This SO post is really good, but some of the points made in the post are not very clear. For example, why is arr['key'] = 5; consider bad? I've read the post an

This SO post is really good, but some of the points made in the post are not very clear. For example, why is arr['key'] = 5; consider bad? I've read the post and the code, but it's still not clear from the example. When does using Object.prototype.hasOwnProperty lead to bugs? Any benchmarks on case statements being slow? Any additional reading on this type of thing (books, etc)?