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cars=new Array("Mercedes","Volvo","BMW","porche");
phones=new Array('Samsung','Nokia','Iphone');

populateSelect();

$(function() {

      $('#cat').change(function(){
        populateSelect();
    });
    
});


function populateSelect(){
    cat=$('#cat').val();
    $('#item').html('');
    
    
    if(cat=='car'){
        cars.forEach(function(t) { 
            $('#item').append('<option>'+t+'</option>');
        });
    }
    
    if(cat=='phone'){
        phones.forEach(function(t) {
            $('#item').append('<option>'+t+'</option>');
        });
    }
    
} 

<select id="cat">
    <option val="car">car</option>
    <option val="phone">phone</option>
</select>

<select id="item">

</select>