triangleMaxSum

// Plain JS:
function triangleMaxSum(triangle, startPos) {
    const [row, column] = startPos || [0, 0];
    const n = triangle.length - row;
    const cap = triangle[row][column];
    // base case
    if (n == 1) return cap;
    // recursive case
    const leftSum = triangleMaxSum(triangle, [row + 1, column]);
    const rightSum = triangleMaxSum(triangle, [row + 1, column + 1]);
    return cap + Math.max(leftSum, rightSum);
}

// "lambda" JS:
const triangleMaxSum = (triangle, row, column) => (row == undefined)
    ? triangleMaxSum(triangle, 0, 0)
    : (triangle.length - row == 1)
        // base case
        ? triangle[row][column]
        // recursive case
        : triangle[row][column] + Math.max(
            triangleMaxSum(triangle, row + 1, column),
            triangleMaxSum(triangle, row + 1, column + 1))