Please add/document `bind:this` setter support

[timephy]

Describe the problem

I am doing something like this:

{#each Object.entries(tabs) as [tabId, tab] (tabId)}
    <TabView selected={tabId === selected} bind:this={tabViews[tabId]}>
        Content
    </TabView>
{/each}

Binding directly to an object const tabViews = {} via bind:this={tabViews[tabId]} does not work when adding more tabs dynamically, then references to existing ("cached" by #each because of given key (tabId)) tabs are lost. See this result after adding more tabs: {player: null, group: Proxy(Object), call: Proxy(Object), chat: Proxy(Object)}. ❗

I then tried binding to a map using getters and setters as described in https://svelte.dev/docs/svelte/bind#Function-bindings like so bind:this={null, (v) => tabViews.set(tabId, v)}. This failed with a runtime error: Binding as described in https://svelte.dev/docs/svelte/bind#Function-bindings ❗

Therefore I changed the code to this bind:this={() => null, (v) => tabViews.set(tabId, v)} which finally worked ✅

Describe the proposed solution

Request

Please add support for the mentioned https://svelte.dev/docs/svelte/bind#Function-bindings for bind:this.

Or if that is not sensible, then maybe add this behaviour to the docs?

Importance

nice to have

[gambo]

Im not seeing an issue with binding to dynamic objects.

Please can you provide a reproduction.
Also is this svelte4 or 5? if 5 then tabViews should be wrapped in the state rune let tabViews = $state({}).

I tried to re-create in a repl, but couldnt see any issue
repl

[timephy]

I agree, I could not reproduce this either/anymore.

However there is one thing left I want to ask about → Isn't bind:this a readonly binding?

Should not therefore bind:this={null, (v) => (elem = v)} work?
Instead currently only bind:this={() => null, (v) => (elem = v)} works.
See https://svelte.dev/docs/svelte/bind#Function-bindings

[7nik]

Isn't bind:this a readonly binding?

It is not completely so. For bind:this in loops, the collection of references must be updated when the array changes, and to do it efficiently (write only changes), the getter is required.

I guess this case is a bug.

[timephy]

the getter is required

Does that mean my way of writing bind:this={() => null, (v) => (elem = v)} is false, or just inefficient?

I guess this case is a bug.

So there does exist a bug, interesting...

[7nik]

Does that mean my way of writing bind:this={() => null, (v) => (elem = v)} is false, or just inefficient?

You should provide a real getter. Otherwise it won't set the variable to null thinking it is overwritten by another bind:this.