纯js实现打字机效果

<h2 id="text-box"></h2>
<!--一行也是代码-->
CSS

        h2 {
   width: 800px;
   line-height: 40px;
   border-bottom: 1px solid;
   margin: 200px auto;
   font-size: 24px;
  }
  
  .animate {
   display: inline-block;
   padding: 0 5px;
   vertical-align: 3px;
   font-size: 20px;
   font-weight: nORMal;
  }
  
  .animate.on {
   animation: fade 1.5s infinite forwards;
  }
  
  @keyframes fade {
   from {
    opacity: 0;
   }
   to {
    opacity: 1;
   }
  }

let textBox = $('#text-box');
let index = 0;
let str = 'Welcome to my WEBsite!';
 
  let len = str.length;
 
  function input() {
 
   textBox.html(str.substr(0, index) + '<span class="animate">|</span>');
 
   setTimeout(function() {
    index++;
 
    if(index === len + 1) {
     $('.animate').addClass('on');
     return;
    }
 
    input();
 
   }, Math.random() * 600)
 
   console.log(index);
  }
 
  input();