diagonals.js

input = [
  [1,2,3],
  [4,5,6],
  [7,8,9],
]

input2 = [
  [1, 2, 3, 4],
  [5, 6, 7, 8],
  [9, 10, 11, 12],
  [13, 14, 15, 16]
]

output = [
  [1],
  [4,2],
  [7,5,3],
  [8,6],
  [9],
]

const getDiagonals = (matrix) => {
	const l = matrix[0].length;
	const n = l * 2 - 1;
  const output = [];
  const mid = Math.ceil(n / 2);
  const count = 0;

  for (let i = 0; i < n; i++) {
		let x = i < l ? i : l - 1;
    let y = i < l ? 0 : i - l + 1;
    let z = (i < mid ? i : n % (i+1)) + 1;
    let diags = [];

    for (let j = 0; j < z; j++) {
      diags.push(matrix[x--][y++]);
    }
    output.push(diags);
  }
  return output;
}

console.log(getDiagonals(input));
console.log(getDiagonals(input2));

Github isn't respecting my request to use 2 spaces instead of tabs, so I apologize for the formatting (also, count variable is unnecessary, but I didn't want to update this a bunch)