返回值:jQueryjQuery(selector, [context])

Accepts a string containing a CSS selector which is then used to match a set of elements.

In the first formulation listed above, jQuery() — which can also be written as $() — searches through the DOM for any elements that match the provided selector and creates a new jQuery object that references these elements: 

$('div.foo');

Selector Context

By default, selectors perform their searches within the DOM starting at the document root. However, an alternate context can be given for the search by using the optional second parameter to the $() function. For example, if within a callback function we wish to do a search for an element, we can restrict that search: 

$('div.foo').click(function() {
  $('span', this).addClass('bar');
});

Since we've restricted the span selector to the context of this, only spans within the clicked element will get the additional class.

Internally, selector context is implemented with the .find() method, so $('span', this) is equivalent to $(this).find('span').

Using DOM elements

The second and third formulations of this function allow us to create a jQuery object using a DOM element or elements that we have already found in some other way. A common use of this facility is to call jQuery methods on an element that has been passed to a callback function through the keyword this: 

$('div.foo').click(function() {
  $(this).slideUp();
});

This example causes elements to be hidden with a sliding animation when clicked. Because the handler receives the clicked item in the this keyword as a bare DOM element, the element must be wrapped in a jQuery object before we can call jQuery methods on it.

When XML data is returned from an Ajax call, we can use the $() function to wrap it in a jQuery object that we can easily work with. Once this is done, we can retrieve individual elements of the XML structure using .find() and other DOM traversal methods.

Cloning jQuery Objects

When a jQuery object is passed as a parameter to the $() function, a clone of the object is created. This new jQuery object references the same DOM elements as the initial one.

Returning an Empty Set

As of jQuery 1.4, calling the jQuery() method with no arguments returns an empty jQuery set. In previous versions of jQuery, this would return a set containing the document node.

示例:

Finds all p elements that are children of a div element.

<!DOCTYPE html>
<html>
<head>
<script src="jquery.min.js"></script>
</head>
<body>

<p>one</p> <div><p>two</p></div> <p>three</p>

<script>

$("div > p").css("border", "1px solid gray");

</script>
</body>
</html>
演示:
结果:
[ <p>two</p> ]

示例:

Finds all inputs of type radio within the first form in the document.

jQuery 代码:
$("input:radio", document.forms[0]);

示例:

Finds all div elements within an XML document from an Ajax response.

jQuery 代码:
$("div", xml.responseXML);

示例:

Sets the background color of the page to black.

jQuery 代码:
$(document.body).css( "background", "black" );

示例:

Hides all the input elements within a form.

jQuery 代码:
$(myForm.elements).hide()

返回值:jQueryjQuery(html, [ownerDocument])

Creates DOM elements on the fly from the provided string of raw HTML.

Creating New Elements

If a string is passed as the parameter to $(), jQuery examines the string to see if it looks like HTML (i.e., it has <tag ... > somewhere within the string). If not, the string is interpreted as a selector expression, as explained above. But if the string appears to be an HTML snippet, jQuery attempts to create new DOM elements as described by the HTML. Then a jQuery object is created and returned that refers to these elements. We can perform any of the usual jQuery methods on this object: 

$('<p id="test">My <em>new</em> text</p>').appendTo('body');

When the HTML is more complex than a single tag without attributes, as it is in the above example, the actual creation of the elements is handled by the browser's innerHTML mechanism. Specifically, jQuery creates a new <div> element and sets the innerHTML property of the element to the HTML snippet that was passed in. When the parameter has a single tag, such as $('<img />') or $('<a></a>'), jQuery creates the element using the native JavaScript createElement() function.

When passing in complex HTML, some browsers may not generate a DOM that exactly replicates the HTML source provided. For example, Internet Explorer prior to version 8 will convert all href properties on links to absolute URLs, and Internet Explorer prior to version 9 will not correctly handle HTML5 elements without the addition of a separate compatibility layer.

To ensure cross-platform compatibility, the snippet must be well-formed. Tags that can contain other elements should be paired with a closing tag: 

$('<a href="http://jquery.com"></a>');

Alternatively, jQuery allows XML-like tag syntax (with or without a space before the slash):

$('<a/>');

Tags that cannot contain elements may be quick-closed or not:

$('<img />');
$('<input>');

As of jQuery 1.4, we can pass a map of properties to the second argument. This argument accepts a superset of properties that can be passed to the .attr() method. Furthermore, any event type can be passed in, and the following jQuery methods can be called: val, css, html, text, data, width, height, or offset. Note that Internet Explorer will not allow you to create an input element and change its type; you must specify the type using '<input type="checkbox" />' for example.

示例:

Creates a div element (and all of its contents) dynamically, and appends it to the body element. Internally, an element is created and its innerHTML property set to the given markup. It is therefore both quite flexible and limited.

jQuery 代码:
$("<div><p>Hello</p></div>").appendTo("body")

示例:

Create some DOM elements.

jQuery 代码:
$("<div/>", {
  "class": "test",
  text: "Click me!",
  click: function(){
    $(this).toggleClass("test");
  }
}).appendTo("body");

$("<input>", {
  type: "text",
  val: "Test",
  focusin: function() {
    $(this).addClass("active");
  },
  focusout: function() {
    $(this).removeClass("active");
  }
}).appendTo("form");

返回值:jQueryjQuery(callback)

Binds a function to be executed when the DOM has finished loading.

This function behaves just like $(document).ready(), in that it should be used to wrap other $() operations on your page that depend on the DOM being ready. While this function is, technically, chainable, there really isn't much use for chaining against it.

示例:

Executes the function when the DOM is ready to be used.

jQuery 代码:
$(function(){
  // Document is ready
});

示例:

Uses both the shortcut for $(document).ready() and the argument to write failsafe jQuery code using the $ alias, without relying on the global alias.

jQuery 代码:
jQuery(function($) {
  // Your code using failsafe $ alias here...
});