Jquery The $() factory function

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本文详细介绍了jQuery中的各种选择器，包括基本的选择器如标签名、ID和类选择器，以及更复杂的选择器组合使用方法。通过实例展示了如何精确地选取HTML文档中的元素。
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jQuery	Description
Tag Name:	Represents a tag name available in the DOM. For example $('p') selects all paragraphs in the document.
Tag ID:	Represents a tag available with the given ID in the DOM. For example $('#some-id') selects the single element in the document that has an ID of some-id.
Tag Class:	Represents a tag available with the given class in the DOM. For example $('.some-class') selects all elements in the document that have a class of some-class.
All the above items can be used either on their own or in combination with other selectors. All the jQuery selectors are based on the same principle except some tweaking.

NOTE: The factory function $() is a synonym of jQuery() function. So in case you are using any other JavaScript library where $ sign is conflicting with some thing else then you can replace $ sign by jQuery name and you can use function jQuery() instead of $().

Example:

Following is a simple example which makes use of Tag Selector. This would select all the elements with a tag name p.

<html>
<head>
<title>the title</title>
   <script type="text/javascript" 
   src="/jquery/jquery-1.3.2.min.js"></script>

   <script type="text/javascript" language="javascript">
   $(document).ready(function() {
      var pars = $("p");
      for( i=0; i<pars.length; i++ ){
         alert("Found paragraph: " + pars[i].innerHTML);
      }
   });
   </script>
</head>
<body>
   <div>
      <p class="myclass">This is a paragraph.</p>
      <p id="myid">This is second paragraph.</p>
      <p>This is third paragraph.</p>
   </div>
</body>
</html>
To understand it in better way you can Try it yourself.

How to use Selectors?
The selectors are very useful and would be required at every step while using jQuery. They get the exact element that you want from your HTML document.

Following table lists down few basic selectors and explains them with examples.

Selector	Description
Name	Selects all elements which match with the given element Name.
#ID	Selects a single element which matches with the given ID
.Class	Selects all elements which match with the given Class.
Universal (*)	Selects all elements available in a DOM.
Multiple Elements E, F, G	Selects the combined results of all the specified selectors E, F or G.
Similar to above syntax and examples, following examples would give you understanding on using different type of other useful selectors:

$('*'): This selector selects all elements in the document.

$("p > *"): This selector selects all elements that are children of a paragraph element.

$("#specialID"): This selector function gets the element with id="specialID".

$(".specialClass"): This selector gets all the elements that have the class of specialClass.

$("li:not(.myclass)"): Selects all elements matched by <li> that do not have class="myclass".

$("a#specialID.specialClass"): This selector matches links with an id of specialID and a class of specialClass.

$("p a.specialClass"): This selector matches links with a class of specialClass declared within <p> elements.

$("ul li:first"): This selector gets only the first <li> element of the <ul>.

$("#container p"): Selects all elements matched by <p> that are descendants of an element that has an id of container.

$("li > ul"): Selects all elements matched by <ul> that are children of an element matched by <li>

$("strong + em"): Selects all elements matched by <em> that immediately follow a sibling element matched by <strong>.

$("p ~ ul"): Selects all elements matched by <ul> that follow a sibling element matched by <p>.

$("code, em, strong"): Selects all elements matched by <code> or <em> or <strong>.

$("p strong, .myclass"): Selects all elements matched by <strong> that are descendants of an element matched by <p> as well as all elements that have a class of myclass.

$(":empty"): Selects all elements that have no children.

$("p:empty"): Selects all elements matched by <p> that have no children.

$("div[p]"): Selects all elements matched by <div> that contain an element matched by <p>.

$("p[.myclass]"): Selects all elements matched by <p> that contain an element with a class of myclass.

$("a[@rel]"): Selects all elements matched by <a> that have a rel attribute.

$("input[@name=myname]"): Selects all elements matched by <input> that have a name value exactly equal to myname.

$("input[@name^=myname]"): Selects all elements matched by <input> that have a name value beginning with myname.

$("a[@rel$=self]"): Selects all elements matched by <p> that have a class value ending with bar

$("a[@href*=domain.com]"): Selects all elements matched by <a> that have an href value containing domain.com.

$("li:even"): Selects all elements matched by <li> that have an even index value.

$("tr:odd"): Selects all elements matched by <tr> that have an odd index value.

$("li:first"): Selects the first <li> element.

$("li:last"): Selects the last <li> element.

$("li:visible"): Selects all elements matched by <li> that are visible.

$("li:hidden"): Selects all elements matched by <li> that are hidden.

$(":radio"): Selects all radio buttons in the form.

$(":checked"): Selects all checked boxex in the form.

$(":input"): Selects only form elements (input, select, textarea, button).

$(":text"): Selects only text elements (input[type=text]).

$("li:eq(2)"): Selects the third <li> element

$("li:eq(4)"): Selects the fifth <li> element

$("li:lt(2)"): Selects all elements matched by <li> element before the third one; in other words, the first two <li> elements.

$("p:lt(3)"): selects all elements matched by <p> elements before the fourth one; in other words the first three <p> elements.

$("li:gt(1)"): Selects all elements matched by <li> after the second one.

$("p:gt(2)"): Selects all elements matched by <p> after the third one.

$("div/p"): Selects all elements matched by <p> that are children of an element matched by <div>.

$("div//code"): Selects all elements matched by <code>that are descendants of an element matched by <div>.

$("//p//a"): Selects all elements matched by <a> that are descendants of an element matched by <p>

$("li:first-child"): Selects all elements matched by <li> that are the first child of their parent.

$("li:last-child"): Selects all elements matched by <li> that are the last child of their parent.

$(":parent"): Selects all elements that are the parent of another element, including text.

$("li:contains(second)"): Selects all elements matched by <li> that contain the text second.

You can use all the above selectors with any HTML/XML element in generic way. For example if selector $("li:first") works for <li> element then $("p:first") would also work for <p> element.