Selectors are what jQuery uses to pick out particular objects in the DOM. While this might start out simply enough, it can appear to be complicated in more testing examples. The trick is to always remember what the selector is doing.

This is an extract from my book Just jQuery The Core UI.

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1. Understanding jQuery
2. Basic jQuery CSS Selectors
      Extract: The DOM 
3. More Selectors
      Extract: Basic Selectors 
4. The JQuery Object 
5. Filters 
6. DOM Traversal Filters 
7. Modifying DOM Objects
      Extract: Modifying The DOM 
8. Creating Objects & Modifying The DOM Hierarchy
   
9. Working With Data
10. Forms 
11. Function Queues 
12. Animation 
13. jQuery UI
14. jQuery UI Custom Control
15. Easy Plugins ***NEW!!!
16. Testing With QUnit
17. Epilog A Bonus Function

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One of the main uses of jQuery is in finding sets of DOM objects so that we can work with them. While this starts out simply enough it can become very complicated. Mastering the use of selectors is the first stage in becoming a jQuery expert. The second stage is finding out how to make changes to the elements you have found.

In this chapter we focus on finding elements, leaving manipulation to another chapter. Of course, jQuery isn't of much use if all you can do is find elements, but it is the first, and necessary, step.

Basic Selectors

In an ideal world any element, or its corresponding DOM object, would be easy to find because it would have a unique id. Even sets of DOM objects that share a set of common properties should be easy to find because they should all belong to the same class.

Before going any deeper, it is worth saying that the three most used jQuery selectors are the ones introduced early in Chapter 2:

• name    all tags like <name>

• #name  the single tag with id="name"

• .name   all tags with class="name"

So, for example:

$('div')

selects all the DOM objects that correspond to div elements;

$('#mydiv') 

selects the one element with id set to mydiv;

$('.bigdiv')

selects all elements with class set to bigdiv.

For many jQuery users these are the only selectors needed because in an ideal world you would be able to find every element you needed using just these.

The reason is that if you are creating the HTML in question and you knew ahead of time that you need to find a particular element then you would give it a unique id. If you knew that you needed to find a set of elements then you would give them a class name. In other words, if you are in charge of the HTML you can nearly always arrange for the elements you want to be easy to find using jQuery.

So when does the need for a complex or sophisticated selector arise?

The answer is when you aren’t in charge of creating the HTML in the first place. This could be because the HTML is obtained from a web page you don’t control or it could be because the HTML is generated by a CMS (Content Management System), a template or a JavaScript function.

Another common situation is where the HTML for a lot of pages was generated in the past and modifying it would take too much time. In this case you might opt to use a JavaScript function to modify it in a systematic way, to add a new logo for example. In this case it is unlikely that the elements you need to modify would have ids or class attributes set.

In other words, it is sometimes not possible or not easy to modify the HTML so that the elements you want to select all have ids or belong to a single class. In such cases you may well have no alternative but to use a sophisticated selector to pick out HTML elements based on their type and their relationship to other elements on the page.

What follows is a full introduction to the range of CSS selectors that jQuery supports. You don’t have to master them all in one go. The best strategy is to read through and get an idea of how everything works and then come back when you have a particular problem and see if there is a selector solution waiting for you. As well as CSS selectors, jQuery adds many of its own selectors and extends the way you can search for objects using filters and traversal functions – more of which in later chapters. In this chapter we focus on the basic CSS selectors.

Type Selectors

Once you graduate beyond the three basic selectors you need to have a framework to organize things – without a framework everything looks like a special case.

Before going on it is worth remembering that jQuery always returns an array of elements that match the selector, even if there is only one match. It is also worth remembering that many jQuery methods operate on the first element of the array – which makes working with a single element much easier - or on all of the elements. 

We already know that that the fundamental selector is the type selector, i.e. name, which selects all elements corresponding to <name>. All other selectors take the form of extra conditions on this basic selector.

For example:

$("p")

selects elements corresponding to all <p> tags. All other selectors are modifications and extensions on this basic selector.

There is also a universal type selector, the asterisk *, which matches every element type. This may not seem to be very useful at first, but it is often used within a larger selector to mean any element type that satisfies other conditions. This will become clear in a moment.

The basic type selector can be modified with additional conditions, usually indicated by a colon or with square brackets or another special symbol. For example, if you want to specify a type with a particular id you can write:

p#name

to select all <p> elements with id equal to name.

Of course, there can only be one element with id equal to name so you don't really need to specify the type. This can be done using the universal type selector, that is:

*#name

selects any type of element with id equal to name.

The convention is that if you don't specify a type then the universal type selector is assumed. This means that you can simply write:

#name

to mean:

*#name

Now you can see that the id selector is just a special case of the more general type selector.

In the same way the .class selector is a shorthand for:

*.name

which selects all elements with the class attribute equal to name. 

In general the class selector is:

T.name

which selects all elements of type T with class set to name.