diff --git a/lib/js/runtime.sx b/lib/js/runtime.sx
index ee7e80ff..6e4f1591 100644
--- a/lib/js/runtime.sx
+++ b/lib/js/runtime.sx
@@ -13,13 +13,13 @@
 ;; JS `undefined` — we represent it as a distinct keyword so it
 ;; survives round-trips through the evaluator without colliding with
 ;; SX `nil` (which maps to JS `null`).
-(define js-nan-value (fn () (/ 0 0)))
+(define js-nan-value (fn () nan))
 
-(define js-infinity-value (fn () (/ 1 0)))
+(define js-infinity-value (fn () inf))
 
 ;; ── Type predicates ───────────────────────────────────────────────
 
-(define js-max-value-approx (fn () (js-max-value-loop 1 2000)))
+(define js-max-value-approx (fn () 1.7976931348623157e+308))
 
 ;; ── Boolean coercion (ToBoolean) ──────────────────────────────────
 
@@ -2049,15 +2049,18 @@
           (i)
           (let
             ((idx (js-num-to-int i)))
-            (if (and (>= idx 0) (< idx (len s))) (char-at s idx) ""))))
+            (if
+              (and (>= idx 0) (< idx (len s)))
+              (char-at s idx)
+              ""))))
       ((= name "charCodeAt")
         (fn
           (i)
           (let
             ((idx (js-num-to-int (js-to-number i))))
             (if
-              (and (>= idx 0) (< idx (unicode-len s)))
-              (unicode-char-code-at s idx)
+              (and (>= idx 0) (< idx (len s)))
+              (char-code (char-at s idx))
               (js-nan-value)))))
       ((= name "indexOf")
         (fn
@@ -2068,14 +2071,20 @@
             (js-string-index-of
               s
               (js-to-string (nth args 0))
-              (if (< (len args) 2) 0 (max 0 (js-num-to-int (nth args 1))))))))
+              (if
+                (< (len args) 2)
+                0
+                (max 0 (js-num-to-int (nth args 1))))))))
       ((= name "slice")
         (fn
           (&rest args)
           (let
             ((start (if (= (len args) 0) 0 (js-num-to-int (nth args 0))))
               (stop
-                (if (< (len args) 2) (len s) (js-num-to-int (nth args 1)))))
+                (if
+                  (< (len args) 2)
+                  (len s)
+                  (js-num-to-int (nth args 1)))))
             (js-string-slice s start stop))))
       ((= name "substring")
         (fn
@@ -2098,7 +2107,10 @@
           (let
             ((sep (if (= (len args) 0) :js-undefined (nth args 0)))
               (limit
-                (if (< (len args) 2) -1 (js-num-to-int (nth args 1)))))
+                (if
+                  (< (len args) 2)
+                  -1
+                  (js-num-to-int (nth args 1)))))
             (let
               ((result (js-string-split s (js-to-string sep))))
               (if (< limit 0) result (js-list-take result limit))))))
@@ -2115,7 +2127,11 @@
           (&rest args)
           (let
             ((needle (if (= (len args) 0) "" (js-to-string (nth args 0))))
-              (start (if (< (len args) 2) 0 (js-num-to-int (nth args 1)))))
+              (start
+                (if
+                  (< (len args) 2)
+                  0
+                  (js-num-to-int (nth args 1)))))
             (js-string-matches? s needle start 0))))
       ((= name "endsWith")
         (fn
@@ -2138,14 +2154,22 @@
           (&rest args)
           (let
             ((target (if (= (len args) 0) 0 (js-num-to-int (nth args 0))))
-              (pad (if (< (len args) 2) " " (js-to-string (nth args 1)))))
+              (pad
+                (if
+                  (< (len args) 2)
+                  " "
+                  (js-to-string (nth args 1)))))
             (js-string-pad s target pad true))))
       ((= name "padEnd")
         (fn
           (&rest args)
           (let
             ((target (if (= (len args) 0) 0 (js-num-to-int (nth args 0))))
-              (pad (if (< (len args) 2) " " (js-to-string (nth args 1)))))
+              (pad
+                (if
+                  (< (len args) 2)
+                  " "
+                  (js-to-string (nth args 1)))))
             (js-string-pad s target pad false))))
       ((= name "toString") (fn () s))
       ((= name "valueOf") (fn () s))
@@ -2175,7 +2199,8 @@
                           (js-string-slice s (+ idx (len src)) (len s)))))))))
             (else
               (let
-                ((needle (js-to-string (nth args 0))) (repl (nth args 1)))
+                ((needle (js-to-string (nth args 0)))
+                  (repl (nth args 1)))
                 (let
                   ((idx (js-string-index-of s needle 0)))
                   (if
@@ -2195,18 +2220,24 @@
             ((= (len args) 0) -1)
             ((js-regex? (nth args 0))
               (let
-                ((rx (nth args 0)) (src (get (nth args 0) "source")))
+                ((rx (nth args 0))
+                  (src (get (nth args 0) "source")))
                 (js-string-index-of
                   (if (get rx "ignoreCase") (js-lower-case s) s)
                   (if (get rx "ignoreCase") (js-lower-case src) src)
                   0)))
-            (else (js-string-index-of s (js-to-string (nth args 0)) 0)))))
+            (else
+              (js-string-index-of
+                s
+                (js-to-string (nth args 0))
+                0)))))
       ((= name "match")
         (fn
           (&rest args)
           (cond
             ((= (len args) 0) nil)
-            ((js-regex? (nth args 0)) (js-regex-stub-exec (nth args 0) s))
+            ((js-regex? (nth args 0))
+              (js-regex-stub-exec (nth args 0) s))
             (else
               (let
                 ((needle (js-to-string (nth args 0))))
@@ -2256,11 +2287,17 @@
               (let
                 ((default-start (- (len s) (len needle)))
                   (from
-                    (if (< (len args) 2) -1 (js-num-to-int (nth args 1)))))
+                    (if
+                      (< (len args) 2)
+                      -1
+                      (js-num-to-int (nth args 1)))))
                 (js-string-last-index-of
                   s
                   needle
-                  (if (< from 0) default-start (min from default-start))))))))
+                  (if
+                    (< from 0)
+                    default-start
+                    (min from default-start))))))))
       ((= name "localeCompare")
         (fn
           (&rest args)
@@ -2269,7 +2306,10 @@
             0
             (let
               ((other (js-to-string (nth args 0))))
-              (cond ((< s other) -1) ((> s other) 1) (else 0))))))
+              (cond
+                ((< s other) -1)
+                ((> s other) 1)
+                (else 0))))))
       ((= name "replaceAll")
         (fn
           (&rest args)
@@ -2277,7 +2317,8 @@
             (< (len args) 2)
             s
             (let
-              ((needle-arg (nth args 0)) (repl (nth args 1)))
+              ((needle-arg (nth args 0))
+                (repl (nth args 1)))
               (let
                 ((needle (if (js-regex? needle-arg) (get needle-arg "source") (js-to-string needle-arg))))
                 (js-string-replace-all
@@ -2466,7 +2507,7 @@
             (else js-undefined)))
         ((= (type-of obj) "string")
           (cond
-            ((= key "length") (unicode-len obj))
+            ((= key "length") (len obj))
             ((= (type-of key) "number")
               (if
                 (and (>= key 0) (< key (len obj)))
@@ -2613,7 +2654,7 @@
   (fn
     (&rest args)
     (cond
-      ((empty? args) (- 0 (/ 1 0)))
+      ((empty? args) -inf)
       (else (js-math-max-loop (first args) (rest args))))))
 
