Arrays: Arrays of arrays

Since we can add an array into an array, we can create multi-dimensional arrays, which have very useful applications (e.g. a matrix):

const matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
]

matrix[0][0] //1
matrix[2][0] //7

0:	Introduction
1:	Number of items in an array
2:	Create a new array from an existing array
3:	Adding an item to an existing array
4:	Adding at the beginning of an array
5:	Adding multiple items to the array
6:	Removing an item from an array
7:	Modifying an existing array without mutating it
8:	▶︎ Arrays of arrays
9:	Filling an array
10:	Array destructuring
11:	Check if an array contains a specific value