$derived exposed through property not necessarily updating

[brunnerh]

Describe the bug

I tried to eliminate an $effect that writes a $derived value to a property by wrapping the derived access in a property. The $derived values appear to run initially but not when dependencies are updated.

Reproduction

• Version with $effect
• Version with wrapped $derived
  (Also made a version without SvelteMap to rule that out)

I tried to make a simpler reproduction from scratch but could not reproduce the issue that way.

Core diff in node.svelte code

-  $effect.pre(() => result = derivedResult);
+  result = {
+    get current() { return derivedResult },
+  };
 </script>
 
 <div>
@@ -35,7 +37,7 @@
             bind:content={content.children[i]}
             bind:result={
               () => undefined,
-              v => childResultMap.set(child, v)
+              v => childResultMap.set(child, v.current)
             }
           />
         </li>

Logs

System Info

REPL

Severity

annoyance

[Ocean-OS]

I found that wrapping the assignment to result in an effect fixes the issue.

[7nik]

So, looks like bind:result={() => undefined, ...} was overriding upcoming result. Doing so:

-        .map((child) => childResultMap.get(child))
+        .map((child) => childResultMap.get(child).current)


            bind:result={
-              () => undefined,
-              v => childResultMap.set(child, v.current)
+              () => childResultMap.get(child),
+              v => childResultMap.set(child, v)
            }

fixes the repl

[brunnerh]

Question being whether this should just work as is, the binding is supposed to be one-way.

[7nik]

One-way binding is bind:value={null, (v) => myVar = v} but it is not supported for props.