Svelte5 not documented breaking changes in error handling, `Error` swallowed, `svelte:boundary` not useful

[rallets]

Describe the problem

Hi, I'm migrating some apps from Svelte4 to Svelte 5 and I found an undocumented breaking change related to error boundaries.
Ref. https://svelte.dev/docs/svelte/v5-migration-guide

In Svelte4, if an error was thrown in a "store" file (not a svelte component, a typescript file containing a Svelte store), the Error was bubbled up and it was possible to catch it within a

window.addEventListener('unhandledrejection', ()=> { /* handle error */});

In Svelte 5 it's now available svelte:boundary but it doesn't work in this case, the error is never catched.

<svelte:boundary onerror={(e) => { /* handle error */}}>
</svelte:boundary>

Now in Svelte5 the Error is been swallowed. Is there any way it could behave like previously? I tried to look in existing issues but I haven't found anything useful.
This is now a big problem for the migration because the error handling is now broken.

Thanks

Describe the proposed solution

Errors thrown outside a svelte:boundary should bubble up like in Svelte4.

Importance

i cannot use svelte without it

[adiguba]

Hello,

It can't be a breaking change since <svelte:boundary> didn't exist in Svelte 4.

=> Could we have a reproduction of your problem ?

[rallets]

Hi @adiguba , it looks like I have explained myself poorly. I didn't mean that <svelte:boundary> had a breaking change, but "how" an Error bubble up in Svelte5 compared with Svelte4.
I mentioned <svelte:boundary> just to make clear that <svelte:boundary> is not a replacement for what I'm looking to, and that I'm aware of this new possibility.

With the error bubbling up in Svelte4, I was able to detect unhandled exceptions, then send a trace to Application Insights, and show a html and vanilla-javascript form to the user (with an useful stacktrace for debugging purposes).
The same code, migrated to svelte5, doesn't allow anymore to catch these runtime exceptions, that are been swallowed instead.
So I'm looking for any way to restore the Svelte4 behaviour somehow.

I have a more complex setup and code, but this is the main concept:

// test.store.ts (svelte4)

import { derived, writable } from 'svelte/store';

type IState = {
    value?: number;
};

const initialState: IState = {
    value: undefined,
};

function create() {
    const store = writable(initialState);
    const { subscribe, update, set: _set } = store;

    return {
        subscribe,

        init: () => {
            update(state => (state = { ...state, value: 0 }));
            throw new Error('Error');
        },
    };
}

export const TestStore = create();

// parent.svelte (main entry point)
<script lang="ts">
    import Throw from './Throw.svelte';

    const handle_rejection = (e: PromiseRejectionEvent) => {
        console.error(e); // <<<--- this in Svelte5 is never called
    };
    window.addEventListener('unhandledrejection', handle_rejection);
</script>

<Throw />

// throw.svelte
<script lang="ts">
    import { TestStore } from './test.store';
    TestStore.init();
</script>

<button on:click="{() => TestStore.init()}">Click me</button>

This is the svelte5 version of the store

// test-store.svelte.ts
export class State {
    value = $state<number | undefined>(undefined);

    init() {
        this.value = 0;

        throw new Error('Error');
    }
};

function create() {
    const store = new State();
    return store;
}

export const TestStore = create();

[adiguba]

I tried your code but I got the same result on both version :

• Svelte 4 REPL
• Svelte 5 REPL

unhandledrejection only catch error on promise without catch/error handler...

[rallets]

Thanks @adiguba I didn't know it was possible to specify which version of Svelte it should be used. I cannot chose that myself, so I'm wondering where that option can be set. Is it only available as query string or there is a UI ?

I tested your REPL and made some changes, but I agree with you that in that setup the code behave the same in both 4 and 5.
My setup is a bit more complex, so I need to dig a bit more on what is the correct combination to be able to reproduce it.

But from my understanding there should not be any known changes about the error handling and bubbling of errors between 4 and 5?

[rallets]

I found why the error was looking as swallowed, it's not related to some incompatibilities between Svelte5 and Svelte4, but rather from a mistake made by me 🤕
Thanks for the support 💯