A JSON.parse reviver from ES6 maps

mapreviver.js

function mapReviver(key, value) {
   if (typeof value != "object") return value;
   switch (value["<kind>"]){
      case undefined: return value;
      case "Map": {
         let newValue = new Map;
         let mapData = value["<mapData>"];
         if (!mapData) return value;
         mapData.forEach(e=>newValue.set(e[0], e[1]));
         return newValue;
         }
      default: return value;
    }
}

Thanks much. Why this isn't just built-in, I have no idea :-(