Why does [5,6,8,7][1,2] = 8 in JavaScript?

Question

I can\'t wrap my mind around this quirk.

[1,2,3,4,5,6][1,2,3]; // 4
[1,2,3,4,5,6][1,2]; // 3

I know [1,2,3] + [1,2] = \"1,2,31,2\"

Answer 1

[1,2,3,4,5,6][1,2,3];
      ^         ^
      |         |
    array       + — array subscript access operation,
                    where index is `1,2,3`,
                    which is an expression that evaluates to `3`.

The second [...] cannot be an array, so it’s an array subscript operation. And the contents of a subscript operation are not a delimited list of operands, but a single expression.

Read more about the comma operator here.