almost_vanilla.js

// see before https://github.com/sergiors/vanilla

(function() {
    'use strict';

    var state = document.getElementById('address_state');

    state.on('change', function() {
        var city = document.getElementById('address_city');
        city.setAttribute('disabled', true);

        if (!state.value) {
            return;
        }

        $.getJSON('http://example.com/cities', {
            state: state.value
        }, function(data) {
            var select = document.createElement('select');
            select.setAttributes(city.getAttributes());

            for (var key in data) {
                var option = document.createElement('option');
                option.value = data[key].id;
                option.text = data[key].name;
                select.options.add(option);
            }

            select.removeAttribute('disabled');
            city.outerHTML = select.outerHTML;
        });
    });
})();