Title Case a Sentence w/ nonfunctional .join

Tell us what’s happening:
It is saying that str[i].join is not a function?
What is the deal?

Your code so far


function titleCase(str) {
  str.toLowerCase();
  str.split(" ");
  for(var i=0; i<str.length; i++) {
    str[i].split("");
    str[i][0].toUpperCase();
    str[i].join("");
    str.join(" ");
  }
  return str;
}

titleCase("I'm a little tea pot");

Your browser information:

Your Browser User Agent is: Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 Safari/537.36 OPR/50.0.2762.67.

Link to the challenge:
https://www.freecodecamp.org/challenges/title-case-a-sentence