mean and standard deviation for node.js

standardDeviation.js

#!/usr/bin/env node
'use strict';

// basic functions for mean and standard deviation
let getMean = (data) => {
    return data.reduce(function (a, b) {
        return Number(a) + Number(b);
    }) / data.length;
};

let getStandardDevitation = (data) => {
    let m = getMean(data);
    return Math.sqrt(data.reduce(function (sq, n) {
            return sq + Math.pow(n - m, 2);
        }, 0) / (data.length - 1));
};

// example
var a = [60, 70, 80, 90];
console.log(getMean(a));  // returns: 75
console.log(getStandardDevitation(a));  // returns: 12.909944487358056