sum-avg-api.js

const sumOf = (data = [], field) => {
  return data.reduce((sum, entry) => {
    return sum += entry[field] || 0;
  }, 0);
};

const averageOf = (data = [], field) => {
  return data.reduce((sum = 0, entry, index) => {
    const sum += (entry[field] || 0);
    if(index === data.length-1) return sum / data.length;
    return sum;
  });
};