class or ID

Question

Duplicate:

• Cascading style sheets use “id” or “class”
• CSS: are class and id interchangeable?
• div class vs id
• CSS: div id VS. div class

Sometime, when i like to make a selection a add an ID (unique) to something (images) and i used jquery to do domething when hover or clic.

But the same thing can be done with classes. I understand that class is for a lot of thing (let say class="selected") but when a class is add to something i can trigger a jQuery action with that class insted of ID...

So the question is, what is "the right way" to use class and ID if i can accomplish anything and everything with both !

Answer 1

A HTML ID may only be applied to a single element, whereas a class can be used on many, which makes it more flexible.

Answer 2

Where to use an ID versus a class

The simple difference between the two is that while a class can be used repeatedly on a page, an ID must only be used once per page. Therefore, it is appropriate to use an ID on the div element that is marking up the main content on the page, as there will only be one main content section. In contrast, you must use a class to set up alternating row colors on a table, as they are by definition going to be used more than once.

IDs are an incredibly powerful tool. An element with an ID can be the target of a piece of JavaScript that manipulates the element or its contents in some way. The ID attribute can be used as the target of an internal link, replacing anchor tags with name attributes. Finally, if you make your IDs clear and logical, they can serve as a sort of “self documentation” within the document. For example, you do not necessarily need to add a comment before a block stating that a block of code will contain the main content if the opening tag of the block has an ID of, say, "main", "header", "footer", etc.

Answer 3

The difference is, that an ID is something unique to a document so it should be only used for one element on a given HTML page.

Classes on the other hand are more like categories of elements. For example if you take a view on a single weblog post where you usually have the comments made to this weblog entry showing up right below it, you could give all these comments the class "comment" and the single weblog entry the ID "entry".

Answer 4

An ID uniquely identifies an element within the entire document, whereas a class name is like a 'tag' - many tags can be added to an element, and the tags can be re-used.

Here's a comparison list:

• You can add multiple class names to an element separated by spaces, but you can only add a single ID.
• You can re-use a class name on multiple elements, but can use an ID only once within an entire document.
• An ID can be used as a fragment identifier on the URL, and the browser will scroll to that ID. For example, http://example.com/mypage.htm#someid. It is not possible to do this with classes.
• An ID makes it much easy and efficient to find a particular element in Javascript. For example, document.getElementById('someid') (or $("#someid") in jQuery). Note that libraries like jQuery make it possible to find elements by their class name too, but it's not nearly as efficient.

Other properties of IDs

• It is possible for an element to contain both an ID and a number of class names, and it doesn't matter, or mean anything, if the ID is the same as one of the class names.
• Frames, iframes and traditional popup windows count as separate documents, so it is possible to use the same ID on elements within these different windows.
• Some browsers do not follow the specification properly, and allow for the same ID to be used multiple times, and for it to behave in some ways like a class name but still kind-of work in document.getElementById(). For instance, a CSS selector based on that ID may match multiple elements as if it were a class. This is a really messy situation and you should never rely on it. I'd be happy if browsers stopped trying to support this.