code: function _join(arr1, arr2) {
  	var len = arr1.length > arr2 ? arr1.length : arr2.length
  	var joinedArr = []
  	while(len --) {
  		joinedArr.push(arr1.shift())
  		joinedArr.push(arr2.shift())
  	}
  	// merge remains
  	return joinedArr.concat(arr1).concat(arr2)
  }
docs: join arr2's items to arr
  @param  {Array} arr1  odd number index items
  @param  {Array} arr2  even number index items
  @return {Array}       new array with join result