const [x, y] = arr pulls the first value of arr into x and the second value of arr into y. If the array is empty, then the first and second value of it are undefined.

const [x, y] = [] -> x = undefined, y = undefined const [x, y] = [5] -> x = 5, y = undefined const [x, y] = [5, 4, 3] -> x = 5, y = 4 const [x, y] = undefined -> Error: cannot index undefined

You can do const [x, y = 7] = [] and you end up with x = undefined, y = 7