Reactive assignment doesn't work when dependent variable is hidden in function

[ChrisS85]

Broken or unsupported
Working

For more enhanced or reusable calculations it would be nice to be able to wrap the reactive assignements inside functions. I didn't find any information about this limitation in the tutorial or the documentation but maybe I missed it.

[Conduitry]

This is intentional actually, but perhaps it should be documented. The functions themselves could also be reassigned at runtime, which would break any dependence that we could figure out at compile time. And, although it's not generally recommended, it's also occasionally convenient to be able to 'hide' dependencies from the compiler.

[Conduitry]

But also, if you do want to use separate functions and have your dependencies still counted, you can make all of the relevant dependencies be arguments to those functions, so the compiler can see them in the reactive block.

[ChrisS85]

But also, if you do want to use separate functions and have your dependencies still counted, you can make all of the relevant dependencies be arguments to those functions, so the compiler can see them in the reactive block.

That's actuually what I'm doing now and it works for me. Thanks for the clarification!
Maybe a warning could be emitted if a function call is used for a reactive assignement and there are no other dependent variables in the expression?

[BennyHinrichs]

If I understand it right, in JS there are four immutable data types: number, string, boolean, and undefined. Everything else is just a reference. So in your "unsupported" example, you're assigning the reactive variable to a reference to an object (functions are objects in JS, yay!). That reference never changes, so your value variable never changes.

Where it behaves differently than I'd expect is that @Conduitry says you can pass the variables as arguments, and value will get updated. That reference still isn't changing, but it apparently updates. Is that a feature of the compiler then?

[btk5h]

@BennyHinrichs I think you have a misunderstanding about how the compiler interprets reactive assignments.

The compiler looks for the dependencies of a reactive statement by determining which reactive variables are read. In the "unsupported" example, even though f() reads the reactive variable count in its body, because the variable count isn't present in the reactive statement itself, the statement is not seen as having a dependency on count.

[lukejoliat]

@Conduitry I'd be happy to take on this issue, if no one else has up to this point!

[dummdidumm]

This is documented now: https://svelte.dev/docs#3_$_marks_a_statement_as_reactive