js-on-sx: arguments object is a mutable list copy

lib/js/transpile.sx

@@ -994,6 +994,12 @@
 
 (define
   js-arguments-build-form
+  (fn
+    (params)
+    (list (js-sym "js-list-copy") (js-arguments-build-form-raw params))))
+
+(define
+  js-arguments-build-form-raw
   (fn
     (params)
     (cond
@@ -1005,7 +1011,7 @@
         (list
           (js-sym "cons")
           (js-param-sym (first params))
-          (js-arguments-build-form (rest params)))))))
+          (js-arguments-build-form-raw (rest params)))))))
 
 (define
   js-param-init-forms

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.
 
+- 2026-05-10 — **`arguments` object inside functions is now a mutable list.** `js-arguments-build-form` produced `(cons p1 (cons p2 __extra_args__))` which yielded a structurally-shared (immutable) list — `arguments[1] = 7; arguments[1]++` raised "set-nth!: list is immutable". Wrapping the build in `js-list-copy` so each function entry constructs a fresh mutable list. Existing reads (`arguments.length`, `arguments[i]`) unaffected. Result: language/expressions/postfix-increment 14/30 → 15/30. conformance.sh: 148/148.
+
 - 2026-05-10 — **`String.prototype.split(undefined)` returns `[wholeString]`; function-expression bodies have spec-correct implicit `undefined` return.** (1) `js-string-method "split"` was calling `js-to-string` on the separator unconditionally, so `"undefinedd".split(undefined)` produced `["", "d"]` (split by `"undefined"`); also `limit=0` returned the whole-string list instead of `[]`. New arms: `undefined` separator → `[s]`, `limit=0` → `[]`, otherwise existing string-split. (2) Function expressions wrapped the body in `(call/cc (fn (__return__) (begin <stmts>)))` and used the begin's last expression as the implicit return value. So `function F(){ this.x = function(){return 99} }` returned the inner lambda (because `js-set-prop` returns the rhs), and `new F()` saw a callable return and replaced the freshly-allocated `this` with it — so `i.x` was missing. Append `nil` to the begin so the implicit completion is always `:js-undefined`; explicit `return` still works via call/cc as before. Result: built-ins/String/prototype/split 8/30 → 10/30. Constructors with function-valued `this.X` now keep their assignments. conformance.sh: 148/148.
 
 - 2026-05-10 — **Number/Boolean primitive method dispatch falls back to `Number.prototype` / `Boolean.prototype`.** When a user assigned a String method onto `Number.prototype` (e.g. `Number.prototype.toUpperCase = String.prototype.toUpperCase; NaN.toUpperCase()`), `js-invoke-number-method` rejected the unknown key with "is not a function (on number)" — it never walked the prototype. Added a fallback in both `js-invoke-number-method` and `js-invoke-boolean-method`: on unknown keys, `js-dict-get-walk` the constructor prototype; if found, `js-call-with-this` it. Result: built-ins/String/prototype/toUpperCase 16/25 → 19/25 (+3). Boolean 29/30. conformance.sh: 148/148.