I have the following code:

function header(){
 experience += '';
 var expimage = '';
 for(var cik=0;cik<experience.length;cik++){
  switch(experience[cik]){
   case '0':
   expimage += 'img0';
   break;
   case '1':
   expimage += 'img1';
      break;
      case '2':
      expimage += 'img2';
      break;
      case '3':
      expimage += 'img3';
      break;
      case '4':
      expimage += 'img4';
      break;
      case '5':
      expimage += 'img5';
      break;
      case '6':
      expimage += 'img6';
      break;
      case '7':
      expimage += 'img7';
      break;
      case '8':
      expimage += 'img8';
      break;
      case '9':
      expimage += 'img9';
      break;
  }
 }
 document.getElementById('level').innerHTML = expimage;
 alert(expimage);
}

But it only work on chrome or mozilla. It shows up an empty alert box, but it work on firefox and chrome.