This function generates progressbar html layout.

progressBarGenerator.js

progressBarHtmlGenerator: (function(){
        var width = progressbar_width,
            cls = 'x-progress',  // default class as you can see in ProgressBar.js of Ext.js
            tpl = new Ext.Template(  // this template also was taken from ProgressBar.js
                '<div class="{cls}-wrap">',
                    '<div class="{cls}-inner">',
                        '<div class="{cls}-bar" style="width: {barWidth}px">',
                            '<div class="{cls}-text">',
                                '<div>{text}</div>',
                            '</div>',
                        '</div>',
                        '<div class="{cls}-text {cls}-text-back">',
                            '<div>{textBack}</div>',
                        '</div>',
                    '</div>',
                '</div>'
            );
        return function(value, text, textBack){
            return tpl.apply({
                cls: cls,
                barWidth: value*width || 0,
                text: text || '&#160;',
                textBack: textBack || '&#160;'
            })
        }
    })()