regarding your edit, you're exactly right, of course a for loop will be faster. Sometimes you really do need a function call though, in which case fast forEach and map implementations become more useful.

The next step for fast.js are some sweet.js macros which will make writing for loops a bit nicer, because it's pretty painful to write this every time you want to iterate over an object:

    var keys = Object.keys(obj),
        length = keys.length,
        key, i;
    for (i = 0; i < length; i++) {
      key = keys[i];
      // ...
    }

I'd rather write:

   every key of obj {
     // ...
   }

and have that expanded at compile time.

Additionally there are some cases where you must use inline for loops (such as when slicing arguments objects, see https://github.com/petkaantonov/bluebird/wiki/Optimization-k...) and a function call is not possible. These can also be addressed with sweet.js macros.