Svelte 5: default property value should be real and not initial|undefined

[dm-de]

Describe the problem

3 months ago, I posted this: https://www.reddit.com/r/sveltejs/comments/16qeg71/svelte_5_fixed_props_issue_you_may_not_aware_of_it/ I was happy about this

It looks like Svelte 5 fixed default props issue. I think most people are not aware about that "export let" with default value is initial value (like constructor), but not default value as replacement for undefined.

Now, I found out that this was changed again. Most people expect that if undefined is passed, the default value will be used. And they are then surprised when this is not the case.

For comparison: React have real default values: const {fontSize = "20px"} = props; https://blog.logrocket.com/complete-guide-react-default-props/

Vue have real default values (see code below). const props = defineProps({ name: {type: String, default: 'Anonymous'} }) https://hackernoon.com/how-to-set-default-value-of-props-in-vue

solid-js have real default values const merged = mergeProps({ greeting: "Hi", name: "John" }, props); https://www.solidjs.com/tutorial/props_defaults?solved

Describe the proposed solution

Simple make default and not initial again.

Demo Svelte 4: https://svelte.dev/repl/393c4caef5fe4297a00da4b7ab9fd4d9?version=4.2.8

Demo Svelte 4 - but with default values as WORKAROUND (using fallback at place) https://svelte.dev/repl/55e8468acb474369931de40d489c0121?version=4.2.8

Demo Svelte 5 - but with default values as WORKAROUND (using $:) https://svelte.dev/repl/649275e475184354a6316111cb451ee3?version=4.2.8

Demo Svelte 5: LINK

Demo Svelte 5 - but with default values as WORKAROUND (using fallback at place) LINK

Demo Svelte 5 - but with default values as WORKAROUND (using $effect) LINK

Alternatives considered

Vue have real default values and not initial values. Test this yourself:

LINK

Importance

svelte would be better with it

[Malix-Labs]

Now, I found out that this was changed again.

Have you found which commit have that effect?

[Malix-Labs]

Strong agree.

I think it's more important than "svelte would be better with it", as it could be the cause of #9764 (which, to me, is a mistake)

[dm-de]

Now, I found out that this was changed again.

Have you found which commit have that effect?

Date of Reddit post is 23. Sep. 2023 Date of 1st Svelte 5 release on GITHUB is: 10. Nov. 2023 I think this wa a pre-proxy release. I remember, at this time, I used online REPL.

[Malix-Labs]

See https://github.com/sveltejs/svelte/issues/9764#issuecomment-1946547707