js-on-sx: String(arr) honours Array.prototype.toString overrides

Was always emitting comma-joined via js-list-join, so user
mutations of Array.prototype.toString had no effect on String(arr)
/ "" + arr. Now look up the override via js-dict-get-walk and call
it on the list as this; fall back to (js-list-join v ",") when the
override doesn't return a string.
String fail count: 11 → 9. conformance.sh: 148/148.

lib/js/runtime.sx

@@ -1402,7 +1402,15 @@
                       (js-to-string result)))
                   "[object Object]"))))
           (cond
-            ((= (type-of v) "list") (js-list-join v ","))
+            ((= (type-of v) "list")
+              (let
+                ((tostr-fn (js-dict-get-walk (get Array "prototype") "toString")))
+                (if
+                  (= (type-of tostr-fn) "lambda")
+                  (let
+                    ((result (js-call-with-this v tostr-fn ())))
+                    (if (= (type-of result) "string") result (js-list-join v ",")))
+                  (js-list-join v ","))))
             ((js-function? v)
               (let
                 ((tostr-fn (js-dict-get-walk (get js-function-global "prototype") "toString")))

lib/js/test262-scoreboard.json

@@ -1,33 +1,37 @@
 {
   "totals": {
-    "pass": 86,
-    "fail": 11,
-    "skip": 1,
-    "timeout": 2,
-    "total": 100,
-    "runnable": 99,
-    "pass_rate": 86.9
+    "pass": 23,
+    "fail": 21,
+    "skip": 5,
+    "timeout": 1,
+    "total": 50,
+    "runnable": 45,
+    "pass_rate": 51.1
   },
   "categories": [
     {
-      "category": "built-ins/String",
-      "total": 100,
-      "pass": 86,
-      "fail": 11,
-      "skip": 1,
-      "timeout": 2,
-      "pass_rate": 86.9,
+      "category": "built-ins/Array",
+      "total": 50,
+      "pass": 23,
+      "fail": 21,
+      "skip": 5,
+      "timeout": 1,
+      "pass_rate": 51.1,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          10
+          18
         ],
         [
-          "Timeout",
+          "TypeError: not a function",
           2
         ],
         [
-          "SyntaxError (parse/unsupported syntax)",
+          "Timeout",
+          1
+        ],
+        [
+          "Unhandled: Not callable: {:2 43 :1 42 :0 41 :length 3}\\",
           1
         ]
       ]
@@ -36,18 +40,22 @@
   "top_failure_modes": [
     [
       "Test262Error (assertion failed)",
-      10
+      18
     ],
     [
-      "Timeout",
+      "TypeError: not a function",
       2
     ],
     [
-      "SyntaxError (parse/unsupported syntax)",
+      "Timeout",
+      1
+    ],
+    [
+      "Unhandled: Not callable: {:2 43 :1 42 :0 41 :length 3}\\",
       1
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 179.5,
+  "elapsed_seconds": 141.9,
   "workers": 1
 }

lib/js/test262-scoreboard.md

@@ -1,26 +1,28 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 179.5s
+Wall time: 141.9s
 
-**Total:** 86/99 runnable passed (86.9%). Raw: pass=86 fail=11 skip=1 timeout=2 total=100.
+**Total:** 23/45 runnable passed (51.1%). Raw: pass=23 fail=21 skip=5 timeout=1 total=50.
 
 ## Top failure modes
 
-- **10x** Test262Error (assertion failed)
-- **2x** Timeout
-- **1x** SyntaxError (parse/unsupported syntax)
+- **18x** Test262Error (assertion failed)
+- **2x** TypeError: not a function
+- **1x** Timeout
+- **1x** Unhandled: Not callable: {:2 43 :1 42 :0 41 :length 3}\
 
 ## Categories (worst pass-rate first, min 10 runnable)
 
 | Category | Pass | Fail | Skip | Timeout | Total | Pass % |
 |---|---:|---:|---:|---:|---:|---:|
-| built-ins/String | 86 | 11 | 1 | 2 | 100 | 86.9% |
+| built-ins/Array | 23 | 21 | 5 | 1 | 50 | 51.1% |
 
 ## Per-category top failures (min 10 runnable, worst first)
 
-### built-ins/String  (86/99 — 86.9%)
+### built-ins/Array  (23/45 — 51.1%)
 
-- **10x** Test262Error (assertion failed)
-- **2x** Timeout
-- **1x** SyntaxError (parse/unsupported syntax)
+- **18x** Test262Error (assertion failed)
+- **2x** TypeError: not a function
+- **1x** Timeout
+- **1x** Unhandled: Not callable: {:2 43 :1 42 :0 41 :length 3}\

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 — **`String(arr)` consults `Array.prototype.toString` (not the hardcoded join).** Was always emitting the comma-joined elements via `js-list-join`, so user-visible mutations of `Array.prototype.toString` had no effect on `String(arr)` / `"" + arr`. Now look up the override via `js-dict-get-walk` and call it on the list as `this`; fall back to `(js-list-join v ",")` when the override doesn't return a string. Default behaviour preserved (Array.prototype.toString already calls `js-list-join`). built-ins/String fail count: 11 → 9. conformance.sh: 148/148.
+
 - 2026-05-08 — **Top-level `this` resolves to the global object.** Per non-strict ES script semantics, `this` at the top level is the global object (window/global/globalThis). Was throwing "Undefined symbol: this" because the SX let-wrap added by `js-eval` didn't bind `this`. Two-part fix: (1) added `js-global-this` runtime variable, set to `js-global` after globals are defined, with `js-this` falling back to it when no `this` is currently active; (2) `js-eval` wraps the transpiled body in `(let ((this (js-this))) ...)` so the JS-source `this` resolves to the function's bound `this` or, at top level, to the global. Fixes `String(this)`, `this.Object === Object`, etc. built-ins/Object: 46/50 → 47/50. conformance.sh: 148/148.
 
 - 2026-05-08 — **Comma operator `(a, b, c)` parses and evaluates left-to-right, returning last.** Was failing with `Expected punct ')' got punct ','` because `jp-try-arrow-or-paren` only consumed a single assignment expression. Added `jp-parse-comma-seq` / `jp-parse-comma-seq-rest` helpers that build a `js-comma` AST node with the list of expressions; the transpiler emits `(begin ...)` which evaluates each in order and returns the last. Fixes `Object((null,2,3),1,2)`-style tests. built-ins/Object: 44/50 → 46/50. conformance.sh: 148/148.