When are you developing for browsers, you will have to deal with user interactions and events associated to them, for example, text typed in the textbox, scrolling of the page, mouse button press, and others. When the user does something on the page, an event takes place. Some events are not triggered by user interaction, for example, load event does not require a user input.
When you are dealing with mouse or keyboard events in the browser, you can't predict when and in which order these events will occur. You will have to constantly look for a key press or mouse move to happen. It's like running an endless background loop listening to some key or mouse event to happen. In traditional programming, this was known as polling. There were many variations of these where the waiting thread used to be optimized using queues; however, polling is still not a great idea in general.
Browsers provide a much better alternative to polling. Browsers provide you with programmatic...