Use `JsonValue` as `JSON.parse` Return Type?

[joealden]

Perhaps this would go further that what is wanted from this library, but I think it'd be nice if JSON.parse could return a type similar to (or the same as) JsonValue from type-fest (see https://github.com/sindresorhus/type-fest#json).

I've never used the optional reviver function in JSON.parse, but it looks like it can modify the return type beyond JsonValue, so perhaps it'd make sense to have a return type of JsonValue if no reviver is passed, then unknown if a reviver is passed?

[mattpocock]

I think unknown here is safer, but I'll leave this issue open!

[j]

interface JSON {
  parse<T = unknown>(
    text: string,
    reviver?: (this: any, key: string, value: any) => any,
  ): T;
}

[joealden]

@j if the aim is type safety, that isn't safe

[mattpocock]

@j Hiding this as not relevant to the issue. Please also take a look at the readme, where I mention in detail the reason why we won't be implementing this.

[j]

@mattpocock

true; understood. I live in a world where I consume self-owned APIs and messages that are type-safe at the point of having to parse it, so my brain went there.

with that said, leaving as unknown and forcing narrowing or casting is better.

[m10rten]

Same applies to JSON.stringify:reviver btw ^.^

[wmertens]

I'd like to see JsonValue when no reviver too. I can't see how it's less safe?

[mattpocock]

OK - I think I prefer unknown here instead of JSONValue. The reason is that narrowing based on unknown is actually more productive. With JSONValue you actually lose a lot of value in the return type, preventing you from narrowing properly:

https://tsplay.dev/WKpQKw

[aaditmshah]

I created a PR, #123, for a type-safe and sound version of JSON.parse if anybody is interested. I also wrote a good description of the problem statement and the solution that I implemented.

[aaditmshah]

@mattpocock Consider the following playground example. With noUncheckedIndexedAccess you can see that the type of result.whatever is just JsonValue, but the type of result.awdkjanwdkjn is JsonValue | undefined. You may not get autocomplete for whatever and an error for awdkjanwdkjn, but this is the next best thing.