Report missing JsVar parents consistently

lib/assets/jaws.js

@@ -477,7 +477,7 @@ function jawsStoreKeys(name) {
 function jawsPathTarget(keys) {
 	let obj = window;
 	for (let i = 0; i < keys.length - 1; i++) {
-		obj = obj[keys[i]];
+		obj = obj?.[keys[i]];
 	}
 	if (obj == null) {
 		throw "jaws: JsVar path undefined: " + keys.join('.');

lib/assets/js_test.go

@@ -616,6 +616,40 @@ process.stdout.write(JSON.stringify({
 	}
 }
 
+func TestJawsJS_MissingJsVarParent(t *testing.T) {
+	raw := runJawsJSSnippet(t, `
+function attach(path) {
+	const elem = {
+		id: "Jid.1",
+		dataset: { jawsstore: path, jawsdata: "1" },
+		hasAttribute: function(name) { return name === "data-jawsstore"; },
+	};
+	try { jawsAttach(elem, true); return ""; }
+	catch (err) { return String(err); }
+}
+window.ok = {};
+process.stdout.write(JSON.stringify({
+	shallow: attach("app.state"),
+	deep: attach("a.b.c"),
+	valid: attach("ok.state"),
+	value: window.ok.state,
+}));
+`)
+	var got struct {
+		Shallow string `json:"shallow"`
+		Deep    string `json:"deep"`
+		Valid   string `json:"valid"`
+		Value   int    `json:"value"`
+	}
+	if err := json.Unmarshal([]byte(raw), &got); err != nil {
+		t.Fatalf("failed to parse snippet output %q: %v", raw, err)
+	}
+	if got.Shallow != "jaws: JsVar path undefined: app.state" ||
+		got.Deep != "jaws: JsVar path undefined: a.b.c" || got.Valid != "" || got.Value != 1 {
+		t.Fatalf("JsVar parent handling = %+v", got)
+	}
+}
+
 func TestJawsJS_StoreReadAndOptimisticProposal(t *testing.T) {
 	raw := runJawsJSSnippet(t, `
 function FakeSocket() { this.readyState = 1; this.sent = []; }

lib/ui/jsvar.go

@@ -140,6 +140,9 @@ type JsVarStore[T any] struct {
 // The name is a dot-separated path from window using JavaScript identifiers
 // with ASCII letters, digits, underscore, or dollar sign. No component may be
 // "__proto__", "constructor", or "prototype"; the limit is 4096 bytes.
+// For dotted names, every parent object must exist on window before jaws.js runs;
+// otherwise the client stops before opening its WebSocket. A top-level name
+// needs no existing property.
 // Initial data and patches assign to that live path, including browser-owned
 // properties and setters.
 //
@@ -168,6 +171,8 @@ func NewJsVarStore[T any](jw *jaws.Jaws, name string, locker sync.Locker, value
 // Render each binding once during the Request's initial page render, outside
 // regions that may later be replaced or removed. Browser names may neither
 // duplicate nor contain one another within a Request.
+// For dotted names, the parent objects must exist on window before jaws.js runs;
+// otherwise the client stops before opening its WebSocket.
 func (store *JsVarStore[T]) Bind() *JsVarBinding[T] {
 	return &JsVarBinding[T]{store: store}
 }