Combobox works when I move my store

[UbuntuPenguin]

Hello Everyone,
I had a custom combobox class that used a store. I declared my store like so

        ds = Ext.create('Ext.data.Store', {
            model: 'KesslerAdmin.model.LineItemType',
            proxy: {
                type:'ajax',
                url:'line_item_type/search_by_product_type.json',
                reader:{
                    type:'json'
                }
            }
        });
      Ext.define('KesslerAdmin.view.AwesomeCombobox', {

This worked fine until I had another instance declared in another view by xtype.

I was confounded for days wondering how my code broke overnight. Then I changed my combobox and moved my store into the initComponent declaration.

...
initComponent:function () {
        var me = this;
        var ds = Ext.create('Ext.data.Store', {
            model: 'KesslerAdmin.model.LineItemType',
            proxy: {
                type:'ajax',
                url:'line_item_type/search_by_product_type.json',
                reader:{
                    type:'json'
                }
            }
        });
        me.store = ds,
        Ext.applyIf(me, {
...

After that my combobox which was declared twice by xtype in my application worked like a charm. My question is, what concept am I missing. Why would declaring a component by its xtype break the combobox. By the way, if I remember correctly, the error was 'Object <object> has no method read.

[evant]

When you put it in the definition, it gets shared across instances. For "primitive" types (strings, numbers, bools) it isn't a problem, however for objects it becomes apparent:

Ext.define('Foo', {
    bar: []
});

var a = new Foo();
a.bar.push(1);
var b = new Foo();
console.log(b.bar[0]);

[UbuntuPenguin]

Thanks Evan. Just one more question, do you know why having the 'store' shared across instances would cause the 'Object <object> has no read method' error ?