JSFiddle - React, Tailwind, and code Playground

HTML

<p class="box">1</p>

<p class="box block">2</p>

<p class="box block">3</p>

CSS

.box {

  height: 100px;

  width: 100px;

  border: 1px solid black;

}

body {

  display: flex;

}
. block {display:block;}

JavaScript

var test = document.getElementsByClassName("box");

var sum = 0;

for (i = 0; i < test.length; i++) {

  sum += parseInt(test[i].textContent);

}

console.log(sum);