In the past maintainers of the major JS runtimes (v8, etc) have stated they avoid documenting performance and optimization tips, because people build code around them and as a result the engine maintainers can't make fundamental changes without regressing performance of those apps. With how widely some JS libraries get distributed, you can imagine how this becomes a problem. It's a nasty situation.

If the difference between a game running at 30fps and 60fps is using weird optimizations to compensate for quirks in v8 and spidermonkey, you better believe I'm going to optimize for them. But then script runtime changes later make my performance tank when other (originally slower) pieces of code speed up.