Grid Traveler function with memoization [JAVASCRIPT]

grid-traveler.js

/// Say that you are a traveler on a 2D grid. You begin in the
/// top-left corner and your goal is to travel to the bottom-right
/// corner. You may only move down or right.

/// In how many ways can you travel to the goal on a grid with
/// dimensions m * n?

/// Write a function 'gridTraveler(m, n)' that calculates this.

const gridTraveler = (n, m, memory = {}) => {
    // LTR check in memory.
    if (`${n},${m}` in memory) return memory[`${n},${m}`];

    // RTL check in memory.
    if (`${m},${n}` in memory) return memory[`${m},${n}`];

    // Return obvious values to stop recursion.
    if (n == 1 || m == 1) return 1;
    if (n == 0 || m == 0) return 0;

    // Store current value in memory.
    memory[`${n},${m}`] = gridTraveler(n - 1, m, memory) + gridTraveler(n, m - 1, memory);

    // Return value.
    return memory[`${n},${m}`];
}

console.log(gridTraveler(18, 18)); // prints => 2333606220