[EB66]

Their decision to switch to a Proxy-based observer is very much welcomed. Other libraries ( https://github.com/ElliotNB/observable-slim ) have shown that Proxy-based observers are robust and performant, but do have some polyfill restrictions for IE11 users. The ability to monitor for properties added dynamically at runtime will allow for a lot more flexibility.

Proxies still behave unusually with more complicated Objects (e.g., Date), but they work brilliantly with plain objects. Implementing an observer with Object.defineProperty is a lot more complicated IMHO.