Index: lib/Format/TokenAnnotator.cpp
===================================================================
--- lib/Format/TokenAnnotator.cpp
+++ lib/Format/TokenAnnotator.cpp
@@ -2981,7 +2981,7 @@
       // We deal with this case later by detecting an entry
       // following a closing paren of this submessage.
     }
-    
+
     // If this is an entry immediately following a submessage, it will be
     // preceded by a closing paren of that submessage, like in:
     //     left---.  .---right
@@ -3027,6 +3027,9 @@
       return false;
     if (Left.is(TT_JsTypeColon))
       return true;
+    // Don't wrap between ":" and "!" of a strict prop init ("field!: type;").
+    if (Left.is(tok::exclaim) && Right.is(tok::colon))
+      return false;
     if (Right.is(Keywords.kw_is))
       return false;
     if (Left.is(Keywords.kw_in))
Index: unittests/Format/FormatTestJS.cpp
===================================================================
--- unittests/Format/FormatTestJS.cpp
+++ unittests/Format/FormatTestJS.cpp
@@ -1540,6 +1540,15 @@
                "}");
 }
 
+TEST_F(FormatTestJS, StrictPropInitWrap) {
+  const FormatStyle &Style = getGoogleJSStyleWithColumns(22);
+  verifyFormat("class X {\n"
+               "  strictPropInitField!:\n"
+               "      string;\n"
+               "}",
+               Style);
+}
+
 TEST_F(FormatTestJS, InterfaceDeclarations) {
   verifyFormat("interface I {\n"
                "  x: string;\n"