diff --git a/lib/js/runtime.sx b/lib/js/runtime.sx
index c88b500f..c77c475f 100644
--- a/lib/js/runtime.sx
+++ b/lib/js/runtime.sx
@@ -88,6 +88,34 @@
       (= name "name")
       (= name "length"))))
 
+(define
+  js-fn-length
+  (fn
+    (f)
+    (let
+      ((t (type-of f)))
+      (cond
+        ((= t "lambda") (js-count-real-params (lambda-params f)))
+        ((= t "function") 0)
+        ((= t "component") 0)
+        ((and (= t "dict") (contains? (keys f) "__callable__"))
+          (js-fn-length (get f "__callable__")))
+        (else 0)))))
+
+(define
+  js-count-real-params
+  (fn
+    (params)
+    (cond
+      ((empty? params) 0)
+      (else
+        (let
+          ((first (first params)))
+          (if
+            (= first "&rest")
+            0
+            (+ 1 (js-count-real-params (rest params)))))))))
+
 (define
   js-invoke-function-method
   (fn
@@ -122,9 +150,16 @@
             (js-call-with-this this-arg recv (js-list-concat bound more)))))
       ((= key "toString") "function () { [native code] }")
       ((= key "name") "")
-      ((= key "length") 0)
+      ((= key "length") (js-fn-length recv))
       (else :js-undefined))))
 
+;; parse a decimal number from a trimmed non-empty string.
+;;   s      — source
+;;   i      — cursor
+;;   acc    — integer part so far (or total for decimals)
+;;   sign   — 1 or -1
+;;   frac?  — are we past the decimal point
+;;   fdiv   — divisor used to scale fraction digits (only if frac?)
 (define
   js-invoke-function-bound
   (fn
@@ -167,13 +202,6 @@
             (js-to-string key)
             " is not a function (on number)"))))))
 
-;; parse a decimal number from a trimmed non-empty string.
-;;   s      — source
-;;   i      — cursor
-;;   acc    — integer part so far (or total for decimals)
-;;   sign   — 1 or -1
-;;   frac?  — are we past the decimal point
-;;   fdiv   — divisor used to scale fraction digits (only if frac?)
 (define
   js-invoke-boolean-method
   (fn
@@ -188,6 +216,8 @@
             (js-to-string key)
             " is not a function (on boolean)"))))))
 
+;; ── String coercion (ToString) ────────────────────────────────────
+
 (define
   js-num-to-str-radix
   (fn
@@ -217,8 +247,9 @@
         ((d (mod n radix)) (rest (js-math-trunc (/ n radix))))
         (js-num-to-str-radix-rec rest radix (str (js-digit-char d) acc))))))
 
-;; ── String coercion (ToString) ────────────────────────────────────
+;; ── Arithmetic (JS rules) ─────────────────────────────────────────
 
+;; JS `+`: if either operand is a string → string concat, else numeric.
 (define
   js-digit-char
   (fn
@@ -262,9 +293,6 @@
                         (js-to-string (js-math-trunc frac-part))
                         d))))))))))))
 
-;; ── Arithmetic (JS rules) ─────────────────────────────────────────
-
-;; JS `+`: if either operand is a string → string concat, else numeric.
 (define
   js-pow-int
   (fn (b e) (if (<= e 0) 1 (* b (js-pow-int b (- e 1))))))
@@ -360,11 +388,14 @@
       ((= name "toLocaleString") "[object Object]")
       (else js-undefined))))
 
+;; Bitwise + logical-not
 (define js-upper-case (fn (s) (js-case-loop s 0 "" true)))
 
 (define js-lower-case (fn (s) (js-case-loop s 0 "" false)))
 
-;; Bitwise + logical-not
+;; ── Equality ──────────────────────────────────────────────────────
+
+;; Strict equality (===): no coercion; js-undefined matches js-undefined.
 (define
   js-case-loop
   (fn
@@ -439,9 +470,9 @@
       ((= code 122) "z")
       (else ""))))
 
