JavaScript is based on an action-oriented model of the World Wide Web

[brumbino]

JavaScript is based on an action-oriented model of the World Wide Web. Elements of a Web page, such as a button or checkbox, may trigger actions or events. When one of these events occurs, a corresponding piece of JavaScript code, usually a JavaScript function, is executed.

[adakist]

That function, in turn, is composed of various statements which perform calculations, examine or modify the contents of the Web page, or perform other tasks in order to respond in some way to that event.