Does not assign a value to a global variable

Question

I'm trying to set value by Ajax request, and it's not working! This is what I doing:

function getStores()
{
    var stores;

    Ext.Ajax.request({
        url: '/handlers/AccountHandler.php?method=list',
        success: function(response, opts)
        {
            stores = Ext.JSON.decode( response.responseText );
        }
    });

    return stores; // null || undefined
}