Ways to think through writing this function!

findLongestWord.js

// Standard Solution
function findLongestWord(word1, word2, word3) {
  if ((word1.length >= word2.length) && (word1.length >= word3.length)) {
    return word1
  } else if ((word2.length >= word3.length) && (word2.length >= word1.length)) {
    return word2
  } else if ((word3.length >= word1.length) && (word3.length >= word2.length)) {
    return word3;
  } else {
    return 'error';
  }
}

// If you're trying to write the function in a more general way, instead of checking every combination
// of words we can just cycle through them and compare them to the current longest.

function findLongestWord(word1, word2, word3) {
  // assign the first word to 'longestWord'
  var longestWord = word1
  var otherWords = [word2, word3]

  // cycle through the other words in the list, and if any of them are bigger, replace the current longest word with that word
  for (var i = 0; i < otherWords.length; i++) {
    var currWord = otherWords[i]
    // if currWord is longer than longestWord then replace longestWord with currWord 
    if (currWord.length > longestWord.length) {
      longestWord = currWord;
    }
  }

  // once done cycling through list, return longestWord!
  return longestWord;
}

// We can actually extrapolate this into a more general function that just takes in any list of words and returns the longest.

function findLongestWordGeneral(listOfWords) {
  // validation check for list
  if (listOfWords.length == 0) return 'error'

  // assign the first word to 'longestWord'
  var longestWord = listOfWords[0]
  // cycle through all the other words in the list, and if any of them are bigger, replace the 
  // current longest word with that word
  for (var i = 1; i < listOfWords.length; i++) {
    var currWord = listOfWords[i]
    // if currWord is longer than longestWord then replace longestWord with currWord 
    if (currWord.length > longestWord.length) {
      longestWord = currWord;
    }
  }
  // once done cycling through list, return longestWord!
  return longestWord;
}

// Now that we've written a general function, we can use that as sub-routine for this function as well as others!

function findLongestWord(word1, word2, word3) {
  var listOfWords = [word1, word2, word3]
  return findLongestWordGeneral(listOfWords)
}

function findLongestWord(word1, word2, word3, word4, word5) {
  var listOfWords = [word1, word2, word3, word4, word5]
  return findLongestWordGeneral(listOfWords)
}

// If you want to do it the most efficient way however, there's probably a faster method than just cycling through each
// item in the array... here's where knowledge of algorithms comes in. But most developers in practice will use a library.

// one of the most popular: Lodash (https://lodash.com/). General rule of thumb -- if you're trying to write something efficiently,
// it's probably better to find a function in this library instead of write it yourself because under the hood it is probably 
// using the most efficient algorithm.

// See https://lodash.com/docs/4.17.15#maxBy
function findLongestWordMostEfficiently(word1, word2, word3) {
  return _.maxBy([word1, word2, word3], function (word) { return word.length; });
}