原生js 首次加载的loading效果

 <div id="loader-wrapper">
        <div id="loader"></div>
        <div class="loader-section section-left"></div>
        <div class="loader-section section-right"></div>
        <div class="load_title">正在加载课题组官网,请稍等……
        </div>
      </div>

 <style>
      html,
      body,
      #app {
        height: 100%;
        margin: 0px;
        padding: 0px;
      }
      .chromeframe {
        margin: 0.2em 0;
        background: #ccc;
        color: #000;
        padding: 0.2em 0;
      }
      #loader-wrapper {
        position: fixed;
        top: 0;
        left: 0;
         100%;
        height: 100%;
        z-index: 999999;
      }
      #loader {
        display: block;
        position: relative;
        left: 50%;
        top: 50%;
         150px;
        height: 150px;
        margin: -75px 0 0 -75px;
        border-radius: 50%;
        border: 3px solid transparent;
        /* COLOR 1 */
        border-top-color: #FFF;
        -webkit-animation: spin 2s linear infinite;
        /* Chrome, Opera 15+, Safari 5+ */
        -ms-animation: spin 2s linear infinite;
        /* Chrome, Opera 15+, Safari 5+ */
        -moz-animation: spin 2s linear infinite;
        /* Chrome, Opera 15+, Safari 5+ */
        -o-animation: spin 2s linear infinite;
        /* Chrome, Opera 15+, Safari 5+ */
        animation: spin 2s linear infinite;
        /* Chrome, Firefox 16+, IE 10+, Opera */
        z-index: 1001;
      }
      #loader:before {
        content: "";
        position: absolute;
        top: 5px;
        left: 5px;
        right: 5px;
        bottom: 5px;
        border-radius: 50%;
        border: 3px solid transparent;
        /* COLOR 2 */
        border-top-color: #FFF;
        -webkit-animation: spin 3s linear infinite;
        /* Chrome, Opera 15+, Safari 5+ */
        -moz-animation: spin 3s linear infinite;
        /* Chrome, Opera 15+, Safari 5+ */
        -o-animation: spin 3s linear infinite;
        /* Chrome, Opera 15+, Safari 5+ */
        -ms-animation: spin 3s linear infinite;
        /* Chrome, Opera 15+, Safari 5+ */
        animation: spin 3s linear infinite;
        /* Chrome, Firefox 16+, IE 10+, Opera */
      }
      #loader:after {
        content: "";
        position: absolute;
        top: 15px;
        left: 15px;
        right: 15px;
        bottom: 15px;
        border-radius: 50%;
        border: 3px solid transparent;
        border-top-color: #FFF;
        /* COLOR 3 */
        -moz-animation: spin 1.5s linear infinite;
        /* Chrome, Opera 15+, Safari 5+ */
        -o-animation: spin 1.5s linear infinite;
        /* Chrome, Opera 15+, Safari 5+ */
        -ms-animation: spin 1.5s linear infinite;
        /* Chrome, Opera 15+, Safari 5+ */
        -webkit-animation: spin 1.5s linear infinite;
        /* Chrome, Opera 15+, Safari 5+ */
        animation: spin 1.5s linear infinite;
        /* Chrome, Firefox 16+, IE 10+, Opera */
      }
      @-webkit-keyframes spin {
        0% {
          -webkit-transform: rotate(0deg);
          /* Chrome, Opera 15+, Safari 3.1+ */
          -ms-transform: rotate(0deg);
          /* IE 9 */
          transform: rotate(0deg);
          /* Firefox 16+, IE 10+, Opera */
        }
        100% {
          -webkit-transform: rotate(360deg);
          /* Chrome, Opera 15+, Safari 3.1+ */
          -ms-transform: rotate(360deg);
          /* IE 9 */
          transform: rotate(360deg);
          /* Firefox 16+, IE 10+, Opera */
        }
      }
      @keyframes spin {
        0% {
