I found answer to your question here, in the future you can do it on your own. Hope you'll find it helpful!

The condition if (document.addEventListener) checks if the addEventListener method is available in the document object. The addEventListener method is used to attach an event listener to an element. If the method is available, it means that the browser supports the W3C DOM standard for event handling, and the code inside the if block will be executed.

In this case, since the condition returns true, it implies that the addEventListener method is supported by the browser. Therefore, the code proceeds to define the add and remove methods on the etHandler object.

The purpose of creating the etHandler.add method is to provide a generic way of adding event handlers to elements. It takes three parameters: element (the target element to attach the event listener to), eventType (the type of event to listen for), and eventFunction (the function to be executed when the event occurs). This method allows you to add event listeners dynamically by calling etHandler.add(element, eventType, eventFunction).

Similarly, the etHandler.remove method is created to remove event handlers from elements. It uses the detachEvent method (for older versions of Internet Explorer) to detach the event handler.

By creating these methods on the etHandler object, you can manage event handling in a modular and reusable way, irrespective of whether the browser supports the modern addEventListener method or the older detachEvent method.