the_hard_part.js

  // words is an array of words (strings)
  getSentiment(words) {
    var score = 0;
    var negator = 1;
    var nrHits = 0;

    words.forEach((token) => {
      var lowerCased = token.toLowerCase();
      if (this.negations.indexOf(lowerCased) > -1) {
        negator = -1;
        nrHits++;
      }
      else {
        // First try without stemming
        if (this.vocabulary[lowerCased] != undefined) {
          score += negator * this.vocabulary[lowerCased];
          nrHits++;
        }
        else {
          if (this.stemmer) {
            var stemmedWord = this.stemmer.stem(lowerCased);
            if(this.vocabulary[stemmedWord] != undefined) {
              score += negator * this.vocabulary[stemmedWord];
              nrHits++;
            }
          }
        }
      }
    });

    score = score / words.length;

    return score;
  }