如何检测浏览器支持requestFullscreen

如何检测浏览器是否支持 requestFullscreen ？

How can I detect a browser supports requestFullscreen or not?

我在下面使用以下代码制作Chrome ，safari，firefox和opera（不完全正常）使文档全屏，但我想检测浏览器是否支持 requestFullscreen 。我该怎么办？

I have these codes below to make chrome, safari, firefox, and opera (not entirely working) to make a document fullscreen, but I want to detect the browser supports requestFullscreen or not. What should I do?

 $('.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;
});