Discussion on: Snail Array Challenge Solution JavaScript

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While I don't think I have an efficient way of generating the spiral, I learnt a thing or two.
My method is to rotate the grid by 90 degrees and take the top row from the grid and add it to the output.

(function(input)
{
  input = input.flat();

  function indexToXY(index, width)
  {
    let x = index % width;
    let y = Math.floor(index / width);
    return [x,y];
  }

  function xyToIndex(x, y, width)
  {
    return (x + (y * width));
  }

  function snail(grid, width, shrinkwidth)
  {
    if(grid.length == 0)
    {
      return [];  
    }
    if(grid.length == 1)
    {
      return grid;
    }
    let part = grid.splice(0, width);
    width = Math.ceil(Math.sqrt(grid.length));
    grid = grid.reduce((oup, item, index) => 
    {
      let [x, y] = indexToXY(index, width);
      let pole = Math.floor(width/2);
      let polex = x - pole;
      let poley = y - pole;
      let r = Math.sqrt((polex*polex)+(poley*poley));
      let fi = Math.atan2(poley, polex);
      fi -= Math.PI/2;
      let newx = Math.round((Math.cos(fi)*r)+pole);
      let newy = Math.round((Math.sin(fi)*r)+pole);
      let newIndex = xyToIndex(newx, newy, width);
      oup[newIndex] = item;
      return oup;
    }, []).filter(x=>x);

    if(shrinkwidth)
    {
      width--;
    }

    return part.concat(snail(grid, width, !shrinkwidth));
  }

  return snail(input, Math.ceil(Math.sqrt(input.length)), true);

})([[1,2,3,1], [4,5,6,4], [7,8,9,7], [7,8,9,7]])