[byroot]

There is no way to be sure it can't be ever referenced:

  function frame() {
    var str = "blah";
    return function() {
      eval("console.log(str)");
    }
  }

Edit: updated to use eval instead of setTimeout because setTimeout(string) do not share the scope.

[masklinn]

eval already triggers a number of deoptimizations and specific behaviors because eval. It's not used in the example and thus entirely irrelevant.