CSE 190M Web Programming

Lecture 14: The Document Object Model (DOM); Timers

Reading: 8.2; 9.2; 11.1

Except where otherwise noted, the contents of this document are Copyright 2012 Marty Stepp, Jessica Miller, Victoria Kirst and Roy McElmurry IV. 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

8.2.6: Debugging Common Errors

Debugging JS code in Firebug

Firebug JS



JavaScript "strict" mode

"use strict";

	your code...
(function() {
	"use strict";

	your code...

Events and Timers

Timer events

method description
setTimeout(functiondelayMS); arranges to call given function after given delay in ms
setInterval(functiondelayMS); arranges to call function repeatedly every delayMS ms
stops the given timer so it will not call its function

setTimeout example

<button onclick="delayMsg();">Click me!</button>
<span id="output"></span>
function delayMsg() {
	setTimeout(booyah, 5000);
	document.getElementById("output").innerHTML = "Wait for it...";

function booyah() {   // called when the timer goes off
	document.getElementById("output").innerHTML = "BOOYAH!";

setInterval example

var timer = null;  // stores ID of interval timer

function delayMsg2() {
	if (timer == null) {
		timer = setInterval(rudy, 1000);
	} else {
		timer = null;

function rudy() {   // called each time the timer goes off
	document.getElementById("output").innerHTML += " Rudy!";

Exercise: Alarm Clock page

We want to add javascript to this alarm page.

  • Write javascript code to decrement the time until zero and then print an alert.
    • The clock should start when the button is clicked.
    • The clock should stop when the time reaches zero.
    • The button should not do anyithng if the alarm is already running.

Passing parameters to timers

function delayedMultiply() {
	// 6 and 7 are passed to multiply when timer goes off
	setTimeout(multiply, 2000, 6, 7);
function multiply(a, b) {
	alert(a * b);

Common timer errors

9.2: DOM Element Objects

DOM element objects

dom object

DOM object properties

<div id="main" class="foo bar">
	<p>Hello, <em>very</em> happy to see you!</p>
	<img id="icon" src="images/borat.jpg" alt="Borat" />
var mainDiv = document.getElementById("main");
var icon    = document.getElementById("icon");
Property Description Example
tagName element's HTML tag mainDiv.tagName is "DIV"
className CSS classes of element mainDiv.className is "foo bar"
innerHTML content in element mainDiv.innerHTML is "\n <p>Hello, <em>ve...
src URL target of an image icon.src is "images/borat.jpg"

DOM properties for form controls

<input id="sid" type="text" size="7" maxlength="7" />
<input id="frosh" type="checkbox" checked="checked" /> Freshman?
var sid   = document.getElementById("sid");
var frosh = document.getElementById("frosh");
Property Description Example
value the text/value chosen by the user sid.value could be "1234567"
checked whether a box is checked frosh.checked is true
disabled whether a control is disabled (boolean) frosh.disabled is false
readOnly whether a text box is read-only sid.readOnly is false

More about form controls

<select id="captain">
	<option value="kirk">James T. Kirk</option>
	<option value="picard">Jean-Luc Picard</option>
	<option value="cisco">Benjamin Cisco</option>
<label> <input id="trekkie" type="checkbox" /> I'm a Trekkie </label>

Abuse of innerHTML

// bad style!
var paragraph = document.getElementById("welcome");
paragraph.innerHTML = "<p>text and <a href="page.html">link</a>";

Adjusting styles with the DOM

<button id="clickme">Color Me</button>
window.onload = function() {
	document.getElementById("clickme").onclick = changeColor;
function changeColor() {
	var clickMe = document.getElementById("clickme");
	clickMe.style.color = "red";
Property Description
style lets you set any CSS style property for an element

Common DOM styling errors

Unobtrusive styling

function okayClick() {
	this.style.color = "red";
	this.className = "highlighted";
.highlighted { color: red; }

Getting/setting CSS classes

function highlightField() {
	// turn text yellow and make it bigger
	if (!document.getElementById("text").className) {
		document.getElementById("text").className = "highlight";
	} else if (document.getElementById("text").className.indexOf("invalid") < 0) {
		document.getElementById("text").className += " highlight";

Problems with reading/changing styles

<button id="clickme">Click Me</button>
window.onload = function() {
	document.getElementById("clickme").onclick = biggerFont;
function biggerFont() {
	var size = parseInt(document.getElementById("clickme").style.fontSize);
	size += 4;
	document.getElementById("clickMe").style.fontSize = size + "pt";

Common bug: incorrect usage of existing styles

document.getElementById("main").style.top =
		document.getElementById("main").style.top + 100 + "px";            // bad!

9.1: Global DOM Objects

The six global DOM objects

Every Javascript program can refer to the following global objects:

document current HTML page and its content
history list of pages the user has visited
location URL of the current HTML page
navigator info about the web browser you are using
screen info about the screen area occupied by the browser
window the browser window

The window object

the entire browser window; the top-level object in DOM hierarchy

Popup windows with window.open

window.open("http://foo.com/bar.html", "My Foo Window",

The document object

the current web page and the elements inside it

The location object

the URL of the current web page

The navigator object

information about the web browser application

The screen object

information about the client's display screen

The history object

the list of sites the browser has visited in this window