Event Handlers

Well, they kind of do what the name implies: they handle events that take place during the execution of a Web page or JS code. Events like what the user does with his/her mouse. They usually work in conjunction with HTML forms, which provide for text boxes, text areas, radio buttons, checkboxes, and other input graphics like that. (You've probably all seen them; go to amazon.com to see a bunch of these things, or any other site which sells goods online.) Some of these event handlers are interactive, which means that they can be applied to events initiated by the user; others are non-interactive, which means they execute when the page loads or unloads--not when the user directly does something like click or move a mouse.

There are four parts to handling events (even though our text only lists three):

• The event itself: the user clicking a mouse, moving a mouse over the screen, or accessing a Web page;
• the event target: the HTML, on-screen form item (a button, a radio button, a text box) with which the event handler is associated;
• the event handler, a special method which "handles" a user event like clicking a mouse;
• the JavaScript function which is invoked when an event handler swings into action.

  Here's a list of JS event handlers, and the HTML form graphics to which they apply.

  Event Handler	HTML form object to which it applies
  onAbort	image
  onBlur	select, text, text area
  onChange	select, text, text area
  onClick	button, checkbox, radio, link, reset, submit, area
  onError	image
  onFocus	select, text, textarea
  onLoad	window, image
  onMouseOver	link, area
  onMouseOut	link, area
  onReset	form
  onSelect	text, textarea
  onSubmit	form
  onUnload	window

  onMouseOver and onMouseOut are often used for some cool graphic effects, like loading and unloading an image when you slide your mouse over a Web page. onClick and onChange are some of the most commonly used event handlers, because this is how you would load user input from an HTML form into your JS code.