Web Programming Step by Step, 2nd Edition

Lecture 22: More Events

Reading: 11.1

Except where otherwise noted, the contents of this document are Copyright 2012 Marty Stepp, Jessica Miller, and Victoria Kirst. All rights reserved. Any redistribution, reproduction, transmission, or storage of part or all of the contents in any form is prohibited without the author's expressed written permission.

Valid HTML5 Valid CSS


JavaScript events

abort blur change click dblclick error focus
keydown keypress keyup load mousedown mousemove mouseout
mouseover mouseup reset resize select submit unload

The keyword this

this.fieldName                  // access field
this.fieldName = value;          // modify field

this.methodName(parameters);    // call method

Event handler binding

window.onload = function() {
	document.getElementById("textbox").onmouseout = booyah;
	document.getElementById("submit").onclick = booyah;       // bound to submit button here

function booyah() {           // booyah knows what object it was called on
	this.value = "booyah";

Fixing redundant code with this

<input id="huey"  type="radio" name="ducks" value="Huey"  /> Huey
<input id="dewey" type="radio" name="ducks" value="Dewey" /> Dewey
<input id="louie" type="radio" name="ducks" value="Louie" /> Louie
function processDucks() {
	if (document.getElementById("huey").checked) {
		alert("Huey is checked!");
	} else if (document.getElementById("dewey").checked) {
		alert("Dewey is checked!");
	} else {
		alert("Louie is checked!");
	alert(this.value + " is checked!");

The event object

function name(event) {
	// an event handler function ...
property name description
type what kind of event, such as "click" or "mousedown"
target the element on which the event occurred
timeStamp when the event occurred

Mouse events

click user presses/releases mouse button on the element
dblclick user presses/releases mouse button twice on the element
mousedown user presses down mouse button on the element
mouseup user releases mouse button on the element
mouseover mouse cursor enters the element's box
mouseout mouse cursor exits the element's box
mousemove mouse cursor moves around within the element's box

Mouse event objects

The event passed to a mouse handler has these properties:

mouse event
property/method description
clientX, clientY coordinates in browser window
screenX, screenY coordinates in screen
offsetX, offsetY coordinates in element (non-standard)
button integer representing which button was pressed (0=Left, 1=Middle, 2=Right)

Mouse event example

<pre id="target">Move the mouse over me!</pre>
window.onload = function() {
	var target = document.getElementById("target");
	target.onmousemove = target.onmousedown = showCoords;

function showCoords(event) {
	document.getElementById("target").innerHTML = 
		+ "screen : (" + event.screenX + ", " + event.screenY + ")\n"
		+ "client : (" + event.clientX + ", " + event.clientY + ")\n"
		+ "button : "  + event.button;
Move the mouse over me!

Keyboard/text events

name description
focus this element gains keyboard focus (attention of user's keyboard)
blur this element loses keyboard focus
keydown user presses a key while this element has keyboard focus
keyup user releases a key while this element has keyboard focus
keypress user presses and releases a key while this element has keyboard focus
select this element's text is selected or deselected
Test key events here:

Key event objects

property name description
keyCode ASCII integer value of key that was pressed
(convert to char with String.fromCharCode)
altKey, ctrlKey, shiftKey true if Alt/Ctrl/Shift key is being held

Key event example

document.getElementById("textbox").onkeydown = textKeyDown;
function textKeyDown(event) {
	var key = String.fromCharCode(event.keyCode);
	if (key == 's' && event.altKey) {
		alert("Save the document!");
		this.value = this.value.split("").join("-");

Some useful key codes

keyboard key event keyCode
Backspace 8
Tab 9
Enter 13
Escape 27
Page Up, Page Down, End, Home 33, 34, 35, 36
Left, Up, Right, Down 37, 38, 39, 40
Insert, Delete 45, 46
Windows/Command 91
F1 - F12 112 - 123
Type a key to see its code:    

Page/window events

contextmenu the user right-clicks to pop up a context menu
error an error occurs when loading a document or an image
load, unload the browser loads the page
resize the browser window is resized
scroll the user scrolls the viewable part of the page up/down/left/right
unload the browser exits/leaves the page

Form events

event name description
submit form is being submitted
reset form is being reset
change the text or state of a form control has changed
Try me to see the events Name

Stopping an event

event method name description
preventDefault stops the browser from doing its normal action on an event; for example, stops the browser from following a link when <a> tag is clicked, or stops browser from submitting a form when submit button is clicked
stopPropagation stops the browser from showing this event to any other objects that may be listening for it

Stopping an event, example

<form id="exampleform" action="http://foo.com/foo.php">...</form>
window.onload = function() {
	var form = document.getElementById("exampleform");
	form.onsubmit = checkData;

function checkData(event) {
	if (document.getElementById("state").length != 2) {
		alert("Error, invalid city/state.");  // show error message
		return false;              // stop form submission

Multiple listeners to the same event

element.addEventListener("event", function);
var button = document.getElementById("mybutton");
button.addEventListener("click", func1);   // button.onclick = func1;
button.addEventListener("click", func2);   // button.onclick = func2;

Multiple window.onload listeners

window.onload = function;
window.addEventListener("load", function);