mean.js

let data = [2, 4, 6, 8];
mean(data); // => 5.0

// common way
const mean = function (xs) {
  let n = xs.length;
  return xs.reduce((m, x) => m + x / n, 0);
};

// using ES6 default parameters and IIFE to emulate
// functional style let-expressions
const mean = xs =>
  ((n = xs.length) => xs.reduce((m, x) => m + x / n, 0))();

// "unprotected" let-expressions (variables can be set
// to unexpected values)
const mean = (xs, n = xs.length) =>
  xs.reduce((m, x) => m + x / n, 0);