How to check whether an element exists using jQuery

this is just a little reminder for myself.
thanks to Aaron Russel.

Variant 1:

if ($("#mydiv").length > 0) {  // do something here }

Variant 2:

if ($('#mydiv').length){ // do something here }

Variant 3:

if($('#mydiv').size()) { //do something }

Variant 4:

if ($('#mydiv')[0]) { // do something }

any more ? let me know ;-)