用js控制进入全屏，退出全屏

需求是这样的 ，有一个图标元素，点击后进入全屏，再次点击退出全屏。
实现代码：

<li id="fullscreen"><a href="#" ><span class="glyphicon glyphicon-resize-full "></span></a></li>

 //控制全屏    function enterfullscreen() {//进入全屏        var docElm = document.documentElement;//W3C        if (docElm.requestFullscreen) {            docElm.requestFullscreen();        }//FireFox        else if (docElm.mozRequestFullScreen) {            docElm.mozRequestFullScreen();        }//Chrome等        else if (docElm.webkitRequestFullScreen) {            docElm.webkitRequestFullScreen();        }//IE11        else if (elem.msRequestFullscreen) {            elem.msRequestFullscreen();        }    }    function exitfullscreen() { //退出全屏        if (document.exitFullscreen) {            document.exitFullscreen();        }        else if (document.mozCancelFullScreen) {            document.mozCancelFullScreen();        }        else if (document.webkitCancelFullScreen) {            document.webkitCancelFullScreen();        }        else if (document.msExitFullscreen) {            document.msExitFullscreen();        }    }    var a=0;    $('#fullscreen').on('click',function () {        a++;        a%2==1?enterfullscreen():exitfullscreen();    })