check childnodes in tree

HTML

<script src="http://cdn.sencha.com/ext/gpl/4.2.1/ext-all-dev.js"></script>
<link rel="stylesheet" href="http://cdn.sencha.com/ext/gpl/4.2.1/resources/css/ext-all.css">

JavaScript

var store = Ext.create('Ext.data.TreeStore', {
    root: {
        expanded: true,
        children: [
            { text: "detention", leaf: true, checked: false },
            { text: "homework", expanded: true, children: [
                { text: "book report", leaf: true, checked: false },
                { text: "algebra", leaf: true, checked: false}
            ], checked: false },
            { text: "buy lottery tickets", leaf: true, checked: false }
        ]
    }
});

Ext.create('Ext.tree.Panel', {
    title: 'Simple Tree',
    width: 200,
    height: 150,
    store: store,
    rootVisible: false,
    renderTo: Ext.getBody(),
    listeners: {
        checkchange: function( node, checked, eOpts ){
if (checked =='true')
alert('true');
else
alert(checked);
        }
    }
    
});