[isntitvacant]

In both examples, the function is only allocated once, no matter how it's instantiated.

Where you have to be careful is if you have a function that's being called repeatedly and it contains a function instantiation -- that function will be reallocated each time.

   [1,2,3].forEach(function(x) {
     return (function(a, b) { return a * b })(x, 2)
   })

Will repeatedly allocate that inner multiplication function, vs:

  mul = function(a, b) { return a * b }
  ;[1,2,3].forEach(function(x) {
    return mul(x, 2)
  })

Will only allocate that multiplication function once.

[cmwelsh]

I think we are in complete agreement. Your example is a little prettier however you still have this "unoptimized" line:

  ;[1,2,3].forEach(function(x) {

If this line is executed more than once, the nested function will be created again and again, wasting memory. However, some people may see my advice here as a micro-optimization. Thus, don't make your code ugly until you identify the real-world bottlenecks. I have updated my code in the grand-parent comment to explain myself a little more clearly (maybe)...