I am using jQuery and I am just wondering, does ID have to be always unique in the whole page? Class, I know, can be repeated as many times as you like, what about ID?
Yes, it must be unique.
HTML4:
https://www.w3.org/TR/html4/struct/global.html#h-7.5.2
Section 7.5.2:
id = name [CS]
This attribute assigns a name to an element. This name must be unique in a document.
HTML5:
https://www.w3.org/TR/html5/dom.html#element-attrdef-global-id
The id attribute specifies its element's unique identifier (ID). The
value must be unique amongst all the IDs in the element's home subtree
and must contain at least one character. The value must not contain
any space characters.
Does an ID have to be unique in the whole page?
No.
Because the HTML Living Standard of March 15, 2022, clearly states:
The class, id, and slot attributes may be specified on all HTML elements. …….
When specified on HTML elements, the id attribute value must be unique amongst all the IDs in the element’s tree and must contain at least one character. The value must not contain any ASCII whitespace.
and a page may have several DOM trees. It does, for example, when you’ve attached (Element.attachShadow()) a shadow DOM tree to an element.
TL; DR
Does an ID have to be unique in the whole page?
No.
Does an ID have to be unique in a DOM tree?
Yes.
from mdn
https://developer.mozilla.org/en/DOM/element.id
so i guess it better be...
Technically, by HTML5 standards ID must be unique on the page - https://developer.mozilla.org/en/DOM/element.id
But I've worked on extremely modular websites, where this is completely ignored and it works. And it makes sense - the most surprising part.
We call it "componentization"
For example, you might have a component on your page, which is some kind of widget. It has stuff inside with their own unique IDs eg "ok-button"
Once there are many of these widgets on the page, you technically have invalid HTML. But it makes perfect sense to componentize your widgets so that they each, internally, reference their own ok button eg if using jquery to search from it's own root it might be: $widgetRoot.find("#ok-button")
This works for us, though technically IDs shouldn't be used at all, once they're not unique.
As cited above, even YouTube does it, so it's not so renegade.
Jan 2018, here is Youtube HTML , you can see id="button" id="info" are duplicated.
That's basically the whole point of an ID. :) IDs are specific, can only be used once per page. Classes can be used as pleased.
Browsers used to be lenient on this (many years ago when css was young) and allow the ID to be used more than once. They have become more strict.
However, yes ID's are to be unique and only used once.
If you need to use css formatting more than once use CLASS.
With Javascript, you can only reference to one element using ID. document.getElementById and jQuery's $ selector will return only the first element matching. So it doesn't make sense using the same ID on multiple elements.
There are great answers for the same question at https://softwareengineering.stackexchange.com/questions/127178/two-html-elements-with-same-id-attribute-how-bad-is-it-really.
One tidbit not mentioned above is that if there are several identical ids one the same page (which happens, even though it violates the standard):
If you have to work around this (that's sad), you can use $("*#foo") which will convince jQuery to use getElementsByTagName and return a list of all matched elements.
Yes, IDs are unique. Class are not.
IDs always have to be unique.
Everybody has a unique identification number (ex. Social Security number), and there are lots of people in a social class
I'm adding to this question, because I feel it has not been answered adequately in any of the above,
As a point reference: I've implemented non-unique id's, and all works just fine (across all browsers). Importantly, when coding, I've not run into any css logic errors, which is where the rubber hits the road (IMO) on this question. Have also not run into any conflicts in js (as one can glean out id's in context with classes)
So, why do id's have to be unique? Obvious answer (as stated and re-stated above) is 'cause the 'standards' say so. The missing part for me is why?
i.e. what actually goes awry (or could theoretically go awry) if (heaven forbid) someone inadvertently used the same id twice?
The reference with all browsers these days?
Makes div possible in such terms of being used multiple times.
There is no rule that it must be unique. When all browsers understand:
<script>div#some {font-size: 20px}</script>
<div id="some"><p>understand</p></div>
<div id="some"><h1>me too</h1></div>
When you add new style CSS codes you have the possibility to use the addition of styles. Since that even is not supposed to be unique it describes the opposite use, so make more styles but do not make more objects? And as you can; assign several div objects, so why didn't they tell you that class must be unique? That's because the class does not need unique value. And that makes the ID in legal terms obsolete if not being unique.
<script>.some {font-size: 25px}</script>
<div class="some"><p>enter</p></div>
<div class="some"><h1>enter</p></div>
"When there is no rule when a rule is said. Then the rule is not fulfilled. It's not inherent to exist. By only in the illusion of all rules that it should have existed only to make life much harder."
