JSFiddle - React, Tailwind, and code Playground

JavaScript

function ConverterOctalParaNumero(txt)
{
    var result = 0, mul = 1;
    for (var i = 0; i < txt.length; i++){
        var ch = txt[txt.length - i - 1];
        result += ch * mul;
        mul *= 8;
    }
    return result;
}

document.write(ConverterOctalParaNumero("10") + "<br>");
document.write(ConverterOctalParaNumero("100") + "<br>");