30-seconds-of-code/snippets/average-by.md

title, tags, cover, firstSeen, lastUpdated

title	tags	cover	firstSeen	lastUpdated
Mapped array average	math	rock-climbing	2018-01-11T12:25:54+02:00	2020-10-21T21:17:45+03:00

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