code: function replace (script){
      var parent = script.parentNode
      if (!parent) return
  
      LARGE = /\?large/.test(script.src)
      var iframe = document.createElement('iframe')
      var iframePath = '/iframe' + (LARGE ? '?large' : '')
      iframe.src = script.src.replace(/\/slackin\.js.*/, iframePath)
      iframe.style.borderWidth = 0
      iframe.className = '__slackin'
  
      // a decent aproximation that we adjust later
      // once we have the knowledge of the actual
      // numbers of users, based on a user count
      // of 3 digits by 3 digits
      iframe.style.width = (LARGE ? 190 : 140) + 'px'
  
      // height depends on target size
      iframe.style.height = (LARGE ? 30 : 20) + 'px'
  
      // hidden by default to avoid flicker
      iframe.style.visibility = 'hidden'
  
      parent.insertBefore(iframe, script)
      parent.removeChild(script)
  
      // setup iframe RPC
      iframe.onload = function (){
        setup(iframe)
      }
    }
docs: replace the script tag with an iframe