While playing with decorators I found a little usability problem. I initially wrote this decorator:

function add(value) {
  return function decorator(element) {
    const { initializer } = element;
    element.initializer = function() {
      return initializer.call(this) + value;
    };
  };
}

class Foo {
  @add(2) num = 3;
}

When run, it throws Cannot read property "kind" of undefined.. It was my fault (I forgot to write return element; at the end of the decorator function), but it has been quite hard to spot.

I propose two ways of improving this:

1. If the decorator function returns undefined, throw an error. (After step 3.d of https://tc39.github.io/proposal-decorators/#sec-decorate-element)
2. If the decorator function returns undefined, default to elementObject. This make it possible to just mutate the descriptor without returning it.