Unexpected behavior with reactive statements

[jmroon]

Describe the bug

The docs state that reactive assignments only trigger when values they depend on have changed. However this does not appear to be the case. Reactive assignments seem to trigger unexpectedly.

This has cropped up in particular when trying to create a copy of a store object when the store changes (which is a common use case with forms where you don't want to mutate the global store until the form is saved).

So far, the only surefire solution appears to be to subscribe to the store manually, as reactive assignments all appear to trigger unexpectedly.

Reproduction

https://svelte.dev/repl/6ffc787bb6fd42f1a2590b98ff8838cd?version=3.46.2

Logs

No response

System Info

System:
    OS: Windows 10 10.0.19042
    CPU: (24) x64 AMD Ryzen 9 5900X 12-Core Processor
    Memory: 18.79 GB / 31.89 GB
  Binaries:
    Node: 16.13.0 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.10 - ~\AppData\Roaming\npm\yarn.CMD
    npm: 8.1.0 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: Spartan (44.19041.1266.0), Chromium (97.0.1072.62)
    Internet Explorer: 11.0.19041.1202
  npmPackages:
    svelte: ^3.44.0 => 3.46.2

Severity

annoyance

[7nik]

We've got a similar problem here #7129 (comment)

As a workaround, you can use setters in every affected reactive block instead of direct assignment.

let localForm = {};
const setLocalForm = (data) => { localForm = data; };
$: setLocalForm({...$formData});

[7nik]

Duplicate of #4933

[jmroon]

Indeed looks like this has been raised for a while now. Will close this and hope it gets addressed at some point.