For minification, as someone else said. I originally wrote this for a JS1K competition some years ago, so it's not geared towards readability, but towards small code size.
I had a really, really ugly trick that saved me a couple of bytes which I removed before posting to HN. I figured out I was using canvas.getImageData() and canvas.putImageData(). So I did this: var tID = "tImageDatA"; canvas["ge"+tID](); canvas["pu"+tID"](); You can see that in the original source: http://gabrielgambetta.com/tiny_raytracer_full.js
Yes, that is (my 5 year old idea of) ZX Spectrum Basic. It follows the format of the listings published in Microhobby (http://microhobby.speccy.cz/mhf/031/MH031_08.jpg - in particular, look at the "NOTAS GRAFICAS" sidebar). I grew up with that :) My programming has improved, my drawing abilities not so much.
That is before minifying. The variable name "math" was chosen for readibility in the non-minified version. The variable math can be renamed during the minifying process to a short identifier, in this case "E". Since the Math object is used quite a lot, it is worth the assignment to rename it.
// Shorten some names. var math = Math;