From 0a3425ba18565eec37c9b92f98224c4d5b7e1a78 Mon Sep 17 00:00:00 2001
From: giles <giles.bradshaw@rose-ash.com>
Date: Thu, 23 Apr 2026 23:24:02 +0000
Subject: [PATCH] js-on-sx: Array.prototype.lastIndexOf

---
 lib/js/runtime.sx | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/lib/js/runtime.sx b/lib/js/runtime.sx
index 3489f095..5fe9a266 100644
--- a/lib/js/runtime.sx
+++ b/lib/js/runtime.sx
@@ -854,6 +854,19 @@
             ((cmp (if (= (len args) 0) nil (nth args 0))))
             (js-list-sort! arr cmp)
             arr)))
+      ((= name "lastIndexOf")
+        (fn
+          (&rest args)
+          (if
+            (= (len args) 0)
+            -1
+            (js-list-last-index-of
+              arr
+              (nth args 0)
+              (if
+                (< (len args) 2)
+                (- (len arr) 1)
+                (js-num-to-int (nth args 1)))))))
       (else js-undefined))))
 
 (define pop-last! (fn (lst) nil))
@@ -1060,6 +1073,15 @@
                 (begin (js-list-set! arr i b) (js-list-set! arr (+ i 1) a)))))
           (js-list-sort-inner! arr cmp (+ i 1) end))))))
 
+(define
+  js-list-last-index-of
+  (fn
+    (arr x i)
+    (cond
+      ((< i 0) -1)
+      ((= (nth arr i) x) i)
+      (else (js-list-last-index-of arr x (- i 1))))))
+
 (define
   js-list-every-loop
   (fn
@@ -1389,6 +1411,7 @@
             ((= key "flat") (js-array-method obj "flat"))
             ((= key "fill") (js-array-method obj "fill"))
             ((= key "sort") (js-array-method obj "sort"))
+            ((= key "lastIndexOf") (js-array-method obj "lastIndexOf"))
             (else js-undefined)))
         ((= (type-of obj) "string")
           (cond