[esrauch]

I think the reason for these is for js minification. With these assignments a simple minifier will replace those three variables with a line like this:

    var a=null,b=true,c=false;

Then your minified js will be smaller because all other instances of true, false and null will be one character. Whether or not this is a reasonable optimization is debatable but in terms of pure code size it would be a win.

[ajanuary]

Couldn't you build it into the minifier rather than polluting the original source?

[jackmoore]

Yes, stuff like this shouldn't be in the source. UglifyJS and Closure compiler make this optimization, but I've used other js minifiers that did not.