js弹出框（第二次及以后不会弹）

<style>
      * {
        padding: 0;
        margin: 0;
      }
      #banner {
        width: 100%;
        height: 100vh;
        background-color: gray;
        position: relative;
      }
      #alert {
        width: 55%;
        height: 800px;
        background-color: aliceblue;
        position: absolute;
        left: 24%;
        top: 20vh;
        display: flex;
        align-items: center;
      }
      #alert img {
        top: -120px;
        left: 490px;
        height: 100px;
        position: absolute;
      }
      #alert p {
        font-size: 80px;
        width: 100%;
        text-align: center;
      }
      #k{
        width: 50%;
        height: 400px;
        margin-left: 25%;
        margin-top: 40vh;
      }
      #k p{
        font-size: 50px;
        text-align: center;
      }
    </style>

<div id="banner">
      <div id="alert">
        <p>等一下</p>
        <i onclick="fn()"><img src="./img/错误提示.svg" alt=""></i>
      </div>
    </div>
    <div id="k">
      <p>恭喜发财</p>
    </div>

<script>
             //获取到id
      let a = document.getElementById('alert');
      let b = document.getElementById('banner');
            //点击切换display
      function fn() {
        sessionStorage.setItem('a', true);
        a,b.style.display = 'none'
      }
            //向临时储存里存一个值
      let data = sessionStorage.getItem('a')
            //判断是否有值
      if (data != null) {
        a,b.style.display = 'none'
      }
    </script>