Project Euler Solution #15 in Javascript

Lattice paths

Starting in the top left corner of a 2×2 grid, and only being able to move to the right and down, there are exactly 6 routes to the bottom right corner.

How many such routes are there through a 20×20 grid?

2x2 = 6 routes

3x3 = 20 routes

4x4 = 70 routes

5x5 = 252 routes

6x6 = 924 routes