フラグを付けたとき ◦ Babel の @babel/plugin-proposal-decorators で legacy フラグを付けたとき • MobX がこれを嫌ってデコレーターを使うのを辞めた How should I use decorators in transpilers today? Unfortunately, we're in the classic trap of, "The old thing is deprecated, and the new thing is not ready yet!" For now, best to keep using the old thing. https://github.com/tc39/proposal-decorators