snail.js

snail = function(array) {

  var side = 'top';
  var result = [];
  
  while(array.length > 0){
    switch(side){
      case 'top': 
        result.push(...cutRow(0, array));
        side = 'right';
        break;
      case 'right': 
        result.push(...cutColumn(array[0].length - 1, array));
        side = 'bottom';
      
        break;
      case 'bottom': 
        result.push(...cutRow(array.length - 1, array, true));
        side = 'left';
      
        break;
      case 'left': 
        result.push(...cutColumn(0, array, true));
        side = 'top';
     
        break;
    }
  }
  
  return result;
}

function cutRow(i, array, reverse){
  const res = array.splice(i, 1)[0];
  return reverse?res.reverse():res;
}

function cutColumn(i, array, reverse){
  const res = array.map(arr => arr.splice(i, 1)[0]);
  return reverse?res.reverse():res;
}