Index: lib/Format/Format.cpp
===================================================================
--- lib/Format/Format.cpp
+++ lib/Format/Format.cpp
@@ -1077,12 +1077,20 @@
 
     checkEmptyNamespace(AnnotatedLines);
 
+    // Trailing or duplicated commas have semantic meaning in JS, e.g.
+    //     x = [a,,b];
+    // creates an array with three elements, one of them undefined.
+    bool CleanUpCommas = Style.Language != FormatStyle::LK_JavaScript;
+
     for (auto &Line : AnnotatedLines) {
       if (Line->Affected) {
-        cleanupRight(Line->First, tok::comma, tok::comma);
+        if (CleanUpCommas)
+          cleanupRight(Line->First, tok::comma, tok::comma);
         cleanupRight(Line->First, TT_CtorInitializerColon, tok::comma);
-        cleanupRight(Line->First, tok::l_paren, tok::comma);
-        cleanupLeft(Line->First, tok::comma, tok::r_paren);
+        if (CleanUpCommas) {
+          cleanupRight(Line->First, tok::l_paren, tok::comma);
+          cleanupLeft(Line->First, tok::comma, tok::r_paren);
+        }
         cleanupLeft(Line->First, TT_CtorInitializerComma, tok::l_brace);
         cleanupLeft(Line->First, TT_CtorInitializerColon, tok::l_brace);
         cleanupLeft(Line->First, TT_CtorInitializerColon, tok::equal);
Index: unittests/Format/CleanupTest.cpp
===================================================================
--- unittests/Format/CleanupTest.cpp
+++ unittests/Format/CleanupTest.cpp
@@ -36,11 +36,12 @@
 
   // Returns code after cleanup around \p Offsets.
   std::string cleanupAroundOffsets(llvm::ArrayRef<unsigned> Offsets,
-                                   llvm::StringRef Code) {
+                                   llvm::StringRef Code,
+                                   const FormatStyle &Style = getLLVMStyle()) {
     std::vector<tooling::Range> Ranges;
     for (auto Offset : Offsets)
       Ranges.push_back(tooling::Range(Offset, 0));
-    return cleanup(Code, Ranges);
+    return cleanup(Code, Ranges, Style);
   }
 };
 
@@ -171,6 +172,14 @@
   EXPECT_EQ(Expected, cleanupAroundOffsets({17, 22}, Code));
 }
 
+TEST_F(CleanupTest, ListRedundantCommaJavaScript) {
+  std::string Code = "function f() { var x = [a, b, , c]; }";
+  std::string Expected = "function f() { var x = [a, b, , c]; }";
+  const FormatStyle &Style = getGoogleStyle(FormatStyle::LK_JavaScript);
+
+  EXPECT_EQ(Expected, cleanupAroundOffsets({30}, Code, Style));
+}
+
 TEST_F(CleanupTest, TrailingCommaInParens) {
   std::string Code = "int main() { f(,1,,2,3,f(1,2,),4,,);}";
   std::string Expected = "int main() { f(1,2,3,f(1,2),4);}";