Why not allow undefined values in serialized JSON returned by getStaticProps() ?

[dotlouis]

Bug report

Describe the bug

I'm not sure it's a bug, in that there may be a reason for this behavior. But I can't wrap my head around it.

When returning a Javascript object from getStaticProps(), NextJS needs to serialize it to send it down to the page it's called from.
When I try to return a somewhat large and nested object that may have "undefined" values in it, Next complains that it needs "serializable" data and that undefined is not. It should be replaced by null values.

However, JSON.stringify() does strips away undefined values. So why not allow undefined values? What serialization does NextJS do in the background?

For large, nested objects, it might be complicated to remove/replace the undefined values except for resorting to JSON.parse(JSON.stringify(myObject)), which is not very nice although it works

To Reproduce

Nothing to do, just observe the error at:
https://codesandbox.io/embed/static-site-generation-ssg-in-nextjs-with-getstaticprops-4yu9h?fontsize=14&hidenavigation=1&module=%2Fpages%2Findex.tsx&theme=dark

Error message

Error serializing `.data.toBe` returned from `getStaticProps` in "/". Reason: `undefined` cannot be serialized as JSON. Please use `null` or omit this value all together.

Expected behavior

Let us return objects with undefined values as JSON.stringify() remove them anyway.

System information

irrelevant/all

[dotlouis]

Relevant discussion: #11209