if (document.fullscreenEnabled || document.webkitFullscreenEnabled) {
  // 全屏功能可用
} else {
  alert("当前浏览器不支持全屏功能");
}

function enterFullscreen() {
  const elem = document.documentElement; // 获取页面根元素
  if (elem.requestFullscreen) {
    elem.requestFullscreen();
  } else if (elem.webkitRequestFullscreen) { // Safari 等旧版浏览器
    elem.webkitRequestFullscreen();
  } else if (elem.msRequestFullscreen) { // IE/Edge
    elem.msRequestFullscreen();
  }
}

function exitFullscreen() {
  if (document.exitFullscreen) {
    document.exitFullscreen();
  } else if (document.webkitExitFullscreen) {
    document.webkitExitFullscreen();
  } else if (document.msExitFullscreen) {
    document.msExitFullscreen();
  }
}

function toggleFullscreen() {
  if (!document.fullscreenElement && 
      !document.webkitFullscreenElement && 
      !document.msFullscreenElement) {
    // 当前不是全屏，进入全屏
    enterFullscreen();
  } else {
    // 当前是全屏，退出全屏
    exitFullscreen();
  }
}

<button onclick="toggleFullscreen()">切换全屏</button>