Lazy loading of a tree example

[menasce]

Dear all,
I found on https://fiddle.sencha.com/#fiddle/243u&view/editor a nice example of a lazy loading of nodes in a tree.

Here's the code:

Ext.onReady(
function()
{ 
 Ext.create(
            {
             xtype    : 'treepanel',
             viewModel: {
                         stores: {
                                  pages: {
                                          type            : 'tree',
                                          parentIdProperty: 'pId',
                                          proxy           : {
                                                             type: 'ajax',
                                                             url : '../JSONFiles/pages.json'
                                                            }
                                 }
                         }
             },

             bind       : '{pages}'    ,
             rootVisible: false        ,
             animate    : false        ,
             renderTo   : Ext.getBody()
            });
});

What is confusing me is the syntax of the pages.json file, which is declared to be (in Fiddle) as:

function(params, req, Fiddle) 
{
 var pages = {
              "root": [
                       {
                        "id"      : 1      ,
                        "pId"     : "root" ,
                        "text"    : "One"  ,
                        "expanded": true
                       }
                      ],

              1     : [
                       {
                        "id"      : 2      ,
                        "pId"     : 1      ,
                        "text"    : "Two"  ,
                        "expanded": true
                       }
                      ],

              2     : [
                       {
                        "id"      : 3      ,
                        "pId"     : 2      ,
                        "text"    : "Three",
                        "leaf"    : true
                       }, 
                       {
                        "id"      : 4      ,
                        "pId"     : 2      ,
                        "text"    : "Four" ,
                        "expanded": true
                       }
                      ],

              4     : [
                       {
                        "id"      : 5      ,
                        "pId"     : 4      ,
                        "text"    : "Five" ,
                        "leaf"    : true
                       }
                      ]
            };

 return pages[params.node];
}

Using this file as pages.json I get the following error: "SyntaxError: JSON.parse: unexpected keyword at line 1 column 1 of the JSON data"
What's confusing me is this is NOT supposed, in fact, to be a correct json file syntax.

What is the EXACT format of the pages.json file in order to make the example work correctly?
Thanks.