small preprocessor - #ifdef like statement ?

Hmm, it's a thought. I wonder whether minification could be modified to do what you want though. For instance:

var DBG = false;

if (DBG) console.log("Debug");
console.log("Foo");

That actually works fine when minified - it goes down to:

var DBG=!1;console.log("Foo");

However when in a function, it doesn't work properly (because it thinks that the state of DBG might get changed). Object.freeze doesn't seem to work either. You could do:

(function() {
  var DBG = false;

  function foo() {
    if (DBG) console.log("Debug");
    console.log("Foo");
  }

  setTimeout(foo,1000);
})();

And that minifies down really nicely:

(function(){setTimeout(function(){console.log("Foo")},1E3)})();