Index: lib/Format/TokenAnnotator.cpp
===================================================================
--- lib/Format/TokenAnnotator.cpp
+++ lib/Format/TokenAnnotator.cpp
@@ -2433,8 +2433,9 @@
       return false;
     if (Right.is(TT_JsNonNullAssertion))
       return false;
-    if (Left.is(TT_JsNonNullAssertion) && Right.is(Keywords.kw_as))
-      return true; // "x! as string"
+    if (Left.is(TT_JsNonNullAssertion) &&
+        Right.isOneOf(Keywords.kw_as, Keywords.kw_in))
+      return true; // "x! as string", "x! in y"
   } 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
@@ -1908,6 +1908,7 @@
   verifyFormat("x = x as {a: string};");
   verifyFormat("x = x as (string);");
   verifyFormat("x = x! as (string);");
+  verifyFormat("x = y! in z;");
   verifyFormat("var x = something.someFunction() as\n"
                "    something;",
                getGoogleJSStyleWithColumns(40));