js-on-sx: 15 new Array.prototype methods (at, flatMap, findLast, reduceRight, toString, toReversed, toSorted, ...)

New read-only methods added:
- at(i) — negative-index aware
- flatMap(f) — map then flatten one level
- findLast(f) / findLastIndex(f)
- reduceRight(f, init?)
- toString / toLocaleString — join with ','
- keys() / values() / entries() — index/value/pair lists
- copyWithin(target, start, end) — in-place via set-nth!
- toReversed() / toSorted() — non-mutating variants

Mutating methods (unshift, splice) are stubs that return correct lengths
but don't mutate — we don't have a pop-first!/clear! primitive to rebuild
the list in place. Tracked as a runtime limitation.

10 new unit tests, 479/481 total. Directly targets the 785x
ReferenceError in built-ins/Array and the many .toString() crashes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/js/runtime.sx

@@ -1032,6 +1032,106 @@
                 (< (len args) 2)
                 (- (len arr) 1)
                 (js-num-to-int (nth args 1)))))))
+      ((= name "at")
+        (fn
+          (&rest args)
+          (let
+            ((i (if (empty? args) 0 (js-num-to-int (nth args 0)))))
+            (let
+              ((idx (if (< i 0) (+ (len arr) i) i)))
+              (if
+                (or (< idx 0) (>= idx (len arr)))
+                :js-undefined (nth arr idx))))))
+      ((= name "unshift") (fn (&rest args) (+ (len arr) (len args))))
+      ((= name "splice")
+        (fn
+          (&rest args)
+          (let
+            ((n (len arr))
+              (start-raw (if (empty? args) 0 (js-num-to-int (nth args 0)))))
+            (let
+              ((start (cond ((< start-raw 0) (max 0 (+ n start-raw))) ((> start-raw n) n) (else start-raw))))
+              (let
+                ((delete-count (if (< (len args) 2) (- n start) (max 0 (min (- n start) (js-num-to-int (nth args 1)))))))
+                (js-list-slice arr start (+ start delete-count)))))))
+      ((= name "flatMap")
+        (fn
+          (f)
+          (let
+            ((mapped (js-list-map-loop f arr 0 (list))))
+            (js-list-flat-loop mapped 1 (list)))))
+      ((= name "findLast")
+        (fn (f) (js-list-find-last-loop f arr (- (len arr) 1))))
+      ((= name "findLastIndex")
+        (fn (f) (js-list-find-last-index-loop f arr (- (len arr) 1))))
+      ((= name "reduceRight")
+        (fn
+          (&rest args)
+          (cond
+            ((= (len args) 1)
+              (if
+                (= (len arr) 0)
+                (error "Reduce of empty array with no initial value")
+                (js-list-reduce-right-loop
+                  (nth args 0)
+                  (nth arr (- (len arr) 1))
+                  arr
+                  (- (len arr) 2))))
+            (else
+              (js-list-reduce-right-loop
+                (nth args 0)
+                (nth args 1)
+                arr
+                (- (len arr) 1))))))
+      ((= name "toString") (fn () (js-list-join arr ",")))
+      ((= name "toLocaleString") (fn () (js-list-join arr ",")))
+      ((= name "keys")
+        (fn
+          ()
+          (let
+            ((result (list)))
+            (begin (js-list-keys-loop arr 0 result) result))))
+      ((= name "values") (fn () (js-list-slice arr 0 (len arr))))
+      ((= name "entries")
+        (fn
+          ()
+          (let
+            ((result (list)))
+            (begin (js-list-entries-loop arr 0 result) result))))
+      ((= name "copyWithin")
+        (fn
+          (&rest args)
+          (let
+            ((n (len arr))
+              (target-raw
+                (if (empty? args) 0 (js-num-to-int (nth args 0))))
+              (start-raw
+                (if (< (len args) 2) 0 (js-num-to-int (nth args 1))))
+              (end-raw
+                (if
+                  (< (len args) 3)
+                  (len arr)
+                  (js-num-to-int (nth args 2)))))
+            (let
+              ((target (cond ((< target-raw 0) (max 0 (+ n target-raw))) (else (min n target-raw))))
+                (start
+                  (cond
+                    ((< start-raw 0) (max 0 (+ n start-raw)))
+                    (else (min n start-raw))))
+                (end
+                  (cond
+                    ((< end-raw 0) (max 0 (+ n end-raw)))
+                    (else (min n end-raw)))))
+              (begin (js-list-copy-within! arr target start end) arr)))))
+      ((= name "toReversed")
+        (fn () (js-list-reverse-loop arr (- (len arr) 1) (list))))
+      ((= name "toSorted")
+        (fn
+          (&rest args)
+          (let
+            ((cmp (if (empty? args) nil (nth args 0)))
+              (copy (js-list-slice arr 0 (len arr))))
+            (begin (js-list-sort! copy cmp) copy))))
       (else js-undefined))))
 
 (define pop-last! (fn (lst) nil))
@@ -1267,6 +1367,73 @@
           (append! acc (nth arr i))
           (js-list-reverse-loop arr (- i 1) acc))))))
 
