Q:

Accessing and returning nested array value

I can't seem to figure this out. I just get an undefined return.
let test = [1, 2, 3, 4, [6, 7, 8]];

How do I return the index [2] of test[4]?
I'm not even sure I'm asking the question properly.
Basically, I want to interact with 8.

A:

To maybe help you understand what is going on.
If you write
let test = [1, 2, 3, 4, [6, 7, 8]];

you create an array (which is more like a list if you compare it to other languages). Every entry has its own datatype. So in the example we have the first 4 elements which are just numbers and the fifth entry which is another Array.
With the [] operator we address certain elements inside the array. If we want the first entry we can use test[0] and should get back 1.
You now want to access an element in the array inside an array. So you first address the array in the array. test[4] this will give you back [6, 7, 8] and now you can do the same thing again and address this new array. You could write it this way
let test = [1, 2, 3, 4, [6, 7, 8]];
let innerArray = test[4];
let element = innerArray[2];

The example above is just to better understand what is going on. In practice you will just do test[4][2] and it will basically to the same as above.

