Index: lib/Format/TokenAnnotator.cpp
===================================================================
--- lib/Format/TokenAnnotator.cpp
+++ lib/Format/TokenAnnotator.cpp
@@ -2158,6 +2158,8 @@
                                                 tok::r_square, tok::r_brace) ||
                                    Left.Tok.isLiteral()))
       return false;
+    if (Left.is(tok::exclaim) && Right.is(Keywords.kw_as))
+      return true; // "x! as string"
   } else if (Style.Language == FormatStyle::LK_Java) {
     if (Left.is(tok::r_square) && Right.is(tok::l_brace))
       return true;
Index: unittests/Format/FormatTestJS.cpp
===================================================================
--- unittests/Format/FormatTestJS.cpp
+++ unittests/Format/FormatTestJS.cpp
@@ -1234,6 +1234,7 @@
   verifyFormat("x = x as [a, b];");
   verifyFormat("x = x as {a: string};");
   verifyFormat("x = x as (string);");
+  verifyFormat("x = x! as (string);");
 }
 
 TEST_F(FormatTestJS, TypeArguments) {