JSFiddle - React, Tailwind, and code Playground

HTML

<h4 id="answer">Ответ</h4>

JavaScript

function sum(items, acc) {
  console.log(items)
  if (items.length > 0) {
    const a = items.pop();    
    return sum(items, acc + a);
  } else {    
    return acc;
  }
}

const arr = [1,2,3,4,5,6,7,8,9];
const res = sum(arr, 0);
console.log("Sum =", res);
$('#answer').text("Sum = " + res);