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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .content {
      height: 200px;
    }
    .one {
      background-color: red;
    }
    .two {
      background-color: blue;
    }
    .there {
      background-color: black;
    }
    .four {
      background-color: cadetblue;
    }
  </style>
</head>
<body>
  <div class="main">
    <div class="content one"></div>
    <div class="content two"></div>
    <div class="content there"></div>
    <div class="content four"></div>
  </div>
</body>
<script>
  window.addEventListener('scroll', function() {
    const clientHeight = document.documentElement.clientHeight;
    const scrollTop = document.documentElement.scrollTop;
    const scrollHeight = document.documentElement.scrollHeight;
    if (clientHeight + scrollTop >= scrollHeight) {
      const eleOne = document.createElement('div');
      eleOne.setAttribute('class', 'content one');
      const eleTwo = document.createElement('div');
      eleTwo.setAttribute('class', 'content two');
      const eleThere = document.createElement('div');
      eleThere.setAttribute('class', 'content there');
      const eleFour = document.createElement('div');
      eleFour.setAttribute('class', 'content four');

      const fragment = document.createDocumentFragment();
      fragment.append(eleOne, eleTwo, eleThere, eleFour);
      document.querySelector('.main').appendChild(fragment);
    }
  }, false);
</script>
</html>
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