JSFiddle - React, Tailwind, and code Playground

HTML

<button id="somar">10 + 5</button>

JavaScript

function somarValores(valor1, valor2)
{
    let soma = valor1 + valor2;
    alert(soma);
}

window.onload=function(){
    let botaoSomar = document.getElementById("somar");

    botaoSomar.onclick = function () { 
        somarValores(10,5) ;
    };
}