`{#each}` block overwrites objects in state array

[marc-eggers]

Describe the bug

First off: I'm not entirely sure this is a bug, so if I'm just misunderstanding some intended behaviour here, please let me know.

The src/routes/+page.svelte file in the reproduction repo contains a heavily simplified version of some state logic I've written in a project of mine. The rough breakdown of the setup is as follows:

• The strat state is an object, which contains an array of steps. The demo contains 3 objects in steps.
• Each step contains two arrays: players and stickies
• The view state is an object with a single number value: activeStep
• The template has an {#each} block, iterating over strat.steps[view.activeStep].players
• There are 3 buttons, which set view.activeStep to 0, 1, or 2 respectively

A possibly important note: The repro I've provided shows no issues in svelte@5.0.0-next.17 and below. The issue starts appearing at version 5.0.0-next.18.

The presence of this {#each} block alone - nothing else is being done anywhere - leads to some very peculiar behaviour. First off, some definitions: I will be referring to the step active on page load as "initial step", and the one targeted by view.activeStep as "current step".

• When switching the current step using one of the buttons, the objects in players of the step being activated overwrite those found in the initial step. (The demo only contains a single player object for brevity, but if there are more, all objects in the array get overwritten).
• For example, when the initial step is 0, and you switch to step 2, the objects in 0 get overwritten and are identical to those in 2. You can tell by the activePlayer.fromStep output in the "UI".
• If you switch to step 1 after that, the objects in step 0 are now those of step 1.
• Switching back to 0 does not revert this.
• If you switch the data source of the {#each} block to stickies instead, you will observe the exact same behaviour, but with the stickies objects, instead.
• The arrays themselves do not share an identity, only the objects within.

I hope this was a decent description of the issue I encountered. Let me know if there are any questions.

Reproduction

https://stackblitz.com/~/github.com/marc-eggers/svelte-5-bug-repro

Logs

No response

System Info

System:
    OS: macOS 13.2.1
    CPU: (8) arm64 Apple M1
    Memory: 85.89 MB / 8.00 GB
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 18.16.1 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 9.5.1 - /usr/local/bin/npm
    pnpm: 7.18.2 - ~/Library/pnpm/pnpm
  Browsers:
    Chrome: 121.0.6167.139
    Safari: 16.3
  npmPackages:
    svelte: ^5.0.0-next.45 => 5.0.0-next.45

Severity

blocking an upgrade

[marc-eggers]

Turns out using a keyed each block fixes this issue, as documented here: https://learn.svelte.dev/tutorial/keyed-each-blocks

I'd still argue that this should be considered a bug, as not properly updating the DOM is one thing, but overwriting objects in state still seems unintended to me.

I will leave this issue open for the time being.

[dummdidumm]

This is definetly a bug, the array shouldn't get mutated here. Svelte 5 REPL reproduction
Wondering if this is in any way related to #10037 in the sense of that there seems to be some code in the each block logic that mutates the original arrays which it shouldn't. cc @trueadm

[cyantree]

I think I stumbled upon the same bug and made another reproduction sample for my usecase:
https://svelte-5-preview.vercel.app/#H4sIAAAAAAAACoWRTW-DMAyG_0qWTSuVENwpIE1aLztshx5LDxDMZi0NKDGTEMp_n8JHKduqwQE79uP4fel5hRIMj449V_kZeMSfmob7nLrGJeYLJAH3ualbLdxJbITGhtJMSSAGijSCYQl7MJQTeMdMMcZYP37cg2XENvnGX05MWyDB2UTsuMk3p6li_b_R4hZa_IuKW6hYoRn9RstbaLlCT9tdpjJVtUoQ1orV6hkkEOwV6c5z7nTbefbi1RQFFUoC7XmoFOj92JykbEkDLNldMgIucbfZTMXh5SdkKm5lOsm_h1x8XO7JzcjZRUgscUnSJRxqRUs0SIiERPGZ9N6wzV-SbDqexeEIuUVofhkbWiPWz2tbFq4bDrOh6xWuBHTBbLrTMcV23T4gc-lRFabZ_ZgXunnX-kOJs1lzzW3WvxzeXgNDGtU7VqNKBLMdvB785T4_1yVWCCWPSLdgT_YbzQYXZT0DAAA=

• When deleting entry "a" the other entrys will all show subitems "d"
• When deleting entry "b" the subitem of entry "a" still shows "a" however the subitems of entry "c" and "d" switch to "d"
• When removing the each block everything is fine (as can be seen in the JSON output on the bottom)

[trueadm]

These issues should be resolved now.

[dummdidumm]

fixed by #10422