去那里:

function longest(str) {
  var words = str.split(' ');
  var longest = ''; // changed

  for (var i = 0; i < words.length; i++) {
    if (words[i].length > longest.length) { // changed
      longest = words[i]; // changed
    }
  }
  return longest;
}
console.log(longest("This is Andela"));