Svelte 5: binding an object to a prop has no effect

[harrisi]

Describe the bug

When passing an object to a child prop, the child can update it as if it were bound, even without using bind:. When not using an object, bind: is required.

Reproduction

Svelte 5 REPL EDIT: This might be a better REPL to show that there's no difference.

Svelte 4 REPL

Logs

No response

System Info

REPL

Severity

annoyance

[Conduitry]

This is a combination of intended behavior and a known limitation with a runtime check.

You are discouraged from mutating state that has been one-way passed into your component. Your original Svelte 5 REPL logs a warning about this at run time in dev mode. (It won't in prod mode so we can avoid the overhead of checking for these mutations at runtime.)

In your revised Svelte 5 REPL, you are both passing it in as a one-way prop and passing it in as a two-way binding to different instances of the child component, and there isn't a good way at runtime for the first instance of the child component to know that it shouldn't own the counter variable while the second one should.

In the Svelte 4 REPL, the parent component's counter value is still mutated (which can be seen by adding something like setInterval(() => console.log(counter), 1000) to the parent component), but it just doesn't find out about it because of a limitation of the compile-time dependency tracking.