Index: lib/Format/ContinuationIndenter.cpp
===================================================================
--- lib/Format/ContinuationIndenter.cpp
+++ lib/Format/ContinuationIndenter.cpp
@@ -845,6 +845,14 @@
         State.Stack[i].NoLineBreak = true;
     State.Stack[State.Stack.size() - 2].NestedBlockInlined = false;
   }
+  if (Style.Language == FormatStyle::LK_JavaScript &&
+      Current.is(tok::l_paren) && Current.MatchingParen &&
+      Current.MatchingParen->Previous &&
+      Current.MatchingParen->Previous->is(tok::comma)) {
+    // llvm::dbgs() << "trailing comma parens\n";
+    State.Stack.back().AvoidBinPacking = true;
+    State.Stack.back().BreakBeforeParameter = true;
+  }
   if (Previous && (Previous->isOneOf(tok::l_paren, tok::comma, tok::colon) ||
                    Previous->isOneOf(TT_BinaryOperator, TT_ConditionalExpr)) &&
       !Previous->isOneOf(TT_DictLiteral, TT_ObjCMethodExpr)) {
@@ -993,6 +1001,7 @@
   bool BreakBeforeParameter = false;
   unsigned NestedBlockIndent = std::max(State.Stack.back().StartOfFunctionCall,
                                         State.Stack.back().NestedBlockIndent);
+
   if (Current.isOneOf(tok::l_brace, TT_ArrayInitializerLSquare)) {
     if (Current.opensBlockOrBlockTypeList(Style)) {
       NewIndent = State.Stack.back().NestedBlockIndent + Style.IndentWidth;
Index: unittests/Format/FormatTestJS.cpp
===================================================================
--- unittests/Format/FormatTestJS.cpp
+++ unittests/Format/FormatTestJS.cpp
@@ -521,6 +521,18 @@
                "}\n");
 }
 
+TEST_F(FormatTestJS, FunctionParametersTrailingComma) {
+  verifyFormat("function trailingComma(\n"
+               "    parameter1,\n"
+               "    parameter2,\n"
+               "    ) {\n"
+               "  a(); //\n"
+               "}\n",
+               "function trailingComma(parameter1, parameter2,) {\n"
+               "  a(); //\n"
+               "}\n");
+}
+
 TEST_F(FormatTestJS, ArrayLiterals) {
   verifyFormat("var aaaaa: List<SomeThing> =\n"
                "    [new SomeThingAAAAAAAAAAAA(), new SomeThingBBBBBBBBB()];");