Svelte 5: Should prop fallbacks based on other props be reactive?

[brunnerh]

Describe the bug

It is possible to define property fallbacks that use other properties like this:

const { a = 1, b = a + 1 } = $props();

The fallback is only evaluated once, so if the property the fallback value is based on changes, there will be no update.

(Workarounds using $derived are possible.)

Reproduction

REPL

Logs

No response

System Info

REPL

Severity

annoyance

[FoHoOV]

I think being explicit (as what the current direction of things are, and I love it), is better. Direct "vanilla" assignments to variables being reactive would kind of break my expectations of svelte-5 reactivity system. thoughts?

[dummdidumm]

No they shouldn't be reactive. The fallback value is meant as a fixed initializer when something else is undefined. It becoming reactive would be confusing and harder to opt out of, whereas using $derived is easier to reason about and easy to opt-in.