-;; ── Equality ──────────────────────────────────────────────────────
-
-;; Strict equality (===): no coercion; js-undefined matches js-undefined.
+;; Abstract equality (==): type coercion rules.
+;; Simplified: number↔string coerce both to number; null == undefined;
+;; everything else falls back to strict equality.
 (define
   js-invoke-method-dyn
   (fn (recv key args) (js-invoke-method recv key args)))
@@ -457,9 +488,11 @@
         (js-call-with-this :js-undefined (get fn-val "__callable__") args))
       (else (js-call-with-this :js-undefined fn-val args)))))
 
-;; Abstract equality (==): type coercion rules.
-;; Simplified: number↔string coerce both to number; null == undefined;
-;; everything else falls back to strict equality.
+;; ── Relational comparisons ────────────────────────────────────────
+
+;; Abstract relational comparison from ES5.
+;; Numbers compare numerically; two strings compare lexicographically;
+;; mixed types coerce both to numbers.
 (define
   js-new-call
   (fn
@@ -488,11 +521,6 @@
           ((proto (js-get-ctor-proto ctor)))
           (js-instanceof-walk obj proto))))))
 
-;; ── Relational comparisons ────────────────────────────────────────
-
-;; Abstract relational comparison from ES5.
-;; Numbers compare numerically; two strings compare lexicographically;
-;; mixed types coerce both to numbers.
 (define
   js-instanceof-walk
   (fn
@@ -562,6 +590,13 @@
           nil)
         this))))
 
+;; ── Property access ───────────────────────────────────────────────
+
+;; obj[key] or obj.key in JS. Handles:
+;;   • dicts keyed by string
+;;   • lists indexed by number (incl. .length)
+;;   • strings indexed by number (incl. .length)
+;; Returns js-undefined if the key is absent.
 (define
   RangeError
   (fn
@@ -598,13 +633,6 @@
           nil)
         this))))
 
-;; ── Property access ───────────────────────────────────────────────
-
-;; obj[key] or obj.key in JS. Handles:
-;;   • dicts keyed by string
-;;   • lists indexed by number (incl. .length)
-;;   • strings indexed by number (incl. .length)
-;; Returns js-undefined if the key is absent.
 (define
   ReferenceError
   (fn
@@ -623,6 +651,7 @@
           nil)
         this))))
 
+;; Setter — mutates the dict. Returns the new value (JS assignment yields rhs).
 (define
   js-function?
   (fn
@@ -635,15 +664,17 @@
         (= t "component")
         (and (= t "dict") (contains? (keys v) "__callable__"))))))
 
-(define __js_proto_table__ (dict))
-
-;; Setter — mutates the dict. Returns the new value (JS assignment yields rhs).
-(define __js_next_id__ (dict))
-
 ;; ── Short-circuit logical ops ─────────────────────────────────────
 
 ;; `a && b` in JS: if a is truthy return b else return a. The thunk
 ;; form defers evaluation of b — the transpiler passes (fn () b).
+(define __js_proto_table__ (dict))
+
+(define __js_next_id__ (dict))
+
+;; ── console.log ───────────────────────────────────────────────────
+
+;; Trivial bridge. `log-info` is available on OCaml; fall back to print.
 (dict-set! __js_next_id__ "n" 0)
 
 (define
@@ -657,9 +688,8 @@
         (else
           (let ((p (dict))) (begin (dict-set! __js_proto_table__ id p) p)))))))
 
-;; ── console.log ───────────────────────────────────────────────────
+;; ── Math object ───────────────────────────────────────────────────
 
