When you use slice and create a new array, the newly created array is no longer a reference to the original array, so the playlists parameter in update in no longer referring to the same array instance as the playlists variable outside the function. Try this:

playlists = [];
results = simpleMysqlQuery();
setinterval{
    playlists = update(playlists, results);
}

function update(playlists, results){
    if(playlists.length != results.length){
        playlists = reWritePlaylists(results, playlists);
    }
    return playlists
}

function reWritePlaylists(results, playlists){
   results.forEach(function(item, i, arr){
      playlists[i] = new Object();
      playlists[i]['id'] = results[i]['id'];
      playlists[i]['name'] = results[i]['name'];
   });
   if(playlists.length > results.length){
      playlists = playlists.slice(0, results.length);
   }
   return playlists;
}

Please update reWritePlaylists function like below:

function reWritePlaylists(results, playlists){
   playlists.splice(0, results.length);
   results.forEach(function(item, i, arr){
      playlists[i] = new Object();
      playlists[i]['id'] = results[i]['id'];
      playlists[i]['name'] = results[i]['name'];
   });
   return playlists;
}

I found de wae!

function reWritePlaylists(results, playlists){
   playlists.splice(0, playlists.length);
   results.forEach(function(item, i, arr){
      playlists[i] = new Object();
      playlists[i]['id'] = results[i]['id'];
      playlists[i]['name'] = results[i]['name'];
   });
   return playlists;
}