Function takes an input string and returns the longest word in the string

longestword.js

function LongestWord(sen) {
  if(typeof sen = "undefined") {
    return "You have to pass a sentence in as a string.";
  } else {
    var words = sen.split(" ");
    if(words.length = 1) {
      return words[0];
    } else {
      var largest = 0;
      var output = "";
      var length = words.length;
      for(var i = 0; i < length; i++) {
        if(words[i].length > largest) {
          largest = words[i].length;
        }
        if(words[i].length === largest) {
          output = words[i];
        }
      }
      return output;
    }
  }
}