Calculating an array's sum and average

sum_and_averge.js

const numbers = [1, 4, 3, 5, 4];

// sum with reduce and first value
numbers.reduce((sum, value) => sum + value, 0); // 17

// sum with reduce, but missing first value
numbers.reduce((sum, value) => sum + value); // 17

// average with reduce, using extra function params
numbers.reduce((average, value, index, array) => average + value / array.length, 0); // 3.4