t.js

var util = require("util");

function url_decode(urlencoded, callback) {
    /*
     * see https://tools.ietf.org/html/rfc3986#section-2.3
     * unreserved  = ALPHA / DIGIT / "-" / "." / "_" / "~"
     */
    var unreserved = /[A-Za-z0-9\-\._~]/;
    /*
     * see https://tools.ietf.org/html/rfc3986#section-2.1
     *  pct-encoded = "%" HEXDIG HEXDIG
     */
    var pct_encoded = /%[A-Fa-f0-9][A-Fa-f0-9]/;

    var correctly_encoded = new RegExp(
        "^(?:" + unreserved.source + "|" + pct_encoded.source + ")*$"
    );

    var splitter = new RegExp(
        "(" + pct_encoded.source + ")"
    );

    if (!urlencoded.match(correctly_encoded))
        return callback(new Error("malformed data"));

    var buffers = urlencoded.split(splitter).map(function (part) {
        if (part.match(pct_encoded)) {
            return Buffer.from(/* remove leading `%' */part.slice(1), "hex");
        } else {
            return Buffer.from(part, "ascii");
        }
    });

    return callback(null, Buffer.concat(buffers));
}

var data = "salut%belol%f1foo%bebar";
url_decode(data, function (err, buffer) {
    if (err)
        throw err;
    console.error(buffer.toString());
});