JSFiddle - React, Tailwind, and code Playground

JavaScript

Test = function() { this.a = 1; }
    
$(document.body).append($.isPlainObject(new Test()) + '<br/>');

Test.prototype = { b: 1 };

$(document.body).append($.isPlainObject(new Test()) + '<br/>');

Test = function() { this.a = 1; }
Test.prototype = $.extend(Test.prototype, {b: 1});
$(document.body).append($.isPlainObject(new Test()) + '<br/>');