Basically, unexpected behavior. One of the "footguns" is that it's easy to write to a Moment object when you mean to be reading from it. And by the way, you can mutate objects declared with `const` in JS - you just can't reassign them.
That said, we wouldn't be in this position if more companies used a shared CDN for libraries instead of always bundling them.
That helps with transfer but clients still have to parse and execute the javascript which can be significant, especially on mobile devices.