jQuery jQuery()

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" );

If no elements match the provided selector, the new jQuery object is "empty"; that is, it contains no elements and has .length property of 0.

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, to do a search within an event handler, the search can be restricted like so:

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

When the search for the span selector is restricted 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 create a jQuery object using one or more DOM elements that were already selected in some other way. A jQuery object is created from the array elements in the order they appeared in the array; unlike most other multi-element jQuery operations, the elements are not sorted in DOM order. Elements will be copied from the array as-is and won’t be unwrapped if they’re already jQuery collections.

Please note that although you can pass text nodes and comment nodes into a jQuery collection this way, most operations don’t support them. The few that do will have an explicit note on their API documentation page.

A common use of single-DOM-element construction 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 passed to the $() function before applying jQuery methods to it.

XML data returned from an Ajax call can be passed to the $() function so individual elements of the XML structure can be retrieved using .find() and other DOM traversal methods.

$.post( "url.xml", function( data ) {
 var $child = $( data ).find( "child" );
});

Cloning jQuery Objects

When a jQuery object is passed 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 (with a .length property of 0). In previous versions of jQuery, this would return a set containing the document node.

Working With Plain Objects

At present, the only operations supported on plain JavaScript objects wrapped in jQuery are: .data(),.prop(),.on(), .off(), .trigger() and .triggerHandler(). The use of .data() (or any method requiring .data()) on a plain object will result in a new property on the object called jQuery{randomNumber} (eg. jQuery123456789).

// Define a plain object
var foo = { foo: "bar", hello: "world" };
 
// Pass it to the jQuery function
var $foo = $( foo );
 
// Test accessing property values
var test1 = $foo.prop( "foo" ); // bar
 
// Test setting property values
$foo.prop( "foo", "foobar" );
var test2 = $foo.prop( "foo" ); // foobar
 
// Test using .data() as summarized above
$foo.data( "keyName", "someValue" );
console.log( $foo ); // will now contain a jQuery{randomNumber} property
 
// Test binding an event name and triggering
$foo.on( "eventName", function () {
 console.log( "eventName was called" );
});
 
$foo.trigger( "eventName" ); // Logs "eventName was called"

Should .trigger( "eventName" ) be used, it will search for an "eventName" property on the object and attempt to execute it after any attached jQuery handlers are executed. It does not check whether the property is a function or not. To avoid this behavior, .triggerHandler( "eventName" ) should be used instead.

$foo.triggerHandler( "eventName" ); // Also logs "eventName was called"