It's designed to easily creating/iterating multi-dimension arrays.
The usage is extremely straightforward:
const x = new ArrayX([3, 4])
x.set([2, 3], 10)
console.log(x.get([2, 3])) // 10
console.log(x.get([0, 0])) // undefined
x.fill(([a, b]) => a * b)
console.log(x) // [0,0,0,0,0,1,2,3,0,2,4,6]