          -webkit-transform: rotate(0deg);
          /* Chrome, Opera 15+, Safari 3.1+ */
          -ms-transform: rotate(0deg);
          /* IE 9 */
          transform: rotate(0deg);
          /* Firefox 16+, IE 10+, Opera */
        }
        100% {
          -webkit-transform: rotate(360deg);
          /* Chrome, Opera 15+, Safari 3.1+ */
          -ms-transform: rotate(360deg);
          /* IE 9 */
          transform: rotate(360deg);
          /* Firefox 16+, IE 10+, Opera */
        }
      }
      #loader-wrapper .loader-section {
        position: fixed;
        top: 0;
         51%;
        height: 100%;
        background: #199eb8;
        /* Old browsers */
        z-index: 1000;
        -webkit-transform: translateX(0);
        /* Chrome, Opera 15+, Safari 3.1+ */
        -ms-transform: translateX(0);
        /* IE 9 */
        transform: translateX(0);
        /* Firefox 16+, IE 10+, Opera */
      }
      #loader-wrapper .loader-section.section-left {
        left: 0;
      }
      #loader-wrapper .loader-section.section-right {
        right: 0;
      }
      /* Loaded */
      .loaded #loader-wrapper .loader-section.section-left {
        -webkit-transform: translateX(-100%);
        /* Chrome, Opera 15+, Safari 3.1+ */
        -ms-transform: translateX(-100%);
        /* IE 9 */
        transform: translateX(-100%);
        /* Firefox 16+, IE 10+, Opera */
        -webkit-transition: all 0.7s 0.3s cubic-bezier(0.645, 0.045, 0.355, 1.000);
        transition: all 0.7s 0.3s cubic-bezier(0.645, 0.045, 0.355, 1.000);
      }
      .loaded #loader-wrapper .loader-section.section-right {
        -webkit-transform: translateX(100%);
        /* Chrome, Opera 15+, Safari 3.1+ */
        -ms-transform: translateX(100%);
        /* IE 9 */
        transform: translateX(100%);
        /* Firefox 16+, IE 10+, Opera */
        -webkit-transition: all 0.7s 0.3s cubic-bezier(0.645, 0.045, 0.355, 1.000);
        transition: all 0.7s 0.3s cubic-bezier(0.645, 0.045, 0.355, 1.000);
      }
      .loaded #loader {
        opacity: 0;
        -webkit-transition: all 0.3s ease-out;
        transition: all 0.3s ease-out;
      }
      .loaded #loader-wrapper {
        visibility: hidden;
        -webkit-transform: translateY(-100%);
        /* Chrome, Opera 15+, Safari 3.1+ */
        -ms-transform: translateY(-100%);
        /* IE 9 */
        transform: translateY(-100%);
        /* Firefox 16+, IE 10+, Opera */
        -webkit-transition: all 0.3s 1s ease-out;
        transition: all 0.3s 1s ease-out;
      }
      /* JavaScript Turned Off */
      .no-js #loader-wrapper {
        display: none;
      }
      .no-js h1 {
        color: #222222;
      }
      #loader-wrapper .load_title {
        font-family: 'Open Sans';
        color: #FFF;
        font-size: 19px;
         100%;
        text-align: center;
        z-index: 9999999999999;
        position: absolute;
        top: 70%;
        opacity: 1;
        line-height: 30px;
      }
      #loader-wrapper .load_title span {
        font-weight: normal;
        font-style: italic;
        font-size: 13px;
        color: #FFF;
        opacity: 0.5;
      }
    </style>

 <script type="text/javascript">
 window.onload = function(){
  // alert('Hello World!');
  let loader_wrapper = document.querySelector('#loader-wrapper');
  loader_wrapper.style.display = 'none';
};
  </script>

window.onload 初始化的时候等于null 参考：
https://www.cnblogs.com/7qin/p/9960987.html
https://baijiahao.baidu.com/s?id=1613225567743061589&wfr=spider&for=pc