[javascript mode] Fix typescript [key in ...] syntax (again)

Issue #5737
Issue #5909

Author: marijnh

mode/javascript/javascript.js

@@ -525,7 +525,7 @@ CodeMirror.defineMode("javascript", function(config, parserConfig) {
       cx.marked = "keyword"
       return cont(objprop)
     } else if (type == "[") {
-      return cont(expression, maybetype, expect("]"), afterprop);
+      return cont(expression, maybetypeOrIn, expect("]"), afterprop);
     } else if (type == "spread") {
       return cont(expressionNoComma, afterprop);
     } else if (value == "*") {
@@ -578,6 +578,9 @@ CodeMirror.defineMode("javascript", function(config, parserConfig) {
       if (value == "?") return cont(maybetype);
     }
   }
+  function maybetypeOrIn(type, value) {
+    if (isTS && (type == ":" || value == "in")) return cont(typeexpr)
+  }
   function mayberettype(type) {
     if (isTS && type == ":") {
       if (cx.stream.match(/^\s*\w+\s+is\b/, false)) return cont(expression, isKW, typeexpr)