How to set the visibility is true of LinkButton using JavaScript

var lbnDemo =

 document.getElementById('ctl00_ContentPlaceHolder_ lbnDemo ')

                       (or)

var lbnDemo=

 document.getElementById(' lbnDemo.ClientID ')

     

         if ( lbnDemo != null) {

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

                     (or)

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

         

         }