A jQuery object contains a collection of Document Object Model (DOM) elements that have been created from an HTML string or selected from a document. Since jQuery methods often use CSS selectors to match elements from a document, the set of elements in a jQuery object is often called a set of "matched elements" or "selected elements".
The jQuery object itself behaves much like an array; it has a
length property and the elements in the object can be accessed by their numeric indices [0] to [length-1]. Note that a jQuery object is not actually a Javascript Array object, so it does not have all the methods of a true Array object such as join().
Most frequently, you will use the jQuery() function to create a jQuery object. jQuery() can also be accessed by its familiar single-character alias of $(), unless you have called jQuery.noConflict() to disable this option. Many jQuery methods return the jQuery object itself, so that method calls can be chained:
Whenever you use a "destructive" jQuery method that potentially changes the set of elements in the jQuery object, such as .filter() or .find(), that method actually returns a new jQuery object with the resulting elements. To return to the previous jQuery object, you use the .end() method.
A jQuery object may be empty, containing no DOM elements. You can create an empty jQuery object with $() (that is, passing no arguments at all). A jQuery object may also be empty if a selector doesn't select any elements, or if a chained method filters out all the elements. It is not an error; any further methods called on that jQuery object simply have no effect since they have no elements to act upon. So, in this example if there are no bad entries on the page then no elements will be colored red:
$(".badEntry").css({color: "red"})
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