ECMAScript 5 introduced a new mechanism for managing and configuring what can be done to an object's properties. New .defineProperty() and .defineProperties() methods were implemented. These methods let you control the behavior of each property.
As you can most certainly guess, they define new object properties, just like the dot-notation property definition with one big difference: these methods let you control the behavior of each property.
By "behavior" I mean the ability to change the value of the property after it's been created (writable), the ability to control the available features of the property once defined (configurable). Last, but most relevant to this article, ES5 added the ability to determine whether the property will be enumerable (shown in the for..in loop).
Read more at: https://imelgrat.me/javascript/object-properties-iteration-in-javascript/