How should the reviver function behave?

[peetklecha]

Should JSON.parseImmutable take a reviver function, and if so, what should its behavior be?

Assuming there is a reviver function: It seems straightforward that in cases where the reviver returns either a primitive or the same object it was passed, that it should behave just like the reviver in JSON.parse. What's less obvious is what do if the reviver returns an object other than the object it was passed. Is it accepted without condition? Does the engine freeze it? Does the engine validate that it is frozen? Or does the engine reject it?

For simplicity, my weak preference is for the engine to simply throw if the reviver attempts to return any object other than the one it was passed.

[ljharb]

then what's the purpose of the reviver? i'd expect it to be able to return a new object so it can change things as it revives them, and that yes, the engine would impose whatever constraints it could (freeze the object, perhaps), and throw if any constraints weren't possible to impose.

[acutmore]

I think we should allow returning the already pre frozen object or any primitives.

The primary functionality this still allows for is inspecting the original source and returning a bigint when that would be more accurate.

Allowing any object to be returned loses one of the major benefits of the proposal, knowing that the returned value is deeply frozen.

Even if the returned object was frozen, that's only one layer. It would need to walk the object and freeze it; we would have snuck a round about way to do Object.deepFreeze into the language.

[ljharb]

Or, it could just throw an error if it had a non-null prototype, or if any object in its prototype chain was not frozen.

[gameroman]

I think we should allow returning the already pre frozen object or any primitives.

The primary functionality this still allows for is inspecting the original source and returning a bigint when that would be more accurate.

Allowing any object to be returned loses one of the major benefits of the proposal, knowing that the returned value is deeply frozen.

Even if the returned object was frozen, that's only one layer. It would need to walk the object and freeze it; we would have snuck a round about way to do Object.deepFreeze into the language.

Doing JSON.parseImmutable(JSON.stringify(...)) would also technically be a Object.deepFreeze for jsonifiable objects

edit: I am wrong, this is not the same thing

[acutmore]

Or, it could just throw an error if it had a non-null prototype, or if any object in its prototype chain was not frozen.

It's not just the prototype, its values could be anything.