spiral-matrix.js

/*

[1] => 1

[1, 2]
[4, 5] => 1, 2, 5, 4

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

*/

console.log(spiral([[1, 2, 3], [4, 5, 6], [7, 8, 9]]));

function spiral(matrix) {
  let result = [];

  while (matrix.length > 0) {
    const topRow = matrix.shift();
    const rightCol = matrix.map(row => row.pop());
    const bottomRow = (matrix.pop() || []).reverse();
    const leftCol = (matrix.map(row => row.shift()) || []).reverse(); 

    result = result
      .concat(topRow)
      .concat(rightCol)
      .concat(bottomRow)
      .concat(leftCol);
  }

  return result.join(", ");
}