Just because some people say div must be unique, this might be right, but at least through their professional perspective to say it, they have to say it. Unless they didn't check the modern browsers, which from nearly the beginning were able to understand the code of several different div objects with the same style.
ID must be unique - One reason for that is, that in the Browser-JavaScript-Context exists a methode: Document.getElementById()
This method can only return one element.
If a Document has not unique IDs, this function behaves in an undocumented and unforeseeable way.
I think, this is reason enough to only use one ID per Document.
Reference:
https://developer.mozilla.org/en-US/docs/Web/API/Document/getElementById
Related
I am using jQuery and I am just wondering, does ID have to be always unique in the whole page? Class, I know, can be repeated as many times as you like, what about ID?
Yes, it must be unique.
HTML4:
https://www.w3.org/TR/html4/struct/global.html#h-7.5.2
Section 7.5.2:
id = name [CS]
This attribute assigns a name to an element. This name must be unique in a document.
HTML5:
https://www.w3.org/TR/html5/dom.html#element-attrdef-global-id
The id attribute specifies its element's unique identifier (ID). The
value must be unique amongst all the IDs in the element's home subtree
and must contain at least one character. The value must not contain
any space characters.
Does an ID have to be unique in the whole page?
No.
Because the HTML Living Standard of March 15, 2022, clearly states:
The class, id, and slot attributes may be specified on all HTML elements. …….
When specified on HTML elements, the id attribute value must be unique amongst all the IDs in the element’s tree and must contain at least one character. The value must not contain any ASCII whitespace.
and a page may have several DOM trees. It does, for example, when you’ve attached (Element.attachShadow()) a shadow DOM tree to an element.
TL; DR
Does an ID have to be unique in the whole page?
No.
Does an ID have to be unique in a DOM tree?
Yes.
from mdn
https://developer.mozilla.org/en/DOM/element.id
so i guess it better be...
Technically, by HTML5 standards ID must be unique on the page - https://developer.mozilla.org/en/DOM/element.id
But I've worked on extremely modular websites, where this is completely ignored and it works. And it makes sense - the most surprising part.
We call it "componentization"
For example, you might have a component on your page, which is some kind of widget. It has stuff inside with their own unique IDs eg "ok-button"
Once there are many of these widgets on the page, you technically have invalid HTML. But it makes perfect sense to componentize your widgets so that they each, internally, reference their own ok button eg if using jquery to search from it's own root it might be: $widgetRoot.find("#ok-button")
This works for us, though technically IDs shouldn't be used at all, once they're not unique.
As cited above, even YouTube does it, so it's not so renegade.
Jan 2018, here is Youtube HTML , you can see id="button" id="info" are duplicated.
That's basically the whole point of an ID. :) IDs are specific, can only be used once per page. Classes can be used as pleased.
Browsers used to be lenient on this (many years ago when css was young) and allow the ID to be used more than once. They have become more strict.
However, yes ID's are to be unique and only used once.
If you need to use css formatting more than once use CLASS.
With Javascript, you can only reference to one element using ID. document.getElementById and jQuery's $ selector will return only the first element matching. So it doesn't make sense using the same ID on multiple elements.
There are great answers for the same question at https://softwareengineering.stackexchange.com/questions/127178/two-html-elements-with-same-id-attribute-how-bad-is-it-really.
One tidbit not mentioned above is that if there are several identical ids one the same page (which happens, even though it violates the standard):
If you have to work around this (that's sad), you can use $("*#foo") which will convince jQuery to use getElementsByTagName and return a list of all matched elements.
Yes, IDs are unique. Class are not.
IDs always have to be unique.
Everybody has a unique identification number (ex. Social Security number), and there are lots of people in a social class
I'm adding to this question, because I feel it has not been answered adequately in any of the above,
As a point reference: I've implemented non-unique id's, and all works just fine (across all browsers). Importantly, when coding, I've not run into any css logic errors, which is where the rubber hits the road (IMO) on this question. Have also not run into any conflicts in js (as one can glean out id's in context with classes)
So, why do id's have to be unique? Obvious answer (as stated and re-stated above) is 'cause the 'standards' say so. The missing part for me is why?
i.e. what actually goes awry (or could theoretically go awry) if (heaven forbid) someone inadvertently used the same id twice?
The reference with all browsers these days?
Makes div possible in such terms of being used multiple times.