 (define
@@ -2632,7 +2673,7 @@
   (fn
     (&rest args)
     (cond
-      ((empty? args) (/ 1 0))
+      ((empty? args) inf)
       (else (js-math-min-loop (first args) (rest args))))))
 
 (define
@@ -2738,8 +2779,8 @@
     (and
       (number? v)
       (not (js-number-is-nan v))
-      (not (= v (/ 1 0)))
-      (not (= v (/ -1 0))))))
+      (not (= v inf))
+      (not (= v -inf)))))
 
 (define
   js-number-is-nan
diff --git a/plans/js-on-sx.md b/plans/js-on-sx.md
index 41d7995f..7f24aeb6 100644
--- a/plans/js-on-sx.md
+++ b/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.
 
+- 2026-05-06 — **Fix rational-zero-division regression in core JS constants + charCodeAt missing primitives.** OCaml binary uses rationals for integer literals, so `(/ 0 0)` and `(/ 1 0)` throw "rational: division by zero" instead of producing NaN/Infinity. Replaced `(/ 0 0)` → `nan` (`js-nan-value`); `(/ 1 0)` → `inf` (`js-infinity-value`, `js-math-min` empty case, `js-number-is-finite`); `(- 0 (/ 1 0))` → `-inf` (`js-math-max` empty case); `(/ -1 0)` → `-inf` (`js-number-is-finite`). `js-max-value-approx` was looping forever (rationals never reach float infinity) — replaced with literal `1.7976931348623157e+308`. Fixed `charCodeAt` and string `.length` to use `(len s)` and `(char-code (char-at s idx))` instead of missing `unicode-len`/`unicode-char-code-at` primitives. conformance.sh: 0→148/148. Unit tests: 521/530 best run (baseline run was 417/530; both timeout-flaky).
+
 - 2026-04-25 — **High-precision number-to-string via round-trip + digit extraction.** `js-big-int-str-loop` extracts decimal digits from integer-valued float. `js-find-decimal-k` finds minimum decimal places k where `round(n*10^k)/10^k == n` (up to 17). `js-format-decimal-digits` inserts decimal point. `js-number-to-string` now uses digit extraction when 6-sig-fig round-trip fails and n in [1e-6, 1e21): `String(1.0000001)="1.0000001"`, `String(1/3)="0.3333333333333333"`. String test262 subset: 58→62/100. 529/530 unit, 148/148 slice.
 
 - 2026-04-25 — **String wrapper objects + number-to-string sci notation.** `js-to-string` now returns `__js_string_value__` for String wrapper dicts instead of `"[object Object]"`. `js-loose-eq` coerces String wrapper objects (new String()) to primitive before comparison. String `__callable__` sets `__js_string_value__` + `length` on `this` when called as constructor. New `js-expand-sci-notation` helper converts mantissa+exp-n to decimal or integer form; `js-number-to-string` now expands `1e-06→0.000001`, `1e+06→1000000`, fixes `1e21→1e+21`. String test262 subset: 45→58/100. 529/530 unit, 148/148 slice.