JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id="divout">0,1,3,7,21</textarea>
<div id="sum"></div>
<div id="square"></div>
<div id="avg"></div>

JavaScript

var Q = function (id) { return document.getElementById(id); };

var arr = Q('divout').textContent.split(',').map(Number);

var sumNumbers = function (p, c) { return p + c; };
var squareNumbers = function (x) { return x * x; };

var sum = arr.reduce(sumNumbers);
var square = arr.map(squareNumbers);
var avg = sum / arr.length;

Q('sum').textContent = sum;
Q('square').textContent = square;
Q('avg').textContent = avg;