Length of array inside array

Question

let arr = [
[1, 2, 3],
[1, 2, 3],
[1, 2, 3],
[1, 2, 3]
];

Here,
If we write arr.length we get 4.
How can we get the length of the array inside the main array (which is 3)?

Answer 1

just simple.

arr[0].length;