Ajax Autocomplete for Jquery : How To Send Dynamic Parameters

Alternatively, you can specify the params using a function which is evaluated just before the ajax request is sent.

$('#q').autocomplete({
    ...
    params: {
        'entity_type': function() {
            return $('#top_search_select').val();
        }
    }
    ...
});

Might be an old question, but I feel that here's the best way to do it:

$('#q').autocomplete({
    ...
    onSearchStart: function(q) {
        q.entity_type = $('#top_search_select').val();
    }
    ...
});