Well, as it turns out, -1 is already a valid key. Arrays are really objects with indices as keys. So arr[-1] is looking at the arr object and the value of the "-1" key, which is undefined.
There can be a "-1" key present in the object, so the behaviour for "negative indices" is already defined by the language.
The article covers this really:
There can be a "-1" key present in the object, so the behaviour for "negative indices" is already defined by the language.
Why would you ever need negative indexes that have their own value? This sounds like a big in the language to me
¯\(ツ)/¯ Any string is a valid key in a javascript object