JSFiddle - React, Tailwind, and code Playground

JavaScript

function formatar(str) {
    str = str.split('');
    var pedacos = [];
    var ultimo = str.pop();
    for (var i = 0; i < str.length; i = i + 4) pedacos.push(str.slice(i, i + 4).join(''));
    pedacos = pedacos.join('.');
    pedacos = pedacos.concat('-', ultimo);
    return pedacos;
}
var novaString = formatar('1234213412341');
alert(novaString);