position sticky的兼容

  let elem = window.document.documentElement;
  let w = elem.clientWidth;
  let h2 = (w/16*9);//使用sticky定位的顶端是一个16比9的div
  function isSupportSticky() {//判断是否支持sticky
    var prefixTestList = [‘‘, ‘-webkit-‘];
    var stickyText = ‘‘;
    for (var i = 0; i < prefixTestList.length; i++ ) {
      stickyText += ‘position:‘ + prefixTestList[i] + ‘sticky;‘;
    }
    // 创建一个dom来检查
    var div = document.createElement(‘div‘);
    div.style.cssText = stickyText;
    document.body.appendChild(div);
    var isSupport = /sticky/i.test(window.getComputedStyle(div).position);
    document.body.removeChild(div);
    div = null;
    return isSupport;
  }
  let fixBottomBar = function(){
    var  topbarTop = h2;
    window.onscroll = function(e){
      let winTop = document.body.scrollTop;
      if(winTop > topbarTop){
          _this.navClass[‘nav-fixed‘] = true;  //vue动态添加class写法
//            topbar.setAttribute(‘class‘,‘nav nav-fixed‘);  //原生js写法
      }else{
        _this.navClass[‘nav-fixed‘] = false;
//            topbar.setAttribute(‘class‘,‘nav‘);
      }
    }

  }
  !isSupportSticky() && fixBottomBar();