Astro unexpectedly stringifies `undefined` class attribute

[delucis]

Astro Info

Astro                    v4.11.5
Node                     v18.20.3
System                   Linux (x64)
Package Manager          npm
Output                   static
Adapter                  none
Integrations             none

If this issue only occurs in one browser, which browser is a problem?

n/a

Describe the Bug

When using the When using the 'where' or 'class' scopedStyleStrategy, an undefined class attribute passed down to a child component is incorrectly stringified.

For example, <ChildComponent class={undefined} /> can receive undefined astro-XXXX as the value of Astro.props.class. (Assuming the parent component is also using scoped styles and therefore has an associated hash ID.)

What's the expected result?

<ChildComponent class={undefined} /> should receive only astro-XXXX as the value of Astro.props.class, without undefined appearing in the class string.

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Notes

• A Starlight user noticed this bug in https://github.com/withastro/starlight/pull/2086
• I’d be happy to try to fix this if someone could point me in the right direction

Link to Minimal Reproducible Example

https://stackblitz.com/edit/github-i2qay8?file=src%2Fpages%2Findex.astro

Participation

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

[delucis]

I knew this seemed familiar — I already reported this once in the past https://github.com/withastro/compiler/issues/1005 😅

Guess I’ll close this in favour of that one then.