From e9c7209f8b6e28f9bb94c44db0e7e0352de1edf0 Mon Sep 17 00:00:00 2001
From: Max Schaefer <max@semmle.com>
Date: Thu, 9 Oct 2014 13:52:08 +0100
Subject: [PATCH] Guard against missing elements in array patterns when
 checking function parameters.

---
 acorn.js              | 6 ++++--
 test/tests-harmony.js | 2 ++
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/acorn.js b/acorn.js
index f1872f4419..2da669c499 100644
--- a/acorn.js
+++ b/acorn.js
@@ -1391,8 +1391,10 @@
         break;
 
       case "ArrayPattern":
-        for (var i = 0; i < param.elements.length; i++)
-          checkFunctionParam(param.elements[i], nameHash);
+        for (var i = 0; i < param.elements.length; i++) {
+          var elem = param.elements[i];
+          if (elem) checkFunctionParam(elem, nameHash);
+        }
         break;
     }
   }
diff --git a/test/tests-harmony.js b/test/tests-harmony.js
index deeb049f65..01010e95ca 100644
--- a/test/tests-harmony.js
+++ b/test/tests-harmony.js
@@ -15389,3 +15389,5 @@ test('function normal(x, y = 10) {}', {
     expression: false
   }]
 }, {ecmaVersion: 6});
+
+test("'use strict'; function f([x,,z]) {}", {}, {ecmaVersion: 6});