Modernizr和Video.h264

Question

I am looking at how Modernizr detects support for Video and H.264 , but it does not make any sense to me. Isn't bool a primitive boolean? why does it become a Boolean object? Why does bool.h264 magically start making any sense at all? thanks

tests['video'] = function() {
        var elem = document.createElement('video'),
            bool = false;

        // IE9 Running on Windows Server SKU can cause an exception to be thrown, bug #224
        try {
            if ( bool = !!elem.canPlayType ) {
                bool      = new Boolean(bool);
                bool.ogg  = elem.canPlayType('video/ogg; codecs="theora"')      .replace(/^no$/,'');

                // Without QuickTime, this value will be `undefined`. github.com/Modernizr/Modernizr/issues/546
                bool.h264 = elem.canPlayType('video/mp4; codecs="avc1.42E01E"') .replace(/^no$/,'');

                bool.webm = elem.canPlayType('video/webm; codecs="vp8, vorbis"').replace(/^no$/,'');
            }

        } catch(e) { }

        return bool;
    };

Answer 1

Initially bool is indeed a primitive boolean, which cannot have properties added to it. Following the if condition, the value of bool gets overwritten via new Boolean(bool) . In Javascript, calling new on a function, creates an empty Object, and uses the function as a constructor for said object. In the case of the Boolean() constructor, the only notable addition to the object, is a valueOf() function which returns the original primitive value. Otherwise, you now have a normal Object which can have arbitrary properties added to it.

See here and here .

Answer 2

I actually wrote a majority of that test.

That is just the format that Modernizr has followed for detects with sub values. In JavaScript, everything is an object, and new Boolean is as well.