pbb64.js

// ==UserScript==
// @name    Pastebin.com Base64
// @include https://pastebin.com/*
// @version 1
// @grant   none
// ==/UserScript==

window.addEventListener("load", function() {
    if (!document.getElementById("code_frame2")) {
        return;
    };

    var container = document.getElementById("paste_code");
    var container_parent = container.parentNode;
    var master = container_parent.parentNode;

    var label = document.createElement("div");
    label.classList.add("content_title");
    label.classList.add("no_border");
    label.textContent = "Base64 decoded";
    master.appendChild(label);

    var new_container_parent = container_parent.cloneNode(true);
    master.appendChild(new_container_parent);
    var new_container = new_container_parent.querySelector("textarea");

    try {
        new_container.textContent = atob(container.textContent);
    } catch (e) {
        new_container.textContent = "Failed to decode ("+e+")";
        throw e;
    }
}, false);