unstable_cache loads Object instance instead of Map instance from cache.

[taylorjohnwood]

Link to the code that reproduces this issue

https://codesandbox.io/p/sandbox/unstable-cache-map-bug-mq2xxm

To Reproduce

1. Start the application in development (next dev)
2. On initial load, the page will correctly display the contents of the Map
3. Refresh the page
4. The page will show an error cachedMap.entries is not a function

Current vs. Expected behavior

Following the steps from the previous section, I expected to still see the contents of the map even after refreshing / loading a cached version of the Map.

It seems to me that the Map is being loaded as an instance of Object. That's why it doesn't have an entries method defined.

If I inspect the cache contents at ./.next/cache/fetch-cache I see

{"kind":"FETCH","data":{"headers":{},"body":"{}","status":200,"url":""},"revalidate":31536000,"tags":[]}

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #1 SMP PREEMPT_DYNAMIC Sun Aug  6 20:05:33 UTC 2023
  Available memory (MB): 4102
  Available CPU cores: 2
Binaries:
  Node: 20.9.0
  npm: 9.8.1
  Yarn: 1.22.19
  pnpm: 8.10.2
Relevant Packages:
  next: 14.2.0-canary.9 // There is a newer canary version (14.3.0-canary.13) available, please upgrade! 
  eslint-config-next: N/A
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.1.3
Next.js Config:
  output: N/A
 ⚠ There is a newer canary version (14.3.0-canary.13) available, please upgrade! 
   Please try the latest canary version (`npm install next@canary`) to confirm the issue still exists before creating a new issue.
   Read more - https://nextjs.org/docs/messages/opening-an-issue

Which area(s) are affected? (Select all that apply)

Runtime

Which stage(s) are affected? (Select all that apply)

next dev (local), Vercel (Deployed)

Additional context

I tried to install latest canary in the sandbox, but the install command was erroring in CodeSandbox.

I've observed this exact issue outside of CodeSandbox environments, I'm certain the issue is not environment related (I've observed in Vercel deployments too).

[hanshs]

probably a similar issue with the Date object, as a Date will be returned as string from the cache

[sannajammeh]

This is not a bug, it is a documentation issue however.

Maps & dates are not JSON serializable. It would be the same as returning new Map() from a server action to the client, it crashes.

[taylorjohnwood]

This is not a bug, it is a documentation issue however.

Maps & dates are not JSON serializable. It would be the same as returning new Map() from a server action to the client, it crashes.

Thanks for clarifying @sannajammeh

1. I could draft a doc's update as my first contribution to Next, but I'm not sure how people feeling about documenting an unstable API (unstable_cache).
2. I would think there should be some warning/runtime error/lint issue when a user attempts to cache a non-serializable instance. Perhaps this could be added as well?