function _getMatchScore(idx, len, isPrefix) {
      let score = 0;

      // increase score exponentially per letter matched so that contiguous matches are ranked higher
      // i.e '[abc]' ranks higher than '[ab]ott [c]hemicals'
      score += SCORE_CONTIGUOUS * len * len;

      if (idx === 0) {
        // matching at the start of string gets a ranking bonus
        score += SCORE_START_STR;
      } else if (isPrefix) {
        // closer to the start, the higher it ranks
        score += SCORE_PREFIX - idx;
      }

      return score;
    }
@param {number} idx - index of the match
@param {number} len - length of the match
@param {boolean} isPrefix - was it a prefix of a word
@returns {number} - score of the match, higher is better