Javascript DOM
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Transcript Javascript DOM
Javascript DOM
Peter Atkinson
1
Objectives
• Understand the nature and structure of the
DOM
• Add and remove content from the page
• Access and change element attributes –
including source and class
• Insert markup into a page using
innerHTML
• Change style attribute using Javascript
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What is the DOM?
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Browser Wars 1997
W3C standardised DOM 1998
End of ‘browser sniffing’
Document Object Model – an API that can
be used by any language. A knowledge of
the DOM will help with PHP or XML
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Nodes Organise the Page
<html>
<head>
<title>My page</title>
</head>
<body>
<p>This is text on my
page</p>
</body>
</html>
html
head
body
title
p
text
text
“my page”
“This is text on
my page”
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What is a Node?
• Element Node – contains an HTML tag
• Text Node – contains text
• Text Nodes are contained in Element
Nodes
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Adding Some Text To A Page
There are five steps:
1. Create a new Element
2. Create new Text
3. Append the new Text to the new Element
4. Find an existing Element
5. Append the new Element to the existing
Element
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1. Create New Element Node
Let us, say create a new <p> tag (element)
so that we can attach some text to it
For convenience, we can put the new object
into a variable
var newNode;
newNode = document.createElement(“p”)
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2. Create a Text Node
Next, create a text node
Again, for convenience, we can put the new
text node into a variable
var newText;
newText = document.createTextNode(“Some text.”)
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3. Attach the New Text Node to the
New Element
To put the text into the page, we have to
attach the text node to the new HTML
element:
newNode.appendChild(newText);
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4.Find an Existing Element
The new element with our text node
attached to it is still floating around in a
Javascript world. We need to find an
existing element so that we can attach it
For convenience, we shall put this existing
element into a variable
var docElement;
docElement = document.getElementById(“thisLocation”);
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5. Append the New Element to the
Existing Element
To insert our text into the page, we now
have to append the new element to the
existing element
docElement.appendChild(newNode);
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Putting the 5 Steps Together
Hands On: Try out this code
<head>
<script language="javascript" type="text/javascript">
var myText;
myText = "This is new text to be added to the page dynamically.";
function addText(location) {
var newNode;
var newText;
var docElement;
newNode = document.createElement("p");
newText = document.createTextNode(myText);
newNode.appendChild(newText);
docElement = document.getElementById(location);
docElement.appendChild(newNode);
}
</script>
</head>
<body>
<p><a href="#" onclick="addText('thisLocation');">Click to add new text to the page</a></p>
<p id="thisLocation">New text will appear below here</p>
<p>Some further text in the page</p>
</body>
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Remove a Node
• To remove a node, we use the element method
removeChild(name of node to be removed)
• For example:
function remText(location) {
var docElement;
docElement = document.getElementById(location);
docElement.removeChild(docElement.lastChild);
}
Hands On
Modify your HTML page so that the user can click on some
text to remove the text that was inserted
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getElementsByTagName()
• getElementById() allows you to work with
elements by their individual id but often
you will want to work with a group of
elements
• getElementsByTagName() allows you to
work with groups of elements. This
method returns an array
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Using getElementsByTagName()
• Hands On
• Open the file JavascriptDOM1.html
• Insert this code at the bottom of the document
body:
<script language="javascript" type="text/javascript">
theseElements = new Array;
theseElements = document.getElementsByTagName("li");
alert(theseElements.length);
</script>
• Now try substituting the tag name li with the wild
card *. Try the code in both IE and Firefox.
• Are you surprised by the number of nodes?
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Stepping Through an Array of
Nodes
• We can step through the array of nodes
and check what kind of node it is:
for (i = 0; i < theseItems.length; i++) {
alert(typeof theseItems[i]);
}
Hands On
Add this code to JavascriptDOM1.html
Execute the code
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Where on the Node Tree?
• childNodes
– nodeList = node.childNodes
• firstChild
– reference = node.firstChild
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lastChild
nextSibling
parentNode
previousSibling
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Attribute Nodes
• We can get at the attributes of an element
through attribute nodes
• Attribute nodes, like text nodes are always
contained in element nodes
• We shall look at methods:
– getAttribute()
– setAttribute()
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Getting Attribute Nodes
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Hands On
Open the file JavascriptDOM2.html
Add this code to alert the attribute of an element:
function dispAttribs() {
var messg;
attribs = new Array;
attribs = document.getElementsByTagName("p");
for (i = 0; i < attribs.length; i++) {
messg = attribs[i].getAttribute("className");
alert(messg);
}
}
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Add this to the bottom of the body:
<p onclick="dispAttribs();">Click here to see class attributes</p>
• Try this in Firefox
• Point to consider: why is this attribute called ‘className’?
