Why my code does not work?

I am learning Ext JS 4 from http://ift.tt/1mt448d

W

hen I tried to run the following code from http://ift.tt/TyzL4z

Code:

Ext.create('Ext.container.Viewport', {


        id: 'SchoolViewport',


        layout: {
            type: 'border'
        },
        items: [
        {
            region: 'north',
            margins: '0 0 5 0',
            title:'Header (north region)',
            height: 100,
            html:'Header information goes here'
        }, {
            region: 'west',
            title: 'Left bar (west region)',
            width: 150,
            html:'Navigation goes here'
        }, {
            region: 'south',
            title: 'Footer (south region)',
            html: 'Footer Information goes here',
            height: 100
                
        }, {
            region: 'east',
            title: 'Right bar (East region)',
            width: 150,
            html:'Important information goes here'
        }, {
            region: 'center',
            title:'center content area (center region)',
            html:'Content area'
        }]
    })

My browser does not show any thing.

As I have run other code in the tutorial thus I don't think it's from my constructing the application. I am wondering whether anyone else has my problem. Thanks!