code sample to determine if the current document library is being used with new experience or not

sharepointlibrarynewexperience.js

function getIsLibraryNewExperience(){
  var keyPairs = document.cookie.split('; ');
  for(var kpIdx in keyPairs){
      var parts = keyPairs[kpIdx].split('=');
      if(parts[0] === 'splnu' && parts[1] === '1'){
          return true;
      }
  }
  return false;
}