Spiral Matrix - Outputs 2 dimensional matrix in spiral sequence

spiralMatrix.js

/* 
 * Spiral Matrix
 * @function toSpiral
 * @description Outputs 2 dimensional matrix in spiral sequence
 * @example
 * const matrix = [[1, 2, 3],[4, 5, 6],[7, 8, 9]]
 * @param {Array} matrix - 2 dimensional matrix
 * @return {Array} given matrix in spiral order
 */
function toSpiral(matrix) {
  let output = [];
  
  // Iterates over the matrix while it has
  // more than 1 item
  // e.g. matrix = [[1, 2, 3],[4, 5, 6],[7, 8, 9]];
  while (matrix.length > 1) {
    // To the right direction
    // Removes the first item of the matrix.
    // e.g. [[1, 2, 3],[4, 5, 6],[7, 8, 9]] => [1, 2, 3]
    output = output.concat(matrix.shift());

    // To the down direction
    // Removes last item of each row
    // e.g. [[4, 5, 6],[7, 8, 9]] => 6, 9
    for (let i = 0; i < matrix.length; i += 1){
      output.push(matrix[i].splice(-1)[0]);
    }

    // To the left direction
    // Removes last item of each row
    // e.g. [[4, 5],[7, 8]] => 8, 7
    output = output.concat(matrix.splice(-1, 1)[0].reverse());

    // To the up direction
    // Removes first item of the first column each time.
    // e.g. [[4, 5]] => 4
    for(let i = matrix.length - 1; i >=0; i-= 1){
      output.push(matrix[i].splice(0,1)[0]);
    }
  }
  
  // If matrix still has value, then extracts it
    // e.g. [[5]] => 5
  if (matrix.length){
  	output.push(matrix.pop()[0]);
  }
  
  // returns matrix in spiral sequence
  // e.g. [1, 2, 3, 6, 9, 8, 7, 4, 5]
  return output;
}

// -------------------------------------------------
// Implementation example

const matrix = [[1, 2, 3],[4, 5, 6],[7, 8, 9]]
console.log(toSpiral(matrix));
// will outputs [1, 2, 3, 6, 9, 8, 7, 4, 5]