Support trailing comma in arrow function params

Fixes #480

Author: dgutov

js2-mode.el

@@ -9892,15 +9892,20 @@ If NODE is non-nil, it is the AST node associated with the symbol."
     (while (and (not oneshot)
                 (js2-match-token js2-COMMA))
       (setq op-pos (- (js2-current-token-beg) pos))  ; relative
-      (setq right (js2-parse-assign-expr)
-            left pn
-            pn (make-js2-infix-node :type js2-COMMA
-                                    :pos pos
-                                    :len (- js2-ts-cursor pos)
-                                    :op-pos op-pos
-                                    :left left
-                                    :right right))
-      (js2-node-add-children pn left right))
+      (if (eq (js2-peek-token) js2-RP)
+          ;; Stop the parser from scanning too far: it's actually
+          ;; valid syntax in arrow fun arguments, and we don't want
+          ;; the RP token to get consumed.
+          (js2-report-error "msg.syntax")
+        (setq right (js2-parse-assign-expr)
+              left pn
+              pn (make-js2-infix-node :type js2-COMMA
+                                      :pos pos
+                                      :len (- js2-ts-cursor pos)
+                                      :op-pos op-pos
+                                      :left left
+                                      :right right))
+        (js2-node-add-children pn left right)))
     pn))
 
 (defun js2-parse-assign-expr ()

tests/parser.el

@@ -449,6 +449,10 @@ the test."
 (js2-deftest-parse arrow-function-recovers-from-error
   "[(,foo) => 1];" :syntax-error "," :errors-count 6)
 
+(js2-deftest-parse arrow-function-with-trailing-comma-in-arguments
+  "(a, b = 1,) => {  c;\n};"
+  :reference "(a, b = 1) => {  c;\n};")
+
 ;;; Automatic semicolon insertion
 
 (js2-deftest-parse no-auto-semi-insertion-after-if