go in <p> tags, headings go into heading tags, and so on.
You never, under any circumstances, what-so-ever need to reproduce the DOM API through hacking in values with ID tags because you didn't know you could just say getAllEmentsByTagName("td")[4] to get the fourth element.
If you can grab getAllEmentsByName("td")[4] you don't need to do <td id="id4"> and then later getElementById("id4") because you didn't want learn just one other API call. I dread the day I ever have to maintain a pile of code left behind by someone who felt the need to stick an ID into every element "just to be sure", especially when I need to go back and insert a new element between the fifth and sixth element in a table of thousands (can you imagine replacing EVERY id? Especially when this feature was accounted for over 10 years ago?! Insanity!)
Tl;dr
HTML isn't actually just a pile of text with one way to access it
rtfm, stop pretending you understand it because you can do a handful of things, you're holding yourself back.
Don't shove IDs everywhere, only use them where absolutely required.
Use classes to describe things, not identify things.
?????
Profit.
However, for readability purposes, it's sometimes nice to have a unique ID for each element instead.
This makes absolutely no sense to me. What makes an ID more readable than a class? There's no point assigning unique identifiers to each of a group of related elements if there's no benefit in having identities.
For what it's worth, realize that a single element can have both classes and an ID. If your elements need to be uniquely identified somehow, give them IDs. If multiple elements should be styled identically and are all similar in purpose anyway, use classes. If your elements fit both criteria, give them both attributes, and use each attribute accordingly.
IDs should not be used for styling. Use classes instead. IDs have a very high specificity, and are difficult to override (leading to more IDs, and longer selector chains). Also, IDs are used for JavaScript DOM selection, so if you're using the same IDs in your CSS that you're using in your JavaScript, you've tied the styles to the scripts, and that's bad separation of concerns.
IDs are for JavaScript. Classes are for CSS.
Note: JavaScript and specificity are not the only reasons. Others include fragment identifiers and code reuse. As I say in the comments, there are several smart people who advise against IDs (start there and follow the links)
I use IDs for elements that have clear responsibility, Classes for element that have same presentations, for example:
HTML:
<div id='sport-news'>
<article class='news'>...</article>
<article class='news'>...</article>
</div>
CSS:
.news { /* global styles */ }
[id=sport-news] .news { /* specific styles */ }
JavaScript:
var sportNews = document.getElementById('sport-news') // faster
, news = sportNews.childNodes;
For me, [id] .class is more readable than .parent-class .child-class.
When designing, I will use both id's and classes. For specific items I will use id only. But if you need to apply same styles for different items, use classes. You cannot use same id for different items because id is specific to one item only.
Why is it bad practice to have more than one HTML element with the same id attribute on the same page? I am looking for a way to explain this to someone who is not very familiar with HTML.
I know that the HTML spec requires ids to be unique but that doesn't sound like a convincing reason. Why should I care what someone wrote in some document?
The main reason I can think of is that multiple elements with the same id can cause strange and undefined behavior with Javascript functions such as document.getElementById. I also know that it can cause unexpected behavior with fragment identifiers in URLs. Can anyone think of any other reasons that would make sense to HTML newbies?
Based on your question you already know what w3c has to say about this:
The id attribute specifies a unique id for an HTML element (the id
attribute value must be unique within the HTML document).
The id attribute can be used to point to a style in a style sheet.
The id attribute can also be used by a JavaScript (via the HTML DOM)
to make changes to the HTML element with the specific id.
The point with an id is that it must be unique. It is used to identify an element (or an anything: if two students had the same student id schools would come apart at the seems). It's not like a human name, which needn't be unique. If two elements in an array had the same index, or if two different real numbers were equal... the universe would just fall apart. It's part of the definition of identity.
You should probably use class for what you are trying to do, I think (ps: what are you trying to do?).
Hope this helps!
Why should I care what someone wrote in some document?
You should care because if you are writing HTML, it will be rendered in a browser which was written by someone who did care. W3C created the spec and Google, Mozilla, Microsoft etc... are following it so it is in your interest to follow it as well.
Besides the obvious reason (they are supposed to be unique), you should care because having multiple elements with the same id can break your application.
Let's say you have this markup:
<p id="my_id">One</p>
<p id="my_id">Two</p>
CSS is forgiving, this will color both elements red:
#my_id { color:red; }
..but with JavaScript, this will only style the first one:
document.getElementById('my_id').style.color = 'red';
This is just a simple example. When you're doing anything with JavaScript that relies on ids being unique, your whole application can fall apart. There are questions posted here every day where this is actually happening - something crucial is broken because the developer used duplicate id attributes.
Because if you have multiple HTML elements with the same ID, it is no longer an IDentifier, is it?
Why can't two people have the same social security number?
You basicaly responded to the question. I think that as long as an elemenet can no longer be uniquely identified by the id, than any function that resides on this functionality will break. You can still choose to search elements in an xpath style using the id like you would use a class, but it's cumbersome, error prone and will give you headaches later.
The main reason I can think of is that multiple elements with the same id can cause strange and undefined behavior with Javascript functions such as document.getElementById.
... and XPath expressions, crawlers, scrapers, etc. that rely on ids, but yes, that's exactly it. If they're not convinced, then too bad for them; it will bite them in the end, whether they know it or not (when their website gets visited poorly).
Why should a social security number be unique, or a license plate number? For the same reason any other identifier should be unique. So that it identifies exactly one thing, and you can find that one thing if you have the id.
The main reason I can think of is that multiple elements with the same
id can cause strange and undefined behavior with Javascript functions
such as document.getElementById.
This is exactly the problem. "Undefined behavior" means that one user's browser will behave one way (perhaps get only the first element), another will behave another way (perhaps get only the last element), and another will behave yet another way (perhaps get an array of all elements). The whole idea of programming is to give the computer (that is, the user's browser) exact instructions concerning what you want it to do. When you use ambiguous instructions like non-unique ID attributes, then you get unpredictable results, which is not what a programmer wants.
Why should I care what someone wrote in some document?
W3C specs are not merely "some document"; they are the rules that, if you follow in your coding, you can reasonably expect any browser to obey. Of course, W3C standards are rarely followed exactly by all browsers, but they are the best set of commonly accepted ground rules that exist.
The short answer is that in HTML/JavaScript DOM API you have the getElementById function which returns one element, not a collection. So if you have more than one element with the same id, it would not know which one to pick.
But the question isn't that dumb actually, because there are reasons to want one id that might refer to more than one element in the HTML. For example, a user might make a selection of text and wants to annotate it. You want to show this with a
<span class="Annotation" id="A01">Bla bla bla</span>
If the user selected text that spans multiple paragraphs, then the needs to be broken up into fragments, but all fragments of that selection should be addressable by the same "id".
Note that in the past you could put
<a name="..."/>
elements in your HTML and you could find them with getElementsByName. So this is similar. But unfortunately the HTML specifications have started to deprecate this, which is a bad idea because it leaves an important use case without a simple solution.
Of course with XPath you can do anything use any attribute or even text node as an id. Apparently the XPointer spec allows you to make reference to elements by any XPath expression and use that in URL fragment references as in
http://my.host.com/document.html#xpointer(id('A01'))
or its short version
http://my.host.com/document.html#A01
or, other equivalent XPath expressions:
http://my.host.com/document.html#xpointer(/*/descendant-or-self::*[#id = 'A01'])
and so, one could refer to name attributes
http://my.host.com/document.html#xpointer(/*/descendant-or-self::*[#name = 'A01'])
or whatever you name your attributes
http://my.host.com/document.html#xpointer(/*/descendant-or-self::*[#annotation-id = 'A01'])
Hope this helps.