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Setting Attribute Nodes
• Hands On
• Open the file JavascriptDOM2.html
• Add this code to change the attribute of an element:
function chngAttribs() {
var messg;
attribs = new Array;
attribs = document.getElementsByTagName("p");
for (i = 0; i < attribs.length; i++) {
attribs[i].setAttribute("className","jazz");
}
}
• Add this to the bottom of the body:
<p onclick="chngAttribs();">Click here to change class
attributes</p>
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User inserts and removes text
• Hands On
• Use file JavascriptDOM3.html
• Place code in this page so that:
– When the user mouseovers on an image, the
relevant text appears
– When the user mouseouts on an image, the
text disappears
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Accessing Images
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Hands On
Open JavascriptDOM4.html
Examine the layout of the page
We are going to modify the behaviour of the
page so that instead of the images displaying in
a new window, they display on the same page
• Write a function that will alter the source of the
placeholder image on the page to another image
• Call this function from the onclick event of each
link
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Suggested Solution
• Hint:
• Get the value of the attribute href from the
link
• Find the placeholder node
• Set the src attribute of the placeholder to
be the same value as the href attribute of
the link
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Code
function showPic(whichpic) {
var source = whichpic.getAttribute("href");
var placeholder =
document.getElementById("placeholder");
placeholder.setAttribute("src",source);
}
In each link, we need an onclick event:
onclick="showPic(this);return false;"
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Web Page Architecture
Javascript for Functionality
CSS for Presentation
HTML for Markup
Progressive Enhancement
Graceful Degradation
• Graceful Degradation = your site is navigable
by users who do not have Javascript
• Progressive Enhancement = page built in
layers:
The page should be constructed using 3 different files – one for each layer
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Graceful Degradation
• Does this degrade gracefully?
<a href=“#” onclick=“popUp(‘http://www.example.com’); return
false;”)>Link to Example</a>
This does degrade gracefully:
<a href=“http://www.example.com”
onclick=“popUp(‘http://www.example.com’); return false;”)>Link
to Example</a>
But it is a little clumsy. There is a shortcut:
<a href=“http://www.example.com” onclick=“popUp(this.href);
return false;”)>Link to Example</a>
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Progressive Enhancement
• Graceful Degradation follows from
Progressive Enhancement
• We need to separate the Javascript from
the markup by removing the event
handlers from the HTML completely
• We can attach events to HTML tags in the
Javascript using the attributes class and id
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Unobtrusive Javascript
• Examine this code:
window.onload = prepareLinks;
function prepareLinks() {
var links = document.getElementsByTagName(‘a’);
for (var i=0; i<links.length; i++) {
if (links[i].className == “popup”) {
links[i].onclick = function() {
popUp(this.getAttribute(“href”));
return false;
}
}
Attaches code to the onclick
event of tags which have
}
been identified by their
}
class name
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Using Unobtrusive Javascript
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Hands On
Open JavascriptDOM4.html
Remove the onclick event handlers
In each a tag put the attribute class=“popup”
Enter the code from the example given so that it
is unobtrusively called from the onclick event
• Also enter this code to open a popup
window:
function popUp(winURL) {
window.open(winURL,”popup”,”width=320,height=480”);
}
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Backwards Compatibility
• Although most browsers fully support the
DOM, some do not support it completely.
• Browser sniffing is too convoluted, so best
to check for specific features
• Put this line of code at the beginning of a
function
if (!document.getElementsByTagName) return false;
• So, if the browser does not support this method
the function will stop
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Tidying Up
• Hands On
• To follow through the principle of completely separating
the three layers, we need to put all our Javascript in a
separate file
• Open JavascriptDOM4.html
• Put the Javascript code into a new file called
javascriptdom4.js
• Put a link into the head:
• <script type=“text/javascript” language=“javascript”
src=“javascriptdom4.js” />
• Put checks into the code to ensure that your page
gracefully degrades if the user’s browser does not have
sufficient level of support for Javascript
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innerHTML
• Hands On
• In the body of a blank HTML page insert a div
tag:
<div id=“test”>This will be replaced</div>
• In the head of the page place this code:
window.onload = function() {
var testdiv = document.getElementBy Id(“testdiv”);
testdiv.innerHTML = “<p>Now we have inserted
<em>this</em> instead!</p>”;
}
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Using innerHTML
• All the HTML in the tag is replaced when
the innerHTML method is used
• innerHTML is not part of the DOM – so it
may one day disappear – though it is
universally recognised by browsers
• Tags within the innerHTML are not part of
the DOM tree so they cannot be
manipulated
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Recap
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Hands On
Now let us use the DOM to insert the same HTML into the div
tag
1. Create an element node “p” assigned to the variable para
2. Create a text node assigned to the variable txt1 (‘Now we
have inserted’)
3. Append txt1 to para
4. Create an element node em assigned to the variable
emphasis
5. Append emphasis to para
6. Create a text node assigned to the variable txt2 (‘this’)
7. Append txt2 to emaphasis
8. Append emphasis to para
9. Create a text node assigned to the variable txt3 (‘instead!’)
10.Append txt3 to para
11.Append para to the element testdiv in the document
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Javascript and CSS
• Hands On
• Open file JavascriptDOM6.html and
examine the code
• Now try out each of the 3 user events
• What do you notice about the difference
between appendChild and insertBefore?
Syntax of insertBefore:
parentElement.insertBefore(newElement,
targetElement)
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Points about CSS
• If style is set inline, it can be manipulated
using Javascript
• If style is set by CSS, Javascript cannot
directly manipulate it
• However, Javascript can set the style of
the element and override the CSS
• Also, Javascript can indirectly manipulate
style using the class tag
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Changing the Class Attribute
• A useful way of manipulating style through Javascript is
to add a second class to an element eg:
thiselement.className += “ newclass”;
– adds the class ‘newclass’ to the class attribute
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Hands On
Open file JavascriptDOM7.html
Create a style for the tags with the class plain
Create a further style with more emphasis called
‘emphatic’
• Write Javascript code that adds the emphatic class to an
element as the user mouseovers and removes it when
the user mouseouts
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Objectives
• Understand the nature and structure of the
DOM
• Add and remove content from the page
• Access and change element attributes –
including source and class
• Insert markup into a page using
innerHTML
• Change style attribute using Javascript
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