const
+function(){ print("[Doesn't compile]"); // → (error before this is printed) const x = 'hoge'; var x = 'fuga'; print(x); }();
+function(){ print("[The substitution is simply ignored]"); const x = 'hoge'; x = 'fuga'; print(x); // → hoge }(); print(this.x) // → undefined (not that it penetrates the scope)
実行時の代入もエラーにしてくれたほうが嬉しいような。