app.js

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost:27017/test', {
  useNewUrlParser: true,
  useUnifiedTopology: true
});

const Score = mongoose.model(
  'score',
  {
    userId: String,
    highScores: {
      type: [Number],
      default: []
    },
    highestScore: {
      type: Number,
      default: 0
    }
  },
  'score'
);

const reportScore = async (userId, score) => {
  let scoreRecord = await Score.findOne({ userId });
  if (!scoreRecord) {
    scoreRecord = await Score.create({
      userId
    });
  }
  scoreRecord.highestScore = Math.max(scoreRecord.highestScore, score);
  if (scoreRecord.highScores.length < 3) {
    scoreRecord.highScores.push(score);
  } else if (score > scoreRecord.highScores[0]) {
    scoreRecord.highScores[0] = score;
    scoreRecord.highScores.sort();
  }
  return await scoreRecord.save();
};

const getUserHighScores = async userId => {
  const scoreRecord = await Score.findOne({ userId });
  if (!scoreRecord) {
    return [];
  }
  return scoreRecord.highScores;
};

const getTopNScore = async n => {
  const records = await Score.find().sort('-highestScore').limit(n);

  const result = records.map(r => ({
    userId: r.userId,
    score: r.highestScore
  }));
  return result;
};

const userId = ['1', '2', '3', '4', '5', '6', '7', '8', '9'];
const scores = [10, 11, 12, 8, 7, 11.5, 13, 9, 15];

const test = async () => {
  await Score.deleteMany({});
  for (let i = 0; i < userId.length; i++) {
    const scoreRecord = await reportScore(userId[i], scores[i]);
    console.log(await getTopNScore(5));
  }
};

test();