Check if an ID exists or not using jQuery [duplicate]

Question

Possible Duplicate:
Finding whether the element exists in whole html page

Is there any way to check for if a ID $('#ID') exists in jQuery

Ex:

$('#cart').append('<li id="456">My Product</li>');

After running append() to something like this I want to check if my ID $('#456') exists or not. If it exits I want to change the text, else I want to append a new

.

$(document).ready(function() {
        $('#RAM,#HDD').change(function() {

            var product = $(this).find("option:selected").attr("product");

            $.ajax({
                url: 'ajax.php',
                type: 'post',
                data: $(this).serialize(),
                dataType: 'json',
                success: function(data) {
                    $('#cart').empty();
                    $.each(data, function(index, value) {
                        $('#cart').append('<li id="'+product+'">'+ value['price'] +'</li>');
                    });
                }
            });
        });
    });

Answer 1

if ($('#456').length)
{
 /* it exists */
}
else
{
 /* it doesn't exist */
}

Answer 2

You can do this to see if the selector does exist or not:

jQuery.fn.exists = function(){return this.length>0;}

if ($(selector).exists()) {
    // Do something
}