How can I write a class instance to a string in JS?

I have a game written in JavaScript that stores lots of game related data in objects. For example, the map of the world is written in a class like this:

const theMap = new GameMap();

class GameMap() {
    constructor() {
        this.rooms = [];
    }
    addRoom(name) {
        this.rooms.push(name);
    }
}

I want to store the variable theMapin a string so I can write it to a file. I can use JSON.stringify(theMap) to convert it to JSON, but it removes the methods. If I were to convert the JSON back to an object, I couldn't use addRoom on it anymore. Is there a way to preserve those methods?