JSFiddle - React, Tailwind, and code Playground

HTML

<p>
    Input:<br/>
    <form id="f1"><input id="input" /></form>
</p>

<p>
    Output:<br/>
    <textarea id="output" cols="50" rows="10"></textarea>
</p>

JavaScript

function Jail() {
    // This is some code inside jail that is user-accessible
    this.hello = function() {
        return "hello world!";
    }
    
    this.eval = function(s) {
        return eval(s);
    }
    
        
    var window = undefined;
    var document = undefined;
    var alert = undefined;
};

(function() {
    var jail = new Jail();
    var hidden = "Some object that I don't want a user to have access to";

    document.getElementById('f1').onsubmit = function() {
        var output = document.getElementById('output');
        try {
            var expr = document.getElementById('input').value;
            var res = jail.eval(expr);
            output.value = res;
        } catch (e) {
            output.value = e.toString();
            console.error(e);
        } finally {
            return false;
        }            
    }
})();