Web Programming Step by Step, 2nd Edition

Chapter 9: Document Object Model (DOM)

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 XHTML 1.1 Valid CSS!

9.1: Global DOM Objects

Unobtrusive JavaScript

Obtrusive event handlers (bad)

<button onclick="okayClick();">OK</button>

// called when OK button is clicked
function okayClick() {
	alert("booyah");
}

Attaching an event handler in JavaScript code

// where element is a DOM element object
element.event = function;

<button id="ok">OK</button>

document.getElementById("ok").onclick = okayClick;

When does my code run?

	<head>
		<script src="myfile.js" type="text/javascript"></script>
	</head>

	<body> ... </body>

// global code
var x = 3;
function f(n) { return n + 1; }
function g(n) { return n - 1; }
x = f(x);

A failed attempt at being unobtrusive

	<head>
		<script src="myfile.js" type="text/javascript"></script>
	</head>

	<body>
		<div><button id="ok">OK</button></div>

// global code
document.getElementById("ok").onclick = okayClick;   // error: document.getElementById("ok") is null

The window.onload event 

// this will run once the page has finished loading
function functionName() {
	element.event = functionName;
	element.event = functionName;
	...
}

window.onload = functionName;   // global code

An unobtrusive event handler

<!-- look Ma, no JavaScript! -->
<button id="ok">OK</button>

// called when page loads; sets up event handlers
function pageLoad() {
	document.getElementById("ok").onclick = okayClick;
}

function okayClick() {
	alert("booyah");
}

window.onload = pageLoad;  // global code

Common unobtrusive JS errors

Anonymous functions 

function(parameters) {
	statements;
}

Anonymous function example

window.onload = function() {
	var okButton = document.getElementById("ok");
	okButton.onclick = okayClick;
};

function okayClick() {
	alert("booyah");
}

window.onload = function() {
	var okButton = document.getElementById("ok");
	okButton.onclick = function() {
		alert("booyah");
	};
};

9.2: DOM Element Objects

Complex DOM manipulation problems

How would we do each of the following in JavaScript code? Each involves modifying each one of a group of elements ...

The DOM tree

DOM tree

Types of DOM nodes 

<p>
	This is a paragraph of text with a 
	<a href="/path/page.html">link in it</a>.
</p>
DOM Tree

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" />
</div>
Property Description Example
tagName element's HTML tag document.getElementById("main").tagName is "DIV"
className CSS classes of element document.getElementById("main").className is "foo bar"
innerHTML content inside element document.getElementById("main").innerHTML is "\n <p>Hello, <em>ve...
src URL target of an image document.getElementById("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?
Property Description Example
value the text in an input control document.getElementById("sid").value could be "1234567"
checked whether a box is checked document.getElementById("frosh").checked is true
disabled whether a control is disabled (boolean) document.getElementById("frosh").disabled is false
readOnly whether a text box is read-only document.getElementById("sid").readOnly is false

Modifying text inside an element 

var paragraph = document.getElementById("welcome");
paragraph.innerHTML = "Welcome to our site!";  // change text on page

DOM element objects have the following properties:

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; }

9.3: The DOM Tree

Traversing the DOM tree

every node's DOM object has the following properties:

name(s) description
firstChild, lastChild start/end of this node's list of children
childNodes array of all this node's children
nextSibling, previousSibling neighboring nodes with the same parent
parentNode the element that contains this node

DOM tree traversal example

<p id="foo">This is a paragraph of text with a 
	<a href="/path/to/another/page.html">link</a>.</p>
navigate tree

Element vs. text nodes

<div>
	<p>
		This is a paragraph of text with a 
		<a href="page.html">link</a>.
	</p>
</div>

Selecting groups of DOM objects

name description
getElementsByTagName returns array of descendents with the given tag, such as "div"
getElementsByName returns array of descendents with the given name attribute (mostly useful for accessing form controls)
querySelector * returns the first element that would be matched by the given CSS selector string
querySelectorAll * returns an array of all elements that would be matched by the given CSS selector string

Getting all elements of a certain type

highlight all paragraphs in the document: 

var allParas = document.querySelectorAll("p");
for (var i = 0; i < allParas.length; i++) {
	allParas[i].style.backgroundColor = "yellow";
}

<body>
	<p>This is the first paragraph</p>
	<p>This is the second paragraph</p>
	<p>You get the idea...</p>
</body>

Complex selectors

highlight all paragraphs inside of the section with ID "address": 

// var addrParas = document.getElementById("address").getElementsByTagName("p");
var addrParas = document.querySelectorAll("#address p");
for (var i = 0; i < addrParas.length; i++) {
	addrParas[i].style.backgroundColor = "yellow";
}

<p>This won't be returned!</p>
<div id="address">
	<p>1234 Street</p>
	<p>Atlanta, GA</p>
</div>

Creating new nodes

name description
document.createElement("tag") creates and returns a new empty DOM node representing an element of that type
document.createTextNode("text") creates and returns a text node containing given text 
// create a new <h2> node
var newHeading = document.createElement("h2");
newHeading.innerHTML = "This is a heading";
newHeading.style.color = "green";

Modifying the DOM tree

Every DOM element object has these methods:

name description
appendChild(node) places given node at end of this node's child list
insertBefore(new, old) places the given new node in this node's child list just before old child
removeChild(node) removes given node from this node's child list
replaceChild(new, old) replaces given child with new node 
var p = document.createElement("p");
p.innerHTML = "A paragraph!";
document.getElementById("main").appendChild(p);

Removing a node from the page

function slideClick() {
	var bullets = document.getElementsByTagName("li");
	for (var i = 0; i < bullets.length; i++) {
		if (bullets[i].innerHTML.indexOf("children") >= 0) {
			bullets[i].parentNode.removeChild(bullets[i]);
		}
	}
}

DOM versus innerHTML hacking

Why not just code the previous example this way?

function slideClick() {
	document.getElementById("thisslide").innerHTML += "<p>A paragraph!</p>";
}
  • Imagine that the new node is more complex:
    • ugly: bad style on many levels (e.g. JS code embedded within HTML)
    • error-prone: must carefully distinguish " and '
    • can only add at beginning or end, not in middle of child list
function slideClick() {
	this.innerHTML += "<p style='color: red; " +
			"margin-left: 50px;' " +
			"onclick='myOnClick();'>" +
			"A paragraph!</p>";
}