JSFiddle - React, Tailwind, and code Playground

HTML

<h4>No Deep</h4>
<div id="noDeep"></div>

<h4>Deep</h4>
<div id="deep"></div>

JavaScript

$(document).ready(function() {
    var obj = $.extend({}, {'a': [1,2,3]}, {'a':[4]});
    $('#noDeep').text(JSON.stringify(obj));
    
    obj = $.extend(true, {}, {'a': [1,2,3]}, {'a':[4]});
    $('#deep').text(JSON.stringify(obj));
});