js-on-sx: delete obj.key actually removes the key

js-delete-prop was setting value to js-undefined instead of
removing the key, so 'key' in obj remained true and proto-chain
lookup didn't fall through. Switched to dict-delete!.
Now delete Boolean.prototype.toString; Boolean.prototype.toString()
walks up to Object.prototype.toString and returns "[object Boolean]".
built-ins/Boolean: 21/27 → 22/27. conformance.sh: 148/148.

lib/js/runtime.sx

@@ -3517,7 +3517,7 @@
     (obj key)
     (cond
       ((dict? obj)
-        (begin (dict-set! obj (js-to-string key) js-undefined) true))
+        (begin (dict-delete! obj (js-to-string key)) true))
       (else true))))
 
 (define

lib/js/test262-scoreboard.json

@@ -1,45 +1,45 @@
 {
   "totals": {
     "pass": 44,
-    "fail": 3,
+    "fail": 6,
     "skip": 0,
-    "timeout": 3,
+    "timeout": 0,
     "total": 50,
     "runnable": 50,
     "pass_rate": 88.0
   },
   "categories": [
     {
-      "category": "built-ins/Number",
+      "category": "built-ins/Object",
       "total": 50,
       "pass": 44,
-      "fail": 3,
+      "fail": 6,
       "skip": 0,
-      "timeout": 3,
+      "timeout": 0,
       "pass_rate": 88.0,
       "top_failures": [
         [
-          "Timeout",
+          "ReferenceError (undefined symbol)",
-          3
+          4
         ],
         [
-          "Test262Error (assertion failed)",
+          "SyntaxError (parse/unsupported syntax)",
-          3
+          2
         ]
       ]
     }
   ],
   "top_failure_modes": [
     [
-      "Timeout",
+      "ReferenceError (undefined symbol)",
-      3
+      4
     ],
     [
-      "Test262Error (assertion failed)",
+      "SyntaxError (parse/unsupported syntax)",
-      3
+      2
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 110.0,
+  "elapsed_seconds": 32.3,
   "workers": 1
 }

lib/js/test262-scoreboard.md

@@ -1,24 +1,24 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 110.0s
+Wall time: 32.3s
 
-**Total:** 44/50 runnable passed (88.0%). Raw: pass=44 fail=3 skip=0 timeout=3 total=50.
+**Total:** 44/50 runnable passed (88.0%). Raw: pass=44 fail=6 skip=0 timeout=0 total=50.
 
 ## Top failure modes
 
-- **3x** Timeout
+- **4x** ReferenceError (undefined symbol)
-- **3x** Test262Error (assertion failed)
+- **2x** SyntaxError (parse/unsupported syntax)
 
 ## Categories (worst pass-rate first, min 10 runnable)
 
 | Category | Pass | Fail | Skip | Timeout | Total | Pass % |
 |---|---:|---:|---:|---:|---:|---:|
-| built-ins/Number | 44 | 3 | 0 | 3 | 50 | 88.0% |
+| built-ins/Object | 44 | 6 | 0 | 0 | 50 | 88.0% |
 
 ## Per-category top failures (min 10 runnable, worst first)
 
-### built-ins/Number  (44/50 — 88.0%)
+### built-ins/Object  (44/50 — 88.0%)
 
-- **3x** Timeout
+- **4x** ReferenceError (undefined symbol)
-- **3x** Test262Error (assertion failed)
+- **2x** SyntaxError (parse/unsupported syntax)

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 — **`delete obj.key` actually removes the key.** `js-delete-prop` was setting the value to `js-undefined` instead of removing the key, so subsequent `'key' in obj` returned true and proto-chain lookup didn't fall through to the parent. Switched to `dict-delete!` (existing SX primitive). Now `delete Boolean.prototype.toString; Boolean.prototype.toString()` correctly walks up to `Object.prototype.toString` and returns `"[object Boolean]"`. built-ins/Boolean: 21/27 → 22/27. conformance.sh: 148/148.
+
 - 2026-05-08 — **`Boolean(NaN) === false` (and `!NaN === true`).** `js-to-boolean` was returning `true` for NaN because NaN ≠ 0 by IEEE semantics, so the `(= v 0)` test fell through to the truthy-else clause. Per ES, NaN is one of the falsy values. Added a `(js-number-is-nan v)` clause. built-ins/Boolean: 19/27 → 21/27. conformance.sh: 148/148.
 
 - 2026-05-08 — **Global `eval(src)` actually evaluates the source.** Was returning the input string unchanged: `eval('1+2')` returned `"1+2"`, not `3`. Per spec, `eval(string)` parses and evaluates as JS; non-string input passes through. Wired the runtime stub through `js-eval` (which already does the lex/parse/transpile/eval pipeline) when the arg is a string. Fixes `String(eval('var x'))`, the harness internal `eval(...)`, and any test that calls `eval` for runtime evaluation. built-ins/String fail count: 13 → 11. conformance.sh: 148/148.