They create a lot of useful content on there site, which they are happy for users to copy and paste for use elsewhere. They wanted to know how often this was happening, on which pages, and what text.

“Once” handlers Say a button does something pretty darn important, like submitting a payment. It would be pretty scary if it was programmed such that clicking the button multiple times submitted multiple payment requests. It is situations like this where you would attach a click handler to a button that only runs once. To make that clear to the user, we’ll disable the button on click as well.

It's possible to have multiple event listeners for the same event

Listeners are often objects, so they are commonly suffixed with Listener.

Event listener and event handler are two terms that cause confusion.

A listener watches for an event to be fired. For example, a KeyListener waits for KeyEvents, a MessageListener waits for messages to arrive on a queue and so on. The handler is responsible for dealing with the event. Normally, listeners and handlers go hand-in-hand. For example, the KeyListener tells the ExitHandler that "the letter Q was pressed" and the handler performs logic such as cleaning up resources and exiting the application gracefully. Similary a ButtonClickListener would tell the same ExitHandler that the "Exit button was clicked". So, in this case you have two different events, two different listeners but a single handler.

The most basic difference is the association Listener is associated with Event Source (Ex: key board) Handler is associated with an Event (Ex: keydown)

enables passive event listeners by default for some events (see list below). It basically will set { passive: true } automatically every time you declare a new event listener.

(The distinction is conceptually similar to passive versus active event listeners.)