gridtravelertabulative.js

const gridTraveler = (m, n) => {

  // create 2 dimensional array
  const grid = new Array(m + 1)
  .fill().map(() => new Array(n + 1).fill(0));
  grid[1][1] = 1;
  for (let i = 0; i < m + 1; i += 1) {
      for (let j = 0; j < n + 1; j += 1) {
          // add current value to right and down if possible
          const currentValue = grid[i][j];
          if (i < m) grid[i + 1][j] += currentValue;
          if (j < n) grid[i][j+1] += currentValue;
      }
  }
  return grid[m][n];
};
console.log(gridTraveler(20, 20));
console.log(gridTraveler(3,3));