js-on-sx: optional chaining ?.

Parser: jp-parse-postfix handles op "?." followed by ident / [ / (
emitting (js-optchain-member obj name), (js-optchain-index obj k),
or (js-optchain-call callee args).

Transpile: each emits (js-optchain-get obj key) or (js-optchain-call
fn args).

Runtime: js-optchain-get and js-optchain-call short-circuit to
js-undefined when receiver is null/undefined.

423/425 unit (+5), 148/148 slice unchanged.

lib/js/parser.sx

@@ -593,6 +593,42 @@
             (jp-call-args-loop st args)
             (jp-expect! st "punct" ")")
             (jp-parse-postfix st (list (quote js-call) left args)))))
+      ((jp-at? st "op" "?.")
+        (do
+          (jp-advance! st)
+          (cond
+            ((jp-at? st "punct" "[")
+              (begin
+                (jp-advance! st)
+                (let
+                  ((k (jp-parse-assignment st)))
+                  (jp-expect! st "punct" "]")
+                  (jp-parse-postfix
+                    st
+                    (list (quote js-optchain-index) left k)))))
+            ((jp-at? st "punct" "(")
+              (begin
+                (jp-advance! st)
+                (let
+                  ((args (list)))
+                  (jp-call-args-loop st args)
+                  (jp-expect! st "punct" ")")
+                  (jp-parse-postfix
+                    st
+                    (list (quote js-optchain-call) left args)))))
+            (else
+              (let
+                ((t (jp-peek st)))
+                (if
+                  (or
+                    (= (get t :type) "ident")
+                    (= (get t :type) "keyword"))
+                  (do
+                    (jp-advance! st)
+                    (jp-parse-postfix
+                      st
+                      (list (quote js-optchain-member) left (get t :value))))
+                  (error "expected ident, [ or ( after ?.")))))))
       ((or (jp-at? st "op" "++") (jp-at? st "op" "--"))
         (let
           ((op (get (jp-peek st) :value)))

lib/js/runtime.sx

@@ -1750,6 +1750,24 @@
 
 (define Object {:entries js-object-entries :values js-object-values :freeze js-object-freeze :assign js-object-assign :keys js-object-keys})
 
+(define
+  js-optchain-get
+  (fn
+    (obj key)
+    (if
+      (or (= obj nil) (js-undefined? obj))
+      js-undefined
+      (js-get-prop obj key))))
+
+(define
+  js-optchain-call
+  (fn
+    (fn-val args)
+    (if
+      (or (= fn-val nil) (js-undefined? fn-val))
+      js-undefined
+      (js-call-plain fn-val args))))
+
 (define
   js-array-spread-build
   (fn

lib/js/test.sh

@@ -1081,6 +1081,18 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 2603)
 (eval "(js-eval \"var pt = {px: 100}; var {px} = pt; px + 1\")")
 
+;; ── Phase 11.optchain: ?. optional chaining ─────────────────────
+(epoch 2700)
+(eval "(js-eval \"var o = {x: 5}; o?.x\")")
+(epoch 2701)
+(eval "(js-eval \"var o = null; o?.x\")")
+(epoch 2702)
+(eval "(js-eval \"var o = undefined; o?.x\")")
+(epoch 2703)
+(eval "(js-eval \"var o = {a:{b:7}}; o?.a?.b\")")
+(epoch 2704)
+(eval "(js-eval \"var o = {a:null}; var r = o?.a?.b; r === undefined\")")
+
 EPOCHS
 
 
@@ -1664,6 +1676,13 @@ check 2601 "arr destructure"               '6'
 check 2602 "arr destructure skip"          '4'
 check 2603 "obj partial+add"               '101'
 
+# ── Phase 11.optchain ──────────────────────────────────────────
+check 2700 "?. obj present"                '5'
+check 2701 "?. obj null"                   'undefined'
+check 2702 "?. obj undef"                  'undefined'
+check 2703 "?. chained"                    '7'
+check 2704 "?. null-chain"                 'true'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "✓ $PASS/$TOTAL JS-on-SX tests passed"

lib/js/transpile.sx

@@ -111,6 +111,12 @@
             (js-transpile-postfix (nth ast 1) (nth ast 2)))
           ((js-tag? ast "js-prefix")
             (js-transpile-prefix (nth ast 1) (nth ast 2)))
+          ((js-tag? ast "js-optchain-member")
+            (js-transpile-optchain-member (nth ast 1) (nth ast 2)))
+          ((js-tag? ast "js-optchain-index")
+            (js-transpile-optchain-index (nth ast 1) (nth ast 2)))
+          ((js-tag? ast "js-optchain-call")
+            (js-transpile-optchain-call (nth ast 1) (nth ast 2)))
           ((js-tag? ast "js-switch")
             (js-transpile-switch (nth ast 1) (nth ast 2)))
           ((js-tag? ast "js-new")
@@ -819,6 +825,30 @@
           (js-collect-funcdecls (rest stmts))))
       (else (js-collect-funcdecls (rest stmts))))))
 
+(define
+  js-transpile-optchain-member
+  (fn
+    (obj-ast name)
+    (list (js-sym "js-optchain-get") (js-transpile obj-ast) name)))
+
+(define
+  js-transpile-optchain-index
+  (fn
+    (obj-ast key-ast)
+    (list
+      (js-sym "js-optchain-get")
+      (js-transpile obj-ast)
+      (js-transpile key-ast))))
+
+(define
+  js-transpile-optchain-call
+  (fn
+    (callee-ast args)
+    (list
+      (js-sym "js-optchain-call")
+      (js-transpile callee-ast)
+      (js-transpile-args args))))
+
 (define
   js-transpile-stmt-list
   (fn