If you only want the addProduct and deleteProduct methods to feature on that single jQuery object, then what you've got will work fine; but you'll have to keep a reference to that jQuery object/ only use it once, to preserve the existance of the addProduct and deleteProduct methods.

However, these addProduct and deleteProduct methods are unique to that particular jQuery object; the methods won't exist on any other jQuery objects you create;

var productList = $.extend($('ul.products'), {

    addProduct: function(productData) {

        // add a new li item

    },

    deleteProduct: function(productId) {

        // delete li with id

    }

});



// Using this particular jQuery object (productList) will work fine.

productList.addProduct();

productList.removeProduct();



// However, addProduct() does not exist on new jQuery objects, even if they use

// the same selector.

$('ul.products').addProduct(); // error; [object Object] has no method 'addProduct'

The best way do to this would be to go-back-to-basics and define separate addProduct and deleteProduct functions, which accept a jQuery object. If you wanted to restrict these functions to they only worked on the ul.products selector, you could do;

function addProduct(obj) {

    obj = obj.filter('ul.products');



    // do something with `obj` (its a jQuery object)

}

This approach would be recommended as it keeps the jQuery.fn API consistent; otherwise you'd be adding addProduct and removeProduct to some jQuery.fn instances but not others, or making their usage redundant in others. With this approach however addProduct and removeProduct are always there, but don't get in anyones way if they don't want to use them.

Historical Notes

This answer was originally written in November 2011, when jQuery 1.7 was released. Since then the API has changed considerably. The answer above is relevant to the current 2.0.0 version of jQuery.

Prior to 1.9, a little used method called jQuery.sub used to exist, which is related to what you're trying to do (but won't help you unless you change your approach). This creates a new jQuery constructor, so you could do;

var newjQuery = jQuery.sub();

newjQuery.fn.addProduct = function () {

    // blah blah

};

newjQuery.fn.deleteProduct = function () {

    // blah blah

};



var foo = newjQuery('ul.products');

foo.deleteProduct();

foo.addProduct();



var bar = jQuery('ul.products');

bar.deleteProduct(); // error

bar.addProduct(); // error

Be careful though, the $ method alias would reference the old jQuery object, rather than the newjQuery instance.

jQuery.sub was removed from jQuery in 1.9. It is now available as a plugin.