JSFiddle - React, Tailwind, and code Playground

HTML

<div id="log"></div>

JavaScript

// working on 35.0.1916.153 m
// NOT working on 37.0.2062.94 m
    var arr = [];
var CONST_SYSTEM_KEY = '___system_callback____';

var logElement = document.getElementById('log');
function log(text){
    logElement.innerHTML += text + "<br>";
}

    function watchKeyChanges(obj, key) {
        
        if (Object.observe){
            // hack for CHROME 37
            // http://stackoverflow.com/questions/25565237/defineproperty-to-array-item-on-chrome-browser
            var cl = function(changes){
                for (var i = 0, l = changes.length; i < l; i++){
                    var item = changes[i];
                    if (item.name === key){
                        // console.log(key);
                        var params = {
                            val: item.object[item.name],
                            old: item.oldValue
                        };
                        handler(params);
                    }
                }
            }
            Object.observe(obj, cl);
            obj[CONST_SYSTEM_KEY] = cl;
        } else {
            
        var oldval = obj[key];
        var newval = oldval;

        function getter() {
            return newval;
        }

        function setter(val) {
            newval = val;
            oldval = val;
        }

        Object.defineProperty(obj, key, {
            enumerable: true,
            configurable: true,
            get: getter,
            set: setter
        });
        }
    }

    function unwatchKeyChanges(obj, key) {
        if (Object.unobserve) {
            var cl = obj[CONST_SYSTEM_KEY];
            cl && Object.unobserve(obj, cl);
        }
        var val = obj[key];
        delete obj[key]; // remove accessors
        obj[key] = val;
    }

    function fillArray() {
        for (var i = 0; i < 100; i++) {
            var item = {
                test: 'test'
            };

            if (arr[i]) {
                log('element "' + i + '" is exist! but it not must!');
        ...