How to set the visibility is true of Button using JavaScript

var btnDemo =

 document.getElementById('ctl00_ContentPlaceHolder_ btnDemo ')

                       (or)

var btnDemo =

 document.getElementById('btnDemo.ClientID ')

    

         if ( btnDemo != null) {

             document.getElementById('ctl00_ContentPlaceHolder_ btnDemo  ').style.display = 'block'

                     (or)

  document.getElementById('btnDemo.ClientID ') .style.display = 'block'

         

         }