There is no rule that it must be unique. When all browsers understand:
<script>div#some {font-size: 20px}</script>
<div id="some"><p>understand</p></div>
<div id="some"><h1>me too</h1></div>
When you add new style CSS codes you have the possibility to use the addition of styles. Since that even is not supposed to be unique it describes the opposite use, so make more styles but do not make more objects? And as you can; assign several div objects, so why didn't they tell you that class must be unique? That's because the class does not need unique value. And that makes the ID in legal terms obsolete if not being unique.
<script>.some {font-size: 25px}</script>
<div class="some"><p>enter</p></div>
<div class="some"><h1>enter</p></div>
"When there is no rule when a rule is said. Then the rule is not fulfilled. It's not inherent to exist. By only in the illusion of all rules that it should have existed only to make life much harder."
Just because some people say div must be unique, this might be right, but at least through their professional perspective to say it, they have to say it. Unless they didn't check the modern browsers, which from nearly the beginning were able to understand the code of several different div objects with the same style.
ID must be unique - One reason for that is, that in the Browser-JavaScript-Context exists a methode: Document.getElementById()
This method can only return one element.
If a Document has not unique IDs, this function behaves in an undocumented and unforeseeable way.
I think, this is reason enough to only use one ID per Document.
Reference:
https://developer.mozilla.org/en-US/docs/Web/API/Document/getElementById
I recently saw a html page, that i thought the id of several html tags was the same, then I realized the ids are unique but it raises the question that what could have happened if the page actually used several tags
As i have heard id attribute of every html tag(if it has one) must be unique,
now i wonder what happens if it is not the case??
what possible errors can it cause?
does different browsers show different reactions for this issue?
does javascript and jquery codes that use duplicated ids run on both tags or what?
Duplicate ids can have various different effects. Which you experience will depend on the method you use to try to access them (and possibly also from browser to browser).
You'll affect all of them
You'll affect the first one
You'll affect the last one
You'll get a collection instead of an element, try to treat it like an element and get an error
Duplicate ids are not allowed in HTML. Don't make trouble for yourself. Use classes for groups and ids for unique identifiers.
Change them as soon as possible, to save a lot of headache in the future. For elements with same property use classes
About your queries,
Now i wonder what happens if it is not the case??
Well, the HTML is not a valid one anymore. Now a days it doesn't hurt much but still not preferred.
What possible errors can it cause?
Errors are little bit hard to predict. But with jQuery you are going to get many.
Does different browsers show different reactions for this issue?
Not sure.
Does javascript and jquery codes that use duplicated ids run on both tags or what?
jQuery will give you trouble. Consider a case where you have two input fields with same ID.
and you try to select second one with out noticing. jQuery('yourID').val() and you'll be selecting the firs value instead. Like this there are a lot of possibilities.
As you said, HTML id, per specs, must be unique. If one where to put duplicated id, the js behavior relative to those ids will be unpredicatable, and could even change between 2 calls.
Any js call on one id (jquery or not) will point to one of the id but without guarentee that :
It will be the same every call
It will have the same order between 2 page refresh
It will have the same behavior beween 2 different browser
It will have the same behavior betwween 2 time the same browser
The problems that could emerge depend on how toghtly the js code is coupled to the underlying element DOM structure anw could mostly point to a undefined exception and stop the js execution.
Is there anything bad in using post arrays for post variables?
<input type="text" id="stuff" name="stuff[text]"/>
instead of
<input type="text" id="stuff" name="stuff"/>
Tips when to use them?
No, there is no reason not to do this.
However, PHP is pretty much the only language which allows to create arrays like this - so if you ever change your backend to a different language you might have to change things.
It isn't a bad way. Typically programmers use array in such manner to post array with non-determined lenght. Still. Don't know for certain, but when You want to change method to GET then on IE <= 8 is a limit to 2048 chars in address lenght. And dynamic generated array can easy depleat this limit. On other browsers limit is much higher or there is none.
Another drawback of this method is that PHP will preceed correctly, but other server side languages may not. This isn't specified in official HTML docs, as far I know.
So it is more convinient to put it in a single cell in post array, that do subarray. If You want to do some namespacing, then You can write name in such way:
name="styff.text"
as do some of forum engines (for certain Vanilla 2 does).
If it has no diffrence to You I would stay to single variable name in html names. Mostly because of backend.
For tips how to use them I could recommend to use such array to cover dynamic generated content on site. Still it can be handled with normal names, but it is pretty ugly. If we have a case that You want to do a picture adding system, that I would name each input file with "pic[]" and on server iterate whole table.
