code: function combineArrays(arr1, arr2) {
      const res = [];
  
      if (arr1.length === 0) {
          return explodeArray(arr2);
      }
      if (arr2.length === 0) {
          return explodeArray(arr1);
      }
      arr1.forEach(x1 => {
          arr2.forEach(x2 => {
              res.push([].concat(x1, x2));
          });
      });
      return res;
  }
docs: Mix two arrays such that each element of the second array is concatenated
  onto each element of the first array.
  
  For example:
  combineArrays([a, [b, c]], [x, y]); // -> [[a, x], [a, y], [b, c, x], [b, c, y]]
  
  @param   {Array} arr1 The first array to combine.
  @param   {Array} arr2 The second array to combine.
  @returns {Array}      A mixture of the elements of the first and second arrays.