Imagine you are listening to a radio program and they announce, "Big event! Huge! Aliens have landed on Earth!" You might think, "Yeah, whatever"; some other listeners might think "They come in peace"; and some might think, "We're all gonna die!". Similarly, the browser broadcasts events, and your code can be notified should it decide to tune in and listen to the events as they happen. Some example events are as follows:
The user clicks a button
The user types a character in a form field
The page finishes loading
You can attach a JavaScript function called event listener or event handler to a specific event and the browser will invoke your function as soon as the event occurs. Let's see how this is done.