JSFiddle - React, Tailwind, and code Playground

HTML

<textarea name="area_1"></textarea>
<textarea name="area_2"></textarea>
<button>x 3</button>

JavaScript

var area_1 = document.querySelector('[name="area_1"]');
var area_2 = document.querySelector('[name="area_2"]');
var botao = document.querySelector('button');

botao.onclick = function () {
    area_2.value = area_1.value * 3;
}