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Comment by Udo | Hacker News Reader

> Right, but it knew you were going to use eval, and to support that, it had to allocate all local variables in the closure.

Wow, so there is actually special handling in the engine for this? So it does static analysis whenever it can, but not in these two cases?

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Yes, the V8 compiler bails out of several optimizations if your function uses eval. You can see this in the profiler: functions which V8 wasn't able to optimize will have an alert sign next to them, and if you click it, it'll tell you what the issue was.

Very cool, thanks for clearing that up!

This is a bit troublesome!

  function f() {var x = 99; return function(a,b) {return a(b)};}
  f()(eval, 'console.log(x);')
  ReferenceError: x is not defined
  function f() {var x = 99; return function(a,b) {return eval(b)};}
  f()(eval, 'console.log(x);')
  99
  undefined

Yep, this is as per spec: http://www.ecma-international.org/ecma-262/5.1/#sec-10.4.2 . "Indirect" calls to eval (i.e. assigning eval to another variable, like you did by passing it as a param) are evaluated in terms of the global environment. "Direct" calls, like in your second example, use the local environment.

Yes, and in articles describing it (v8), they explicitly warn you to not use "eval" or "with" for the performance impact.

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