js-URI lib from https://code.google.com/archive/p/js-uri/

URI.js

/*
 * An URI datatype.  Based upon examples in RFC3986.
 *
 * TODO %-escaping
 * TODO split apart authority
 * TODO split apart query_string (on demand, anyway)
 *
 * @(#) $Id$
 */

// Constructor for the URI object.  Parse a string into its components.
function URI(str) {
    if (!str) str = "";
    // Based on the regex in RFC2396 Appendix B.
    var parser = /^(?:([^:\/?\#]+):)?(?:\/\/([^\/?\#]*))?([^?\#]*)(?:\?([^\#]*))?(?:\#(.*))?/;
    var result = str.match(parser);
    this.scheme    = result[1] || null;
    this.authority = result[2] || null;
    this.path      = result[3] || null;
    this.query     = result[4] || null;
    this.fragment  = result[5] || null;
}

// Restore the URI to it's stringy glory.
URI.prototype.toString = function () {
    var str = "";
    if (this.scheme) {
        str += this.scheme + ":";
    }
    if (this.authority) {
        str += "//" + this.authority;
    }
    if (this.path) {
        str += this.path;
    }
    if (this.query) {
        str += "?" + this.query;
    }
    if (this.fragment) {
        str += "#" + this.fragment;
    }
    return str;
};

// Introduce a new scope to define some private helper functions.
(function () {
    // RFC3986 §5.2.3 (Merge Paths)
    function merge(base, rel_path) {
        var dirname = /^(.*)\//;
        if (base.authority && !base.path) {
            return "/" + rel_path;
        }
        else {
            return base.path.match(dirname)[0] + rel_path;
        }
    }

    // Match two path segments, where the second is ".." and the first must
    // not be "..".
    var DoubleDot = /\/((?!\.\.\/)[^\/]*)\/\.\.\//;

    function remove_dot_segments(path) {
        if (!path) return "";
        // Remove any single dots
        var newpath = path.replace(/\/\.\//g, '/');
        // Remove any trailing single dots.
        newpath = newpath.replace(/\/\.$/, '/');
        // Remove any double dots and the path previous.  NB: We can't use
        // the "g", modifier because we are changing the string that we're
        // matching over.
        while (newpath.match(DoubleDot)) {
            newpath = newpath.replace(DoubleDot, '/');
        }
        // Remove any trailing double dots.
        newpath = newpath.replace(/\/([^\/]*)\/\.\.$/, '/');
        // If there are any remaining double dot bits, then they're wrong
        // and must be nuked.  Again, we can't use the g modifier.
        while (newpath.match(/\/\.\.\//)) {
            newpath = newpath.replace(/\/\.\.\//, '/');
        }
        return newpath;
    }

    // RFC3986 §5.2.2. Transform References;
    URI.prototype.resolve = function (base) {
        var target = new URI();
        if (this.scheme) {
            target.scheme    = this.scheme;
            target.authority = this.authority;
            target.path      = remove_dot_segments(this.path);
            target.query     = this.query;
        }
        else {
            if (this.authority) {
                target.authority = this.authority;
                target.path      = remove_dot_segments(this.path);
                target.query     = this.query;
            }
            else {
                // XXX Original spec says "if defined and empty"…;
                if (!this.path) {
                    target.path = base.path;
                    if (this.query) {
                        target.query = this.query;
                    }
                    else {
                        target.query = base.query;
                    }
                }
                else {
                    if (this.path.charAt(0) === '/') {
                        target.path = remove_dot_segments(this.path);
                    } else {
                        target.path = merge(base, this.path);
                        target.path = remove_dot_segments(target.path);
                    }
                    target.query = this.query;
                }
                target.authority = base.authority;
            }
            target.scheme = base.scheme;
        }

        target.fragment = this.fragment;

        return target;
    };
})();