sumBy

Calculates the sum 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.

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

sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], x => x.n); // 20
sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 20