diff --git a/lib/js/runtime.sx b/lib/js/runtime.sx
index 4ccb6b3b..7b62670c 100644
--- a/lib/js/runtime.sx
+++ b/lib/js/runtime.sx
@@ -2284,11 +2284,50 @@
             (>=
               (js-list-index-of arr (nth args 0) 0)
               0))))
-      ((= name "find") (fn (f) (js-list-find-loop f arr 0)))
+      ((= name "find")
+        (fn (&rest args)
+          (let
+            ((f (if (empty? args) :js-undefined (nth args 0)))
+              (this-arg
+                (cond
+                  ((< (len args) 2) js-global-this)
+                  ((js-undefined? (nth args 1)) js-global-this)
+                  ((= (nth args 1) nil) js-global-this)
+                  (else (nth args 1)))))
+            (js-list-find-loop f arr this-arg 0))))
       ((= name "findIndex")
-        (fn (f) (js-list-find-index-loop f arr 0)))
-      ((= name "some") (fn (f) (js-list-some-loop f arr 0)))
-      ((= name "every") (fn (f) (js-list-every-loop f arr 0)))
+        (fn (&rest args)
+          (let
+            ((f (if (empty? args) :js-undefined (nth args 0)))
+              (this-arg
+                (cond
+                  ((< (len args) 2) js-global-this)
+                  ((js-undefined? (nth args 1)) js-global-this)
+                  ((= (nth args 1) nil) js-global-this)
+                  (else (nth args 1)))))
+            (js-list-find-index-loop f arr this-arg 0))))
+      ((= name "some")
+        (fn (&rest args)
+          (let
+            ((f (if (empty? args) :js-undefined (nth args 0)))
+              (this-arg
+                (cond
+                  ((< (len args) 2) js-global-this)
+                  ((js-undefined? (nth args 1)) js-global-this)
+                  ((= (nth args 1) nil) js-global-this)
+                  (else (nth args 1)))))
+            (js-list-some-loop f arr this-arg 0))))
+      ((= name "every")
+        (fn (&rest args)
+          (let
+            ((f (if (empty? args) :js-undefined (nth args 0)))
+              (this-arg
+                (cond
+                  ((< (len args) 2) js-global-this)
+                  ((js-undefined? (nth args 1)) js-global-this)
+                  ((= (nth args 1) nil) js-global-this)
+                  (else (nth args 1)))))
+            (js-list-every-loop f arr this-arg 0))))
       ((= name "reverse")
         (fn () (js-list-reverse-loop arr (- (len arr) 1) (list))))
       ((= name "flat")
@@ -2590,29 +2629,32 @@
 (define
   js-list-find-loop
   (fn
-    (f arr i)
+    (f arr this-arg i)
     (cond
       ((>= i (len arr)) js-undefined)
-      ((js-to-boolean (f (nth arr i))) (nth arr i))
-      (else (js-list-find-loop f arr (+ i 1))))))
+      ((js-to-boolean (js-call-with-this this-arg f (list (nth arr i) i arr)))
+        (nth arr i))
+      (else (js-list-find-loop f arr this-arg (+ i 1))))))
 
 (define
   js-list-find-index-loop
   (fn
-    (f arr i)
+    (f arr this-arg i)
     (cond
       ((>= i (len arr)) -1)
-      ((js-to-boolean (f (nth arr i))) i)
-      (else (js-list-find-index-loop f arr (+ i 1))))))
+      ((js-to-boolean (js-call-with-this this-arg f (list (nth arr i) i arr)))
+        i)
+      (else (js-list-find-index-loop f arr this-arg (+ i 1))))))
 
 (define
   js-list-some-loop
   (fn
-    (f arr i)
+    (f arr this-arg i)
     (cond
       ((>= i (len arr)) false)
-      ((js-to-boolean (f (nth arr i))) true)
-      (else (js-list-some-loop f arr (+ i 1))))))
+      ((js-to-boolean (js-call-with-this this-arg f (list (nth arr i) i arr)))
+        true)
+      (else (js-list-some-loop f arr this-arg (+ i 1))))))
 
 (define
   js-list-flat-loop
@@ -2688,11 +2730,12 @@
 (define
   js-list-every-loop
   (fn
-    (f arr i)
+    (f arr this-arg i)
     (cond
       ((>= i (len arr)) true)
-      ((not (js-to-boolean (f (nth arr i)))) false)
-      (else (js-list-every-loop f arr (+ i 1))))))
+      ((not (js-to-boolean (js-call-with-this this-arg f (list (nth arr i) i arr))))
+        false)
+      (else (js-list-every-loop f arr this-arg (+ i 1))))))
 
 (define
   js-list-reverse-loop
diff --git a/plans/js-on-sx.md b/plans/js-on-sx.md
index 370dd6d6..2f66349d 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.
 
+- 2026-05-09 — **`Array.prototype.find`/`findIndex`/`some`/`every` honour `thisArg` and pass `(value, index, array)`.** Same shape as the previous `forEach`/`map`/`filter` fix — these were calling `(f x)` directly. Updated each prototype method to extract optional `thisArg` (defaulting to globalThis when null/undefined) and route through `js-call-with-this` with the full `(value, index, array)` triple. Updated `js-list-find-loop` / `js-list-find-index-loop` / `js-list-some-loop` / `js-list-every-loop` to match. Result: built-ins/Array/prototype/find 5/30 → 6/30. Modest delta this round (most remaining failures need deeper Array semantics — sparse arrays, ToLength on `length`, etc.). Object 30/30, Map 18/30 unchanged. conformance.sh: 148/148.
+
 - 2026-05-09 — **`Array.prototype.forEach`/`map`/`filter` honour `thisArg` and pass `(value, index, array)` to callback.** Was calling the callback with just `(value)` from a bare `(f x)` and ignoring the optional second `thisArg` parameter. Per spec, the callback receives `(value, index, array)` and `this` is `thisArg ?? globalThis` in non-strict. Updated the prototype methods to take `&rest args`, extract `thisArg` (defaulting to globalThis when null/undefined), and route through `js-call-with-this` with the full triple. Updated `js-list-foreach-loop` / `js-list-map-loop` / `js-list-filter-loop` accordingly. Result: built-ins/Array/prototype/forEach 2/30 → 9/30, filter 5/30 → 10/30. Array 18/30, Object 30/30, Map 18/30 unchanged. conformance.sh: 148/148.
 
 - 2026-05-09 — **`Map.prototype.forEach` / `Set.prototype.forEach` honour `thisArg` and pass `(value, key, collection)` to callback.** Was hardcoding `js-undefined` as the callback receiver and only passing `(value, key)`. Per spec, the callback receives `(value, key, collection)` and `this` is `thisArg ?? globalThis` in non-strict. Updated `js-map-do-foreach` / `js-set-do-foreach` to accept an optional `thisArg`, defaulting to `globalThis` when null/undefined; the prototype methods now route the second positional arg through. Result: built-ins/Map/prototype 11/30 → 13/30, built-ins/Set/prototype +similar. Map 18/30 holds. conformance.sh: 148/148.