Javascript jQuery dblclick（）事件和Html ondblclick（）事件之间的区别

我正在做一个项目，我需要在双击视频后将其全屏显示。我尝试了两种方法。第一个很好，但第二个不行。我不明白为什么会这样

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第一种方法（使用dblclick jquery可以很好地工作）： js

html

html

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尝试在

.ready

内部进行操作，因为应该首先加载DOM元素。还可以使用

.on（）

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相关事件不同，但概念相同，将解决该问题。因此标记为dupeThe

elem

它来自何处？还请注意，

elem

没有在任何地方定义；这应该是

这个
instead@Pointy看起来是的。
var elem = document.getElementsByClassName("video")[0];
function makefullscreen(){
      if (elem.requestFullscreen) {
        elem.requestFullscreen();
      } else if (elem.mozRequestFullScreen) { /* Firefox */
        elem.mozRequestFullScreen();
      } else if (elem.webkitRequestFullscreen) { /* Chrome, Safari and Opera */
        elem.webkitRequestFullscreen();
      } else if (elem.msRequestFullscreen) { /* IE/Edge */
        elem.msRequestFullscreen();
      }
    });

      <video class="video" autoplay ondblclick="makefullscreen()">
          <source src="http://e14aaeb709f7cde1ae68-a1d0a134a31b545b257b15f8a8ba5726.r70.cf3.rackcdn.com/projects/31432/1427815464209-bf74131a7528d0ea5ce8c0710f530bb5/1280x720.mp4" type="video/mp4" />
      </video>

$(".video").dblclick(function(){
      if (elem.requestFullscreen) {
        elem.requestFullscreen();
      } else if (elem.mozRequestFullScreen) { /* Firefox */
        elem.mozRequestFullScreen();
      } else if (elem.webkitRequestFullscreen) { /* Chrome, Safari and Opera */
        elem.webkitRequestFullscreen();
      } else if (elem.msRequestFullscreen) { /* IE/Edge */
        elem.msRequestFullscreen();
      }
    });

        <video class="video" autoplay>
          <source src="http://e14aaeb709f7cde1ae68-a1d0a134a31b545b257b15f8a8ba5726.r70.cf3.rackcdn.com/projects/31432/1427815464209-bf74131a7528d0ea5ce8c0710f530bb5/1280x720.mp4" type="video/mp4" />
        </video>

$(document).ready(function(){
$(".video").on("dblclick",function(){

    if (this.requestFullscreen) {
        this.requestFullscreen();
      } else if (this.mozRequestFullScreen) { /* Firefox */
        this.mozRequestFullScreen();
      } else if (this.webkitRequestFullscreen) { /* Chrome, Safari and Opera */
        this.webkitRequestFullscreen();
      } else if (this.msRequestFullscreen) { /* IE/Edge */
        this.msRequestFullscreen();
      }
    });

});

});