Unexpected Reactive Updates

[NfNitLoop]

Describe the bug

In a component, a reactive update like this:

$: bar = foo.clone()

Ends up causing reactivity to treat foo as updated when it hasn't been changed (and can not have been changed, since it's not passed/bound to any other components.)

Is this a bug, or undocumented behavior?

These kinds of things are difficult to debug and eat a lot of time. There's not good visibility into why Svelte is choosing to act as if some value was updated.

Reproduction

REPL link: https://svelte.dev/repl/65a48991ca7b46218e67b933edca77b4?version=4.0.1

Logs

See logs in the REPL. 😊

System Info

Reproduced in the REPL with v4.0.1, and locally with 3.59.2.

Severity

annoyance

[NfNitLoop]

reactivity-bug.zip

Just in case there's an issue w/ the reproduction in the REPL, here's the export.

[mpopovic4116]

Ran into this as well. Seems to be a duplicate of #4933. Undocumented behaviour intended to fix #2444, but causes surprising behaviour in some other cases and was marked as a bug.

Workaround: Hide the dependencies from the compiler. In your REPL's BrokenExample.svelte:11, instead of

$: bar = foo ? foo.clone() : new Example("default bar")

you should do this instead (or make bar a store and use bar.set(...) instead of $bar = ...):

let bar;
function setBar(x) {
    bar = x;
}

$: setBar(foo ? foo.clone() : new Example("default bar"));

That way, the compiler won't know bar depends on foo and won't invalidate both when you modify bar.
This only seems to happen when using two-way bindings or bar = bar (bar = identity(bar) doesn't trigger it).

[NfNitLoop]

@mpopovic4116 Nice! I'd done something similar in my Workaround.svelte, but it depended on me knowing exactly where foo gets updated. I like that yours still allows for reactivity. Thanks! 👍

And thanks for the issue link. I'd done some searching but didn't find #4933. I think that fully captures this case, so I'll close this as a duplicate and watch the other.

[alexspurling]

$: bar := foo.clone()

What is the meaning of := here? Does this syntax have a name or is it a typo?