JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.16/angular.min.js"></script>
<div ng-app ng-controller="MyController">
    <button ng-click="addObj()">Add new item</button>
    <ul>
        <li ng-repeat="obj in main.list">{{obj.$$hashKey}}</li>
    </ul>
</div>

JavaScript

function Obj()
{
	this.id = Math.floor(Math.random() * 100000);
}

var MyController = function($scope, $timeout)
{
    $scope.main = {
        list: []
    };

    $scope.$watch("main", function(newValue, oldValue)
    {
        if(newValue == oldValue)
            return;
        var lastObj = $scope.main.list[$scope.main.list.length - 1];
        /*******************************
        /   HERE, IT'S UNDEFINED HERE!
        ********************************/
        $timeout(function(){console.log(lastObj.$$hashKey)}); // undefined
        /*******************************
        /   ^^^^^^^^^^^^^^^^^^^^^^^^
        ********************************/
    }, true);

    $scope.addObj = function()
    {
        $scope.main.list.push(new Obj());
    };
};