Index: lib/Format/ContinuationIndenter.cpp
===================================================================
--- lib/Format/ContinuationIndenter.cpp
+++ lib/Format/ContinuationIndenter.cpp
@@ -855,6 +855,7 @@
 
   moveStatePastFakeLParens(State, Newline);
   moveStatePastScopeCloser(State);
+
   if (Current.is(TT_TemplateString) && Current.opensScope())
     State.Stack.back().LastSpace =
         (Current.IsMultiline ? Current.LastLineColumnWidth
@@ -865,6 +866,20 @@
   moveStatePastScopeOpener(State, Newline);
   moveStatePastFakeRParens(State);
 
+  if (Style.Language == FormatStyle::LK_JavaScript &&
+      Current.is(tok::l_paren) && Current.MatchingParen &&
+      Current.MatchingParen->Previous &&
+      Current.MatchingParen->Previous->is(tok::comma)) {
+    // JavaScript param lists allow trailing commas, which trigger wrapping
+    // parameters one-per-line.
+    // TODO(martinprobst): this only causes one-per-line formatting only if
+    // lines overflow.
+    State.Stack.back().AvoidBinPacking = true;
+    State.Stack.back().BreakBeforeParameter = true;
+    // TODO(martinprobst): the line below triggers "Could not find a solution".
+    // Current.MatchingParen->MustBreakBefore = true;
+  }
+
   if (Current.is(TT_ObjCStringLiteral) && State.StartOfStringLiteral == 0)
     State.StartOfStringLiteral = State.Column + 1;
   else if (Current.isStringLiteral() && State.StartOfStringLiteral == 0)
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()];");