fix #2065 forward initial value to on

.changeset/bright-maps-sing.md

@@ -0,0 +1,5 @@
+---
+"solid-js": patch
+---
+
+fix #2065 forward initial value to `on`

packages/solid/src/reactive/signal.ts

@@ -953,7 +953,7 @@ export function on<S, Next extends Prev, Prev = Next>(
     } else input = deps();
     if (defer) {
       defer = false;
-      return undefined;
+      return prevValue;
     }
     const result = untrack(() => fn(input, prevInput, prevValue));
     prevInput = input;

packages/solid/test/signals.spec.ts

@@ -81,6 +81,17 @@ describe("Create signals", () => {
     set("minds");
     expect(temp!).toBe("impure minds");
   });
+  test("Create a Effect with explicit deps, lazy evaluation, and initial value", () => {
+    let temp: string;
+    const [sign, set] = createSignal("thoughts");
+    createRoot(() => {
+      const fn = on(sign, (v, _, p) => (temp = `impure ${p} ${v}`), { defer: true });
+      createEffect(fn, "numbers");
+    });
+    expect(temp!).toBeUndefined();
+    set("minds");
+    expect(temp!).toBe("impure numbers minds");
+  });
 });
 
 describe("Update signals", () => {