I'm currently doing the Redux Essentials tutorial and stuck in chapter 7: When switching from the classical thunk to the RTK query, the mapping will not work:

content = posts.map(post => <PostExcerpt key={post.id} post={post} />)

The error message is:

Unhandled Rejection (TypeError): Cannot read properties of undefined (reading 'title')

To find the reason, I've broken the code down to something simple as follows:

} else if (isSuccess) {
    content = '';
    //content = posts.map(post => <PostExcerpt key={post.id} postId={post}/>);
    if (posts !== undefined) {
        for (const post1 in posts)
            if (post1 !== undefined) content +=
                ' loop: ' + post1.toString() + ' typ: ' + typeof(post1) + ' ID: ' + post1.title + '!!';
            else content += 'kein post';
    }

The output of my curious code is:

loop: 0 typ: string ID: undefined!! loop: 1 typ: string ID: undefined!! loop: 2 typ: string ID: undefined!!

So it is looping but not having the right values in the variable. This is weird as the posts variable seems to have the right content. Here is the output of the evaluation:

Opening the Sandbox at the end of the mentioned tutorial page also does not render the posts - so there might be a bug somewhere.

As I didn't find any similar problem description, I hope, you may give me a hint of what is wrong.