diff --git a/clang/lib/Format/TokenAnnotator.cpp b/clang/lib/Format/TokenAnnotator.cpp
--- a/clang/lib/Format/TokenAnnotator.cpp
+++ b/clang/lib/Format/TokenAnnotator.cpp
@@ -1248,7 +1248,7 @@
     case tok::question:
       if (Style.isJavaScript() && Tok->Next &&
           Tok->Next->isOneOf(tok::semi, tok::comma, tok::colon, tok::r_paren,
-                             tok::r_brace)) {
+                             tok::r_brace, tok::r_square)) {
         // Question marks before semicolons, colons, etc. indicate optional
         // types (fields, parameters), e.g.
         //   function(x?: string, y?) {...}
diff --git a/clang/unittests/Format/FormatTestJS.cpp b/clang/unittests/Format/FormatTestJS.cpp
--- a/clang/unittests/Format/FormatTestJS.cpp
+++ b/clang/unittests/Format/FormatTestJS.cpp
@@ -2822,5 +2822,9 @@
                Style);
 }
 
+TEST_F(FormatTestJS, TupleTypeWithOptionalLastElement) {
+  verifyFormat("type T = [number?];");
+}
+
 } // namespace format
 } // end namespace clang