When/why to prefix variables with "$" when using jQuery?

> Possible Duplicate:
> Why would a javascript variable start with a dollar sign?

I see people using the dollar sign in front of variables when using jQuery. Is there any reason behind this? I'm I missing something basic or is it just a common practice?

It's a common reference to a jQuery wrapped object. It makes reading the code easier to know which variables are jQuery wrapped.

//Item has been "cached" for later use in the script as a jQuery object.
var $item = $(this);

For me a common practice is this:

If a variable is private I use an underscore like this:

(function(){
var _foo = "bar";
})()

If it's public I'll use no underscore:

var foo = "bar"

And if it's a jQuery selector I'll use the $:

var $foo = $('bar');
//then you can access it like this
$foo.attr('id')

It's just coding convention and it allows you to quickly reference what type the variable is later in the code.

Many people using jQuery will prefix variables that contain a jQuery object with a $, so that they are easily identified. Consider this example:

var $img = $(".someclass span.otherclass img");
/* somewhere later in the code */
$img.bind("click", function() {/*...*/});

In my experience this is just a readability. Some developers like to prefix their variables so that they are easy to spot. It could also be a PHP habit creeping it's way in to Javascript.

Dollar signs in code that uses JQuery commonly means that the variable in question is a jQuery variable (an object wrapped by jquery).