Populate one dropdown list based on the selection of other dropdown list

WORKING DEMO http://jsfiddle.net/kasperfish/r7MN9/3/ (with jquery)

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>');
        });
    }

} 

UPDATED: using eval() to be able to add as much arrays as you want.
JSFIDDLE DEMO

cars=new Array("Mercedes","Volvo","BMW","porche");
phones=new Array('Samsung','Nokia','Iphone');
names=new Array('Kasper','Elke','Fred','Bobby','Frits');
colors=new Array('blue','green','yellow');

populateSelect();

$(function() {

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

});


function populateSelect(){
    cat=$('#cat').val();
    $('#item').html('');

       eval(cat).forEach(function(t) { 
            $('#item').append('<option>'+t+'</option>');
        });
    }