text: // var listDiv=document.getElementsByTagName('div');
  // console.log(listDiv[1]);
  // var listH1=document.getElementsByTagName("h1");
  // console.log(listH1[0].innerHTML);
  // for (let index = 0; index < listH1.length; index++) {
  //     listH1[index].innerHTML="ABCD";
  // }
  var listTitle=document.getElementsByTagName('h4');
  for (let i = 0; i < listTitle.length; i++) {
     listTitle[i].innerHTML="MRAMRA";
  }
  var card=document.getElementById('card');
  console.log(card);
  var h4=document.getElementById('h4');
  console.log(h4);
  var cardText=document.getElementsByClassName('card-text');
  console.log(cardText);
  for (let i = 0; i < cardText.length; i++) {
      cardText[i].innerHTML='MRA Study JS';
  }
  var listGroup=document.querySelector('.list-group-item');//tra ve phan tu dau tien
  var listGroupAll=document.querySelectorAll('.list-group-item');
  console.log(listGroup);
  console.log(listGroupAll);
  
  document.addEventListener("DOMContentLoaded",function(){
      var b1=document.getElementById('b1');
      console.log(b1);
      var cardText=document.getElementsByClassName('card-text');
      b1.onclick=function(){
          console.log("you are click "+b1);
          b1.classList.add('down');
          for (let i = 0; i < cardText.length; i++) {
              cardText[i].classList.add('disappear');
              
          }
      }
  },false)