The same thing for generating documents on client side. I would then do names like "content[][name]" "content[][type]" "content[][value]" and so on. Whathever I woud have in document part class I would throw in this kind of naming, and on server just check is set and do certain things for certain block of document.
This could be talked for a long time since every programmer have own technics and they tend to stick with it. For example I throw a in every form I have on site, and each action is parsed by a generall controller, and then passed by do specific controllers.
Nothing bad. If it is useful four you, use it.
In my project I have submissions and comments, each with an ID. Currently the ID's are just numeric and correspond to their database ID's. Everything is working fine but when I run it through the W3 validator I get the error:
value of attribute "id" invalid: "1" cannot start a name
I suppose instead that I could just precede all ids with some sort of string but then whenever I was using or manipulating the id in JQuery or PHP I have to do a id.replace('string', '') before using it. This seems rather cumbersome. Any advice?
Yes, using numbers as HTML element IDs is bad practice.
It violates W3C specification.
You noted this in your question, and it is true for every HTML specification except HTML5
It is detrimental to SEO.
Search-engine optimized HTML element IDs SHOULD reflect the content of the identified element. Check out How To Compose HTML ID and Class Names like a Rockstar by Meitar Moscovitz. It provides a good overview of this concept.
There can be server-side scripting issues.
Back when I first started programming in ASP classic, I had to access submitted form fields by a syntax like Request.Form("some_id"). However, if I did Request.Form(1) it would return the value of the second field in the form collection, instead of the element with an Id equal to 1. This is a pretty standard behavior for working with collections. Its also similar with javascript, and could make your client side scripting more complicated to maintain as well.
I suggest you to use prefixes "comment-ID" or "post-ID".
If you need the id in JavaScript, you just have to id.substring(8) (for "comment-")
The HTML 5 Specification lifts this restriction. If you're worried about validity you might simply consider changing the DTD to HTML5's.
http://www.w3.org/TR/html5/elements.html#the-id-attribute
If you're manipulating the element then you can just use $(this).jQueryOperation() - therefore you can have a prefix without having to replace anything!
The best way for your need is having the prefix for your class, I mean something like item-x and x is the number that you need.
But from my personal experience, it is better to use classes for your elements, and you know that you must use classes if the item is not unique in the page
I was wondering if their was any sort of way to detect a pages genre/category.
Possibly their is a way to find keywords or something?
Unfortunately I don't have any idea so far, so I don't have any code to show you.
But if anybody has any ideas at all, let me know.
Thanks!
EDIT #Nican
Perhaps their is a way to set, let's say 10 category's (Entertainment, Funny, Tech).
Then creating keywords for these category's (Funny = Laughter, Funny, Joke etc).
Then searching through a webpage (maybe using a cUrl) for these keywords and assigning it to the right category.
Hope that makes sense.
What you are talking about is basically what Google Adsense and similar services do, and it's based on analyzing the content of a page and matching it to topics. Generally, this kind of stuff is beyond what you would call simple programming / development and would require significant resources to be invested to get it to work "right".
A basic system might work along the following lines:
Get page content
Get X most commonly used words (omitting stuff like "and" "or" etc.)
Get words used in headings
Assign weights to different words according to a set of factors (is used in heading, is used in more than one paragraph, is used in link anchors)
Match the filtered words against a database of words related to a specific "category"
If cumulative score > treshold, classify site as belonging to category
Rinse and repeat
Folksonomy may be a way of accomplishing what you're looking for:
http://en.wikipedia.org/wiki/Folksonomy
For instance, in Drupal they have a Folksonomy module:
http://drupal.org/node/19697 (Note this module appears to be dead, see http://drupal.org/taxonomy/term/71)
Couple that with a tag cloud generator, and you may get somewhere:
http://drupal.org/project/searchcloud
Plus, a little more complexity may be able to derive mapped relationships to other terms, especially if you control the structure of the tagging options.
http://intranetblog.blogware.com/blog/_archives/2008/5/22/3707044.html
EDIT
In general, the type of system you're trying to build relies on unique word values on a page. So you would need to...
Get unique word values from your content (index values or create a bot to crawl your site)
Remove all words and symbols you can't use (at, the, or, and, etc...)
Count the number of times the unique words appear on the page
Add them to some type of datastore so you can call them based on the relationships you're mapping
If you have a root label system in place, associate those values with the word counts on the page (such as a query or derived table)
This is very general, and there are a number of ways this can be implemented/interpreted. Folksonomies are meant to "crowdsource" much of the effort for you, in a "natural way", as long as you have a user base that will contribute.