CachedLongestWord.js

function LongestWord(sen) {
  if (typeof LongestWord.cache === 'undefined') {
      LongestWord.cache = [];
  } else {
    if (LongestWord.cache.hasOwnProperty(sen)) {
      return LongestWord.cache[sen];
    }
  }
  LongestWord.cache[sen] = sen
    .match(/[\w]+/g)
    .sort(function(a,b){return b.length - a.length})
    .shift();
  return LongestWord.cache[sen];
}

console.log(LongestWord('one two three'));
console.log(LongestWord('three one two'));
console.log(LongestWord('two one three'));