Why is the object value not changing?

Question

I am trying to pull a Post object from my MongoDB. This Post has an author field, which is an ObjectID referencing who authored the blog post. Once I fetch the blog post, I want to fetch the Author's username and replace the ID in the post object before sending it out to as a response to clients.

When running the code, the correct Post shows when I log it, and the correct Username is shown when it is fetched, but I can never modify the object itself.

BlogController.getPost(id, (error, result) => {
    if (error) res.send({success:false, message:error})
    else {
        var post = result
        AuthController.getUserByID(post.author, (error, resultNested) => {
            if (error) res.send({success:false, message:error})
            else {
                post.author = resultNested.username
                console.log(post)
                console.log(resultNested)
                res.send({success:true, message:post})
            }
        }) 
    }
})

I would include the console log outputs to show you the structure of the objects and that the values I am trying to modify xist, but the new StackOverflow UI makes inputting code even more of a pain in the ass than the past. Somehow...

I expect for the post object to have it's authorfield modified, but the author field remains an ObjectID.