Async document.writeln

script.js

const evalCode = (code) => {
    const oldWrite = document.writeln;
    const sandbox = document.createElement('div');
    document.body.appendChild(sandbox);
    document.writeln = (content) => {
        sandbox.innerHTML += content;
    };

    try {
        (window.execScript || function(data) {
            window['eval'].call(window, data);
        })(code);
    } finally {
        document.writeln = oldWrite;
        document.body.removeChild(sandbox);
    }

    return sandbox.innerHTML;
};