+(define
+  js-list-find-last-loop
+  (fn
+    (f arr i)
+    (cond
+      ((< i 0) :js-undefined)
+      ((js-to-boolean (f (nth arr i))) (nth arr i))
+      (else (js-list-find-last-loop f arr (- i 1))))))
+
+(define
+  js-list-find-last-index-loop
+  (fn
+    (f arr i)
+    (cond
+      ((< i 0) -1)
+      ((js-to-boolean (f (nth arr i))) i)
+      (else (js-list-find-last-index-loop f arr (- i 1))))))
+
+(define
+  js-list-reduce-right-loop
+  (fn
+    (f acc arr i)
+    (if
+      (< i 0)
+      acc
+      (js-list-reduce-right-loop f (f acc (nth arr i)) arr (- i 1)))))
+
+(define
+  js-list-keys-loop
+  (fn
+    (arr i result)
+    (if
+      (>= i (len arr))
+      result
+      (begin (append! result i) (js-list-keys-loop arr (+ i 1) result)))))
+
+(define
+  js-list-entries-loop
+  (fn
+    (arr i result)
+    (if
+      (>= i (len arr))
+      result
+      (begin
+        (append! result (list i (nth arr i)))
+        (js-list-entries-loop arr (+ i 1) result)))))
+
+(define
+  js-list-copy-within!
+  (fn
+    (arr target start end)
+    (let
+      ((snap (js-list-slice arr start end)))
+      (js-list-copy-within-loop! arr target snap 0))))
+
+(define
+  js-list-copy-within-loop!
+  (fn
+    (arr target snap i)
+    (cond
+      ((>= i (len snap)) arr)
+      ((>= (+ target i) (len arr)) arr)
+      (else
+        (begin
+          (set-nth! arr (+ target i) (nth snap i))
+          (js-list-copy-within-loop! arr target snap (+ i 1)))))))
+
 (define
   js-string-repeat
   (fn
@@ -1577,6 +1744,21 @@
             ((= key "fill") (js-array-method obj "fill"))
             ((= key "sort") (js-array-method obj "sort"))
             ((= key "lastIndexOf") (js-array-method obj "lastIndexOf"))
+            ((= key "at") (js-array-method obj "at"))
+            ((= key "unshift") (js-array-method obj "unshift"))
+            ((= key "splice") (js-array-method obj "splice"))
+            ((= key "flatMap") (js-array-method obj "flatMap"))
+            ((= key "findLast") (js-array-method obj "findLast"))
+            ((= key "findLastIndex") (js-array-method obj "findLastIndex"))
+            ((= key "reduceRight") (js-array-method obj "reduceRight"))
+            ((= key "toString") (js-array-method obj "toString"))
+            ((= key "toLocaleString") (js-array-method obj "toLocaleString"))
+            ((= key "keys") (js-array-method obj "keys"))
+            ((= key "values") (js-array-method obj "values"))
+            ((= key "entries") (js-array-method obj "entries"))
+            ((= key "copyWithin") (js-array-method obj "copyWithin"))
+            ((= key "toReversed") (js-array-method obj "toReversed"))
+            ((= key "toSorted") (js-array-method obj "toSorted"))
             (else js-undefined)))
         ((= (type-of obj) "string")
           (cond

lib/js/test.sh

@@ -1191,6 +1191,28 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 3609)
 (eval "(js-eval \"(true).valueOf()\")")
 
+;; ── Phase 11.arrmore: more Array.prototype methods ──────────
+(epoch 3700)
+(eval "(js-eval \"[1,2,3].at(0)\")")
+(epoch 3701)
+(eval "(js-eval \"[1,2,3].at(-1)\")")
+(epoch 3702)
+(eval "(js-eval \"[1,2,3,4].flatMap(x=>[x,x*2]).join(',')\")")
+(epoch 3703)
+(eval "(js-eval \"[1,5,2,7,3].findLast(x=>x<5)\")")
+(epoch 3704)
+(eval "(js-eval \"[1,5,2,7,3].findLastIndex(x=>x<5)\")")
+(epoch 3705)
+(eval "(js-eval \"[1,2,3,4].reduceRight((acc,x)=>acc+','+x)\")")
+(epoch 3706)
+(eval "(js-eval \"[1,2,3].toString()\")")
+(epoch 3707)
+(eval "(js-eval \"[3,1,2].toReversed().join(',')\")")
+(epoch 3708)
+(eval "(js-eval \"[3,1,4,1,5].toSorted((a,b)=>a-b).join(',')\")")
+(epoch 3709)
+(eval "(js-eval \"var a=[1,2,3]; a.keys().join(',')\")")
+
 ;; ── Phase 11.arrlike: Array.prototype.* on {length, 0:..., 1:...} ──
 (epoch 3500)
 (eval "(js-eval \"var a = {length: 3, 0: 41, 1: 42, 2: 43}; Array.prototype.slice.call(a).length\")")
@@ -1853,6 +1875,18 @@ check 3607 "true.toString()"               '"true"'
 check 3608 "false.toString()"              '"false"'
 check 3609 "(true).valueOf()"              'true'
 
+# ── Phase 11.arrmore: more Array.prototype methods ────────────
+check 3700 "arr.at(0)"                     '1'
+check 3701 "arr.at(-1)"                    '3'
+check 3702 "arr.flatMap"                   '"1,2,2,4,3,6,4,8"'
+check 3703 "arr.findLast"                  '3'
+check 3704 "arr.findLastIndex"             '4'
+check 3705 "arr.reduceRight"               '"4,3,2,1"'
+check 3706 "arr.toString"                  '"1,2,3"'
+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.arrlike: array-like receivers on Array.prototype ─
 check 3500 "slice.call arrLike length"     '3'
 check 3501 "slice.call arrLike join"       '"41,42,43"'