Using underscore.js, return the average value from an array of Numbers.

average.js

function average (arr)
{
	return _.reduce(arr, function(memo, num)
	{
		return memo + num;
	}, 0) / arr.length;
}

or without underscore/lodash

prices.reduce(
  (memo, num) => memo + num,
  0
) / prices.length || 1

or as a one-liner

prices.reduce((memo, num) => memo + num, 0) / prices.length || 1

or as a prototype of array

Array.prototype.avg = function () { return this.reduce((memo, num) => memo + num, 0) / this.length || 1 };

// and then doing prices.avg() which returns the result