Add value to hidden form field onclick

HTML

<form action="" name="testform" method="post" id="testform">
    <input type="text" value="" name="a[]" class='input_text'>
    <input type="text" value="" name="b[]" class='input_text'>
    <input type="text" value="" name="c[]" class='input_text'>
    <select data-placeholder="Add a value to the letter">
        <option value="" data-value=""></option>
        <option value="1" data-value="11,12,13">1</option>
        <option value="2" data-value="21,22,23">2</option>
        <option value="3" data-value="31,32,33">3</option>
        <option value="4" data-value="41,42,43">4</option>
        <option value="5" data-value="51,52,53">5</option>
    </select>     
    <input type="text" id="gg" class="input_text">
</form>

JavaScript

$(function() {
    $('select').on('change', function() {
        var select_val = $(this).find('option:selected').data('value');
        if( select_val != '' ) {
        	var arr = [];
          arr.push(1);
        	arr.push(2);
            var a = select_val.split(',');
            $('input[name="a[]"]').val(a[0]);
            $('input[name="b[]"]').val(a[1]);
            $('input[name="c[]"]').val(a[2]);
        } else {
            $('input.input_text').val('');
        }
        $('input[name="b[]"]').val(arr);
        var result = $('input[name="b[]"]').val();
        result.forEach(function(tee){
           alert(tee);
        });
        alert($('input[name="b[]"]').val());
    });
});