如何检测浏览器支持requestFullscreen

如何检测浏览器支持requestFullscreen or not?

我有下面这些代码可以使 chrome、safari、firefox 和 opera（不完全工作）使文档全屏显示，但我想检测浏览器支持requestFullscreen或不。我应该怎么办？

 $('.button-fullscreen').click(function(){ 

    var docElm = document.documentElement;

    // W3C Proposal
    if (docElm.requestFullscreen) {
        docElm.requestFullscreen();
    }

    // mozilla proposal
    else if (docElm.mozRequestFullScreen) {
        docElm.mozRequestFullScreen();          
    }

    // Webkit (works in Safari and Chrome Canary)
    else if (docElm.webkitRequestFullScreen) {
        docElm.webkitRequestFullScreen(); 
    }

    return false;
});

$('.button-sound').click(function(){               

    // W3C Proposal
    if (document.exitFullscreen) {
        document.exitFullscreen();
    }

    // mozilla proposal
    else if (document.mozCancelFullScreen) {
        document.mozCancelFullScreen();
    }

    // Webkit (works in Safari and Chrome Canary)
    else if (document.webkitCancelFullScreen) {
        document.webkitCancelFullScreen();
    }

    return false;
});

当你把if (docElm.requestFullscreen)，您正在检测浏览器是否支持该方法，因为如果requestFullscreen定义在docElmobject.