gistfile1.txt

var CustomError = function (msg, code) {
    Error.call(this, msg);
    this.code = code;
}

CustomError.prototype = Object.create(Error.prototype);

function throwError() {
    throw new CustomError("Throwing custom error", 123);
}

try {
    throwError();
} catch (err) {
    if (err instanceof CustomError) {
        console.log("Custom error, code: " + err.code);
    }
    if (err instanceof Error) {
        console.log("Custom error is still an error");
    }
}