JavaScript is powerful addition to HTML5. It is better understood if we look at JavaScript events and ways it handles events being fired as part of HTML5 web page. 

For instance, events can be fired when HTML5 page loads for the first time or when user click on the button or canvas. JavaScript is smart enough to figure out event that was fired as a result of user action and will lunch a script that will perform action it is designed to do.

There is very definitive list of built-in functions in HTML5 that will recognize an event and trigger JavaScript to process this function call. 

List o event function below is a complete set of event handlers available in HTML5.