[xilun]

Stupid microoptimisations. Not only one should never optimize prematurely, and that is noted in the slides, but one should also never optimize like that (at least in the JS context). The interpreters/compilers/whatever will evolve for the readable cases, not for obscure idiocies, and programs using these techniques today will just run slower tomorrow.

[kwamenum86]

On a large Javascript codebase these optimizations actually make a difference. It could take SECONDS off your page load time, which is huge.