jQuery selectors: multiselect vs select

The correct attribute name for a <select> element with multiple selectable options is multiple. You can use the "has attribute" selector to select elements with the multiple attribute, and combine it with :not() to select elements that only allow a single selection.

Therefore, your jQuery selector should be:

$("select#categories[multiple]")  // <select> with multiple 
$("select#categories:not([multiple])")  // <select> with single only 

http://www.w3.org/TR/html401/interact/forms.html#edef-OPTION