Code:

/**
 * @param {number} m
 * @param {number} n
 * @return {number}
 */
var uniquePaths = function(m, n) {
    const dp = Array(m).fill().map(() => Array(n).fill(1))

    for (let i = 1; i < m; i++) {
        for (let j = 1; j < n; j++) {
            dp[i][j] = dp[i - 1][j] + dp[i][j - 1]            
        }
    }

    return dp[m - 1][n - 1]
};


console.log(uniquePaths(3, 7))