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

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", but I can’t find what type or operation is being performed.