`undefined` props are serialized as `null`

[wackbyte]

What version of astro are you using?

2.7.2

Are you using an SSR adapter? If so, which one?

Node

What package manager are you using?

pnpm

What operating system are you using?

Linux (NixOS)

What browser are you using?

Firefox (Librewolf)

Describe the Bug

When a Svelte component is hydrated, any props that are undefined become null. This has caused unexpected behavior and unsound typing (prop === null vs. prop === undefined) for me in the past.

In my example, it is shown that undefined props are indeed undefined when rendered on the server, but are hydrated with such props being null instead.

I have not tested whether this happens in other frameworks.

What's the expected result?

Any undefined props should remain undefined when hydrated.

Link to Minimal Reproducible Example

https://gitlab.com/wackbyte/astro-serializes-undefined-as-null

Participation

• I am willing to submit a pull request for this issue.

[wackbyte]

This happens because JSON.stringify([undefined]) returns "[null]". This code is what creates such an array.