toFullScreen：全屏------exitFullscreen：退出全屏

toFullScreen：全屏

function toFullScreen(){    
　　let elem = document.body;    
　　elem.webkitRequestFullScreen    
　　? elem.webkitRequestFullScreen()    
　　: elem.mozRequestFullScreen    
　　? elem.mozRequestFullScreen()    
　　: elem.msRequestFullscreen    
　　? elem.msRequestFullscreen()    
　　: elem.requestFullScreen    
　　? elem.requestFullScreen()    
　　: alert("浏览器不支持全屏");
}

exitFullscreen：退出全屏

function exitFullscreen(){    
　　let elem = parent.document;    
　　elem.webkitCancelFullScreen    
　　? elem.webkitCancelFullScreen()    
　　: elem.mozCancelFullScreen    
　　? elem.mozCancelFullScreen()    
　　: elem.cancelFullScreen    
　　? elem.cancelFullScreen()    
　　: elem.msExitFullscreen    
　　? elem.msExitFullscreen()    
　　: elem.exitFullscreen    
　　? elem.exitFullscreen()    
　　: alert("切换失败,可尝试Esc退出");
}