Eval breaks both JIT[1] and garbage collection [2] (unless used very carefully). It also affects Google Closure Compiler [3]. What is even better - it won't ever be fixed, at least not in the terms of performance.

[1] http://echo.nextapp.com/site/node/6097

[2] http://stackoverflow.com/questions/5368048/how-are-closures-...

[3] http://groups.google.com/group/closure-compiler-discuss/brow...

Garbage collection was brought up during the Q/A and I would dig a jsPerf revision to explore that.

One of the tests included in the slides was to see if simply using `eval` in a method would disable optimizations: http://jsperf.com/eval-kills

Chris (@robodynamo) included the slide about `eval` because he found it performant/useful in his day job.

Eval has it's uses, for example it's used in json2.js as part of the JSON fallback.

The same is true with undefined. You should use void 0 rather than undefined because undefined is just a normal variable which happens to be undefined unless somebody defines it, while void 0 is always going to be undefined.

There is absolutely no need to use `void 0` you can simply use `typeof` or create a local `undefined` var. Many libs do this in their IIFE `(function(undefined) { /* your code */ }());`. The point was there isn't a real world performance concern to justify using one over the other.