How do you say that state updates are merged?

For example, let us take a facebook user with posts and comments details as state variables,

      constructor(props) {
        super(props);
        this.state = {
          posts: [],
          comments: []
        };
      }

Now you can update them independently with separate `setState()` calls as below,

     componentDidMount() {
        fetchPosts().then(response => {
          this.setState({
            posts: response.posts
          });
        });

        fetchComments().then(response => {
          this.setState({
            comments: response.comments
          });
        });
      }

As mentioned in the above code snippets, `this.setState({comments})` updates only comments variable without modifying or replacing `posts` variable.