JSFiddle - React, Tailwind, and code Playground

JavaScript

function Counter() { 
    var count = 0; 
    return Object.freeze({ 
        increment: function() { return ++count; },  
        decrement: function() { return --count; } 
    }); 
}

var counter = new Counter;
alert(counter.increment()); // 1

//malicious code below
counter.increment = function() { return 10; } 
alert(counter.increment()); // 10

var counter = new function(){var count=10; return Object.freeze({increment:function(){return count+=4;}});};
alert(100e0);