feat(csr): diagnose missing output binding functions

packages/csr/src/reactive-runtime.ts

@@ -210,6 +210,17 @@ export const REACTIVE_RUNTIME = String.raw`
       // CustomEvent can be unavailable in minimal DOM test environments.
     }
   }
+
+  function warnMissingBindingFunction(statement, resolve, element) {
+    var match = /^\s*([A-Za-z_$][\w$]*)\s*\(/.exec(statement || "");
+    if (!match || typeof resolve(match[1]) === "function") return;
+    warnOnce(
+      "WRN-DEV-BINDING-MISSING",
+      "Component binding calls '" + match[1] + "', but that function does not exist in the parent scope.",
+      element,
+      { binding: statement, functionName: match[1] },
+    );
+  }
   /*__WRNEXUS_DEV_END__*/
 
   function scheduleUpdateHook(element, callback) {
@@ -2514,6 +2525,9 @@ export const REACTIVE_RUNTIME = String.raw`
             locals.event = undefined;
             locals.$event = undefined;
             try {
+              /*__WRNEXUS_DEV_START__*/
+              warnMissingBindingFunction(outStmt, function (name) { return peekScope(name); }, componentRoot);
+              /*__WRNEXUS_DEV_END__*/
               return runStmt(outStmt, locals);
             } catch (error) {
               console.error(

packages/csr/test/reactive.test.ts

@@ -689,6 +689,41 @@ test("development runtime warns once for an output with no parent binding", () =
   expect(String(warnings[0]?.[0])).toContain("WRN-DEV-OUTPUT-UNHANDLED");
 });
 
+test("development runtime warns when a component binding names a missing function", () => {
+  const win = new Window() as unknown as Window & Record<string, unknown>;
+  win.document.body.innerHTML =
+    `<div data-scope="">` +
+      `<div data-scope="" data-wrn-hydration="Child:x">` +
+      `<div data-wrn-events="save" data-wrn-out-save="missingSave(payload)"></div>` +
+      `</div></div>`;
+  (globalThis as Record<string, unknown>).window = win;
+  (globalThis as Record<string, unknown>).document = win.document;
+  (globalThis as Record<string, unknown>).location = win.location;
+  (globalThis as Record<string, unknown>).NodeFilter = (win as unknown as { NodeFilter: unknown }).NodeFilter;
+  (globalThis as Record<string, unknown>).MutationObserver = (win as unknown as { MutationObserver: unknown }).MutationObserver;
+  (globalThis as Record<string, unknown>).CustomEvent = (win as unknown as { CustomEvent: unknown }).CustomEvent;
+  const warnings: unknown[][] = [];
+  const originalWarn = console.warn;
+  const originalError = console.error;
+  console.warn = (...args: unknown[]) => warnings.push(args);
+  console.error = () => {};
+  try {
+    (0, eval)(getReactiveRuntime(true));
+    (win as unknown as { __wrnexusHydrateScopes?: (root: unknown) => void }).__wrnexusHydrateScopes?.(win.document);
+    const target = win.document.querySelector("[data-wrn-events]") as unknown as {
+      __wrnexusOutputHandlers?: Record<string, Set<(payload: unknown) => unknown>>;
+    };
+    target.__wrnexusOutputHandlers?.save?.forEach((handler) => handler({ id: 1 }));
+    target.__wrnexusOutputHandlers?.save?.forEach((handler) => handler({ id: 2 }));
+  } finally {
+    console.warn = originalWarn;
+    console.error = originalError;
+  }
+  expect(warnings).toHaveLength(1);
+  expect(String(warnings[0]?.[0])).toContain("WRN-DEV-BINDING-MISSING");
+  expect(String(warnings[0]?.[0])).toContain("missingSave");
+});
+
 test("production runtime strips development diagnostics", () => {
   const production = getReactiveRuntime();
   expect(production).not.toContain("WRN-DEV-");