diff --git a/lib/js/lexer.sx b/lib/js/lexer.sx
index 1e72bce1..abf28b75 100644
--- a/lib/js/lexer.sx
+++ b/lib/js/lexer.sx
@@ -94,7 +94,7 @@
   (fn
     (src)
     (let
-      ((tokens (list)) (pos 0) (src-len (len src)))
+      ((tokens (list)) (pos 0) (src-len (len src)) (nl-before false))
       (define
         js-peek
         (fn
@@ -109,11 +109,7 @@
           (let
             ((sl (len s)))
             (and (<= (+ pos sl) src-len) (= (slice src pos (+ pos sl)) s)))))
-      (define
-        js-emit!
-        (fn
-          (type value start)
-          (append! tokens (js-make-token type value start))))
+      (define js-emit! (fn (type value start) (append! tokens {:pos start :value value :type type :nl nl-before})))
       (define
         skip-line-comment!
         (fn
@@ -136,7 +132,13 @@
           ()
           (cond
             ((>= pos src-len) nil)
-            ((js-ws? (cur)) (do (advance! 1) (skip-ws!)))
+            ((js-ws? (cur))
+              (do
+                (when
+                  (or (= (cur) "\n") (= (cur) "\r"))
+                  (set! nl-before true))
+                (advance! 1)
+                (skip-ws!)))
             ((and (= (cur) "/") (< (+ pos 1) src-len) (= (js-peek 1) "/"))
               (do (advance! 2) (skip-line-comment!) (skip-ws!)))
             ((and (= (cur) "/") (< (+ pos 1) src-len) (= (js-peek 1) "*"))
@@ -568,6 +570,7 @@
         (fn
           ()
           (do
+            (set! nl-before false)
             (skip-ws!)
             (when
               (< pos src-len)
diff --git a/lib/js/parser.sx b/lib/js/parser.sx
index a3cc71ce..5b1d1bb7 100644
--- a/lib/js/parser.sx
+++ b/lib/js/parser.sx
@@ -835,6 +835,12 @@
   jp-eat-semi
   (fn (st) (if (jp-at? st "punct" ";") (do (jp-advance! st) nil) nil)))
 
+(define
+  jp-token-nl?
+  (fn
+    (st)
+    (let ((tok (jp-peek st))) (if tok (= (get tok :nl) true) false))))
+
 (define
   jp-parse-vardecl
   (fn
@@ -1166,6 +1172,7 @@
         (or
           (jp-at? st "punct" ";")
           (jp-at? st "punct" "}")
+          (jp-token-nl? st)
           (jp-at? st "eof" nil))
         (do (jp-eat-semi st) (list (quote js-return) nil))
         (let
diff --git a/lib/js/test.sh b/lib/js/test.sh
index de6caea5..751da07b 100755
--- a/lib/js/test.sh
+++ b/lib/js/test.sh
@@ -1323,6 +1323,16 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 3505)
 (eval "(js-eval \"var a = {length: 3, 0: 10, 1: 20, 2: 30}; var sum = 0; Array.prototype.forEach.call(a, function(x){sum += x;}); sum\")")
 
+;; ── Phase 1.ASI: automatic semicolon insertion ─────────────────
+(epoch 4200)
+(eval "(js-eval \"function f() { return\n42\n} f()\")")
+(epoch 4201)
+(eval "(js-eval \"function g() { return 42 } g()\")")
+(epoch 4202)
+(eval "(let ((toks (js-tokenize \"a\nb\"))) (get (nth toks 1) :nl))")
+(epoch 4203)
+(eval "(let ((toks (js-tokenize \"a b\"))) (get (nth toks 1) :nl))")
+
 EPOCHS
 
 
@@ -2042,6 +2052,12 @@ check 3503 "indexOf.call arrLike"          '1'
 check 3504 "filter.call arrLike"           '"2,3"'
 check 3505 "forEach.call arrLike sum"      '60'
 
+# ── Phase 1.ASI: automatic semicolon insertion ────────────────────
+check 4200 "return+newline → undefined"    '"js-undefined"'
+check 4201 "return+space+val → val"        '42'
+check 4202 "nl-before flag set after newline" 'true'
+check 4203 "nl-before flag false on same line" 'false'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "✓ $PASS/$TOTAL JS-on-SX tests passed"