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
@@ -2221,6 +2221,7 @@
                "  aaaaaaaaaaaaaaa?: boolean,\n"
                "  aaaaaa?: List<string>\n"
                "}) {}");
+  verifyFormat("type X = [y?];");
 }
 
 TEST_F(FormatTestJS, IndexSignature) {