In JavaScript programming, you can use code block functions to handle events. 0:00 There are two types of events that can occur. 0:05 A user event or a system event. 0:08 An example of a user event is a mouse click. 0:11 When a mouse click happens on the button, the callback is triggered and 0:14 then an alert is shown. 0:18 An example of a system event is the setTimeout function. 0:20 A call back gets executed after a given time. 0:24 The developer may define the time when the call back gets executed, but 0:27 the system is what triggers it. 0:32 Another example of a system event is the ready state change of an Ajax request. 0:34 [SOUND] The ready state change handler gets triggered by the system when 0:40 it goes through different stages of the request. 0:44 For example, when it sends the request, when it loads data in, and 0:48 when it's finished. 0:53 Node.js has a number of system events like reading data from a URL or 0:55 file, when an action is completed, and various error handling events. 1:00 We'll cover these throughout this course. 1:07