code: function interleave(items, char) {
    return items.reduce((res, item, index) => {
      if (index !== items.length - 1) {
        return res.concat(item, char);
      }
      return res.concat(item);
    }, []);
  }
docs: Returns new array in which `char` are interleaved between the original items.
  
  @param {Array} items
  @param {String} char
  @returns Array