用js来实现页面全屏和退出全屏的功能

function openFullscreen(elem) {
  if (elem.requestFullscreen) {
    elem.requestFullscreen();
  } else if (elem.mozRequestFullScreen) { /* Firefox */
    elem.mozRequestFullScreen();
  } else if (elem.webkitRequestFullscreen) { /* Chrome, Safari 和 Opera */
    elem.webkitRequestFullscreen();
  } else if (elem.msRequestFullscreen) { /* IE/Edge */
    elem.msRequestFullscreen();
  }
}
function closeFullscreen() {
  if (document.exitFullscreen) {
    document.exitFullscreen();
  } else if (document.mozCancelFullScreen) { /* Firefox */
    document.mozCancelFullScreen();
  } else if (document.webkitExitFullscreen) { /* Chrome, Safari and Opera */
    document.webkitExitFullscreen();
  } else if (document.msExitFullscreen) { /* IE/Edge */
    document.msExitFullscreen();
  }
}

let elem = document.documentElement; /* 全屏整个页面 */
openFullscreen(elem);