-;; Trivial bridge. `log-info` is available on OCaml; fall back to print.
 (define
   js-reset-ctor-proto!
   (fn
@@ -667,15 +697,11 @@
     (let
       ((id (js-ctor-id ctor)) (p (dict)))
       (begin (dict-set! __js_proto_table__ id p) p))))
-
 (define
   js-set-ctor-proto!
   (fn
     (ctor proto)
     (let ((id (js-ctor-id ctor))) (dict-set! __js_proto_table__ id proto))))
-
-;; ── Math object ───────────────────────────────────────────────────
-
 (define
   js-ctor-id
   (fn
@@ -766,8 +792,14 @@
                   (or (= prev "e") (= prev "E"))
                   (js-is-numeric-loop s (+ i 1) sawdigit sawdot sawe)
                   false)))
-            (else false)))))))
+            (else false)))))))   ; deterministic placeholder for tests
+
 (define js-parse-num-safe (fn (s) (cond (else (js-num-from-string s)))))
+
+;; The global object — lookup table for JS names that aren't in the
+;; SX env. Transpiled idents look up locally first; globals here are a
+;; fallback, but most slice programs reference `console`, `Math`,
+;; `undefined` as plain symbols, which we bind as defines above.
 (define
   js-num-from-string
   (fn
@@ -776,14 +808,10 @@
       ((trimmed (js-trim s)))
       (cond
         ((= trimmed "") 0)
-        (else (js-parse-decimal trimmed 0 0 1 false 0))))))   ; deterministic placeholder for tests
+        (else (js-parse-decimal trimmed 0 0 1 false 0))))))
 
 (define js-trim (fn (s) (js-trim-left (js-trim-right s))))
 
-;; The global object — lookup table for JS names that aren't in the
-;; SX env. Transpiled idents look up locally first; globals here are a
-;; fallback, but most slice programs reference `console`, `Math`,
-;; `undefined` as plain symbols, which we bind as defines above.
 (define
   js-trim-left
   (fn (s) (let ((n (len s))) (js-trim-left-at s n 0))))
@@ -1985,7 +2013,7 @@
           (cond
             ((= key "prototype") (js-get-ctor-proto obj))
             ((= key "name") "")
-            ((= key "length") 0)
+            ((= key "length") (js-fn-length obj))
             (else (js-invoke-function-bound obj key))))
         (else js-undefined))))
   (define
diff --git a/lib/js/test.sh b/lib/js/test.sh
index 8625521e..a9e7a83c 100755
--- a/lib/js/test.sh
+++ b/lib/js/test.sh
@@ -1213,6 +1213,20 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 3709)
 (eval "(js-eval \"var a=[1,2,3]; a.keys().join(',')\")")
 
+;; ── Phase 11.fnlen: fn.length uses arity from lambda-params ─
+(epoch 4100)
+(eval "(js-eval \"function f(){} f.length\")")
+(epoch 4101)
+(eval "(js-eval \"function f(a){} f.length\")")
+(epoch 4102)
+(eval "(js-eval \"function f(a,b,c){} f.length\")")
+(epoch 4103)
+(eval "(js-eval \"function f(a, ...rest){} f.length\")")
+(epoch 4104)
+(eval "(js-eval \"Math.abs.length\")")
+(epoch 4105)
+(eval "(js-eval \"Math.floor.length\")")
+
 ;; ── Phase 11.coerce2: Number coercion edge cases ────────────
 (epoch 4000)
 (eval "(js-eval \"Number('abc')\")")
@@ -1969,6 +1983,14 @@ check 3707 "arr.toReversed"                '"2,1,3"'
 check 3708 "arr.toSorted"                  '"1,1,3,4,5"'
 check 3709 "arr.keys"                      '"0,1,2"'
 
+# ── Phase 11.fnlen: fn.length arity ───────────────────────────
+check 4100 "fn () length"                  '0'
+check 4101 "fn (a) length"                 '1'
+check 4102 "fn (a,b,c) length"             '3'
+check 4103 "fn rest length"                '1'
+check 4104 "Math.abs.length"               '1'
+check 4105 "Math.floor.length"             '1'
+
 # ── Phase 11.coerce2: Number coercion edge cases ─────────────
 check 4001 "isNaN(Number('abc'))"          'true'
 check 4002 "parseInt leading digits"       '123'