An extension is a feature that is not part of the tablefilter core bundle. It is loaded asynchronously from a separate bundle but only if required by the configuration. The extensions are now stored in the extensions registry. To get the instance of an extension you can use the extension('name') method:

var tf = new TableFilter('tableId', {
    extensions: [{ name: 'sort' }]
});
var sort = tf.extension('sort');