Alter a object variable inside cloud functions

I am trying to alter a object variable after query in cloud functions, but before sending it to my app, but not sure how to do so! Here is what I am trying to do:

Parse.Cloud.define("getUserData", async(request) => {

    // Getting the users data
    const userQuery = new Parse.Query("UserData");
    userQuery.equalTo("userId", request.params.userId);
    const userData = await userQuery.first();

    // Getting the groups
    const groupQuery = new Parse.Query("GroupMembers");
    groupQuery.equalTo("userId", request.params.userId);
    groupQuery.include('pointerObject'); // including the pointer object 
    const groups = await groupQuery.find();

    const allGroups = [];

    for (let i = 0; i < groups.length; ++i) {
        var thisGroup = groups[i].get("pointerObject");

        thisGroup.isFavorite = true;

        allGroups.push(thisGroup);
    }

    var returnObject = {
        "playerData": userData,
        "playerGroups": allGroups
    }

    const jsonString = JSON.stringify(returnObject);

    return jsonString;
});

It is "thisGroup.isFavorite" i am trying to set to true, but when receiving the jsonString, it is still set to false? How do I alter a variable in cloud functions?

Any help is appreciated and thanks in advance :-)

Try with:

thisGroup.set('isFavorite', true);
await thisGroup.save();