>"This object will only ever have these properties; it will never have properties created or deleted" >"This prototype will never be modified at runtime" You

>"This object will only ever have these properties; it will never have properties created or deleted" >"This prototype will never be modified at runtime" You can already give these hints with Object.freeze/Object.seal Not really hints though, they enforce a certain behavior. Because hints aren't enough. A JS engine may already assume you'll never change the objects, but it still has to support cases where you then do so anyways. I don't think these alone would enable any substantial speedups, since the performance issues arise where unknown objects are used.