A lot of these use cases are achievable using Object.defineProperty/Properties. In most cases I think defineProperty even provides a nicer API because it's per-property instead of per-trap. The 'has' trap can't be achieved completely with defineProperty (although you can make a property non-enumerable, which achieves the same goal in some cases).

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...