JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://cdn.sencha.com/ext/gpl/4.2.0/resources/css/ext-all.css">

JavaScript

Ext.define('My.view.Contact', {
    extend: 'Ext.window.Window',
    alias: 'widget.mywin',
    title: 'Contact',
    renderTo: Ext.getBody(),
    width: 300,
    height:200,
    items:[
        {
            xtype: 'combobox',
            itemId: 'continent',
            fieldLabel: 'My combo',
            valueField: 'value',
            queryMode: 'local',
            store: new Ext.data.ArrayStore({
                fields: ['value', 'text'],
                data: [
                    [1, 'US'],
                    [2, 'Europe']
                ]
            })
        }
    ],
    buttons: [
        {
        xtype: 'button',
        text: 'Save',
        action: 'save'
        }
    ]
});

Ext.define('My.controller.Contact', {
    extend: 'Ext.app.Controller',
    views: ['Contact'],
    init: function() {
       
        //reference the view
        var view = this.getView('Contact');
        
        //reference the combobox change event
        this.control({
            'mywin combobox': {
                change: this.onChangeContinent
            }
         });
       
    },
    onChangeContinent:function (field, value, options) {
        //here you can get combobox component and his value
        Ext.Msg.alert('Continent', value);
    }
});

Ext.application({
    name: 'My',
    controllers: ['Contact'],
    launch: function() {
        Ext.create('My.view.Contact').show();
    }
});