Question

Console debug shows me that array is ex. [“2”], but I need [2].

Why casting doesnt’work?

function filterElements(deals) {

    var result = deals,
        categories= $('#deals-categories').data('current_category');
        if (categories != undefined && categories.length > 0) {
            for (var i; i < categories.length; i++) {
                categories[i] = parseInt(categories[i]);
            }
            console.log(categories, 'cats');
                result = $.grep(result, function(e) {
                    return $.inArray(e.category_id, categories) != -1;
     });                
        }
    return result;
}

Answer 1

You need to initialize var i = 0 in the loop declaration.

Full code cleanup:

function filterElements(deals) {

    var result = deals,
        categories = $('#deals-categories').data('current_category');

        if (categories && categories.length) {
            for (var i=0; i<categories.length; i++) {
                categories[i] = parseInt(categories[i], 10);
            }
            console.log(categories, 'cats');
            result = $.grep(result, function(e) {
                return $.inArray(e.category_id, categories) !== -1;
            });                
        }

    return result;
}