js-on-sx: Error.isError + [[ErrorData]] slot + verifyEqualTo

lib/js/runtime.sx

@@ -834,10 +834,36 @@
                 (= (len args) 0)
                 ""
                 (js-to-string (nth args 0))))
-            (dict-set! this "name" "Error"))
+            (dict-set! this "name" "Error")
+            (dict-set! this "__js_error_data__" true))
           nil)
         this))))
 
+(define
+  js-error-is-error
+  (fn
+    (&rest args)
+    (cond
+      ((= (len args) 0) false)
+      (else
+        (let
+          ((v (nth args 0)))
+          (and
+            (= (type-of v) "dict")
+            (or
+              (contains? (keys v) "__js_error_data__")
+              (js-error-proto-walk? v))))))))
+
+(define
+  js-error-proto-walk?
+  (fn
+    (v)
+    (cond
+      ((not (= (type-of v) "dict")) false)
+      ((contains? (keys v) "__js_error_data__") true)
+      ((not (contains? (keys v) "__proto__")) false)
+      (else (js-error-proto-walk? (get v "__proto__"))))))
+
 ;; ── Math object ───────────────────────────────────────────────────
 
 (define
@@ -857,7 +883,8 @@
                 (= (len args) 0)
                 ""
                 (js-to-string (nth args 0))))
-            (dict-set! this "name" "TypeError"))
+            (dict-set! this "name" "TypeError")
+            (dict-set! this "__js_error_data__" true))
           nil)
         this))))
 (define
@@ -877,7 +904,8 @@
                 (= (len args) 0)
                 ""
                 (js-to-string (nth args 0))))
-            (dict-set! this "name" "RangeError"))
+            (dict-set! this "name" "RangeError")
+            (dict-set! this "__js_error_data__" true))
           nil)
         this))))
 (define
@@ -897,7 +925,8 @@
                 (= (len args) 0)
                 ""
                 (js-to-string (nth args 0))))
-            (dict-set! this "name" "SyntaxError"))
+            (dict-set! this "name" "SyntaxError")
+            (dict-set! this "__js_error_data__" true))
           nil)
         this))))
 (define
@@ -917,7 +946,8 @@
                 (= (len args) 0)
                 ""
                 (js-to-string (nth args 0))))
-            (dict-set! this "name" "ReferenceError"))
+            (dict-set! this "name" "ReferenceError")
+            (dict-set! this "__js_error_data__" true))
           nil)
         this))))
 (define
@@ -935,7 +965,8 @@
               this
               "message"
               (if (empty? args) "" (js-to-string (nth args 0))))
-            (dict-set! this "name" "URIError")))
+            (dict-set! this "name" "URIError")
+            (dict-set! this "__js_error_data__" true)))
         this))))
 (define
   EvalError
@@ -952,8 +983,13 @@
               this
               "message"
               (if (empty? args) "" (js-to-string (nth args 0))))
-            (dict-set! this "name" "EvalError")))
+            (dict-set! this "name" "EvalError")
+            (dict-set! this "__js_error_data__" true)))
         this))))
+
+(define AggregateError :js-undefined)
+
+(define SuppressedError :js-undefined)
 (define
   js-function?
   (fn
@@ -2965,6 +3001,8 @@
           (js-dict-get-walk obj (js-to-string key)))
         ((and (= obj Promise) (dict-has? __js_promise_statics__ (js-to-string key)))
           (get __js_promise_statics__ (js-to-string key)))
+        ((and (= obj Error) (= (js-to-string key) "isError"))
+          js-error-is-error)
         ((and (js-function? obj) (or (= key "prototype") (= key "name") (= key "length") (= key "call") (= key "apply") (= key "bind") (= key "constructor")))
           (cond
             ((= key "prototype") (js-get-ctor-proto obj))

lib/js/test262-runner.py

@@ -134,6 +134,9 @@ var verifyProperty = function (obj, name, desc, opts) {
   }
 };
 var verifyPrimordialProperty = verifyProperty;
+var verifyEqualTo = function (obj, name, value) {
+  assert.sameValue(obj[name], value, name + " equals");
+};
 var verifyNotEnumerable = function (o, n, v, w, x) { };
 var verifyNotWritable = function (o, n, v, w, x) { };
 var verifyNotConfigurable = function (o, n, v, w, x) { };

plans/js-on-sx.md

@@ -158,6 +158,8 @@ Each item: implement → tests → update progress. Mark `[x]` when tests green.
 
 Append-only record of completed iterations. Loop writes one line per iteration: date, what was done, test count delta.
 
+- 2026-05-08 — **`Error.isError` static + `[[ErrorData]]` slot + `verifyEqualTo` harness helper.** Added `Error.isError(v)` per the Stage-3 proposal: returns `true` only for objects with the internal `[[ErrorData]]` slot. Implemented as `__js_error_data__: true` set on `this` by every Error subclass constructor (Error/TypeError/RangeError/SyntaxError/ReferenceError/URIError/EvalError); `js-error-is-error` walks `__proto__` looking for the marker. Wired through the lambda-static-prop path next to the existing `Promise.resolve` / `Promise.reject` lookup. Defined `AggregateError` and `SuppressedError` as `:js-undefined` so `typeof AggregateError !== 'undefined'` resolves cleanly (without these, the bare ident lookup throws ReferenceError). Added `verifyEqualTo` to the harness — `propertyHelper.js` includes it, used by `Error/message_property.js` etc. Result: built-ins/Error 6/30 → 11/30 (+5), Error/isError sub-suite 0/9 → 5/9. Map/Object unchanged. conformance.sh: 148/148.
+
 - 2026-05-08 — **Harness: `$DONE` / `asyncTest` and `checkSequence` / `checkSettledPromises` stubs added.** Async-flagged Promise tests call `$DONE(err?)` to signal completion — we run synchronously and drain microtasks, so the stub just throws a `Test262Error` if `err` is passed. `asyncTest(fn)` wraps the test fn in `Promise.resolve().then(..., $DONE)`. `checkSequence(arr, msg)` (from `promiseHelper.js`) verifies `arr[i] === i+1` — used by ordering tests on `Promise.all` / `Promise.race`. `checkSettledPromises(actual, expected, msg)` matches what `Promise.allSettled` tests expect. Result: built-ins/Promise 1/30 → 15/30 (50%, 14 new passes from previously ReferenceError'ing on `$DONE`/`checkSequence`). conformance.sh: 148/148.
 
 - 2026-05-08 — **`Map` and `Set` constructors with full instance API.** Both were undefined globally — every test in those categories died at `new Map()` / `new Set()` with ReferenceError. Implemented as plain SX storage on the instance dict (`__map_keys__` + `__map_vals__` parallel lists for Map, `__set_items__` for Set) using SX `=` for key/value comparisons. Wired prototype methods: `.get`, `.set`, `.has`, `.delete`, `.clear`, `.forEach`, `.keys`, `.values`, `.entries` for Map; `.add`, `.has`, `.delete`, `.clear`, `.forEach`, `.keys`, `.values`, `.entries` for Set. `.size` is a real own property updated on every mutation (no getters). Constructors use the dict-with-`__callable__` pattern (like `Object`) so `Map.length`, `Map.name`, `Map.prototype` work as regular dict reads. Constructor accepts an iterable of `[k,v]` pairs (Map) or values (Set). Added `Map`/`Set` to `js-global` and to the prototype-chain post-init block. Result: built-ins/Map 1/30 → 18/30 (60%), built-ins/Set 0/30 → 15/30 (50%, rest mostly timeouts on iterator-protocol tests). conformance.sh: 148/148.