averageBy

Calculates the average of an array, after mapping each element to a value using the provided function.

• Use Array.prototype.map() to map each element to the value returned by fn.
• Use Array.prototype.reduce() to add each value to an accumulator, initialized with a value of 0.
• Divide the resulting array by its length.

const averageBy = (arr, fn) =>
  arr
    .map(typeof fn === 'function' ? fn : val => val[fn])
    .reduce((acc, val) => acc + val, 0) / arr.length;

averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 5
averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 5