(アルゴリズム)CodeWars:Thinkful-String Drills:Poem formatter

Description:
You have a collection of lovely poems. Unfortuantely they aren't formatted very well. They're all on one line, like this:
Beautiful is better than ugly. Explicit is better than implicit. Simple is better than complex. Complex is better than complicated.
What you want is to present each sentence on a new line, so that it looks like this:

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.

Write a function, format_poem() that takes a string like the one line example as an argument and returns a new string that is formatted across multiple lines with each new sentence starting on a new line when you print it out.
Tip
Try to solve this challenge with the str.split() and the str.join() string methods.
Every sentence will end with a period, and every new sentence will have one space before the previous period. Be careful about trailing whitespace in your solution.
問題を解く
ヒントに示すように,split()とjoin()法を適用して解くことができる.

function formatPoem(poem) {
  //your code here
  return poem.split(". ").join(".\n");
}