create new key and value for existing map in document firestore code example

Example: document.set() firebasefirestore java

// Create an initial document to update.var frankDocRef = db.collection("users").doc("frank");frankDocRef.set({    name: "Frank",    favorites: { food: "Pizza", color: "Blue", subject: "recess" },    age: 12});// To update age and favorite color:db.collection("users").doc("frank").update({    "age": 13,    "favorites